gcc/extend.texi

@c Copyright (C) 1988,89,92,93,94,96,98,99,2000,01 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

@node C Extensions
@chapter Extensions to the C Language Family
@cindex extensions, C language
@cindex C language extensions

GNU C provides several language features not found in ISO standard C.
(The @samp{-pedantic} option directs GNU CC to print a warning message if
any of these features is used.)  To test for the availability of these
features in conditional compilation, check for a predefined macro
@code{__GNUC__}, which is always defined under GNU CC.

These extensions are available in C and Objective C.  Most of them are
also available in C++.  @xref{C++ Extensions,,Extensions to the
C++ Language}, for extensions that apply @emph{only} to C++.

@c FIXME: document clearly which features are in ISO C99, but also
@c accepted as extensions for -std=gnu89 and possibly for C++.
@c See PR other/930.

@c FIXME: the documentation for preprocessor extensions here is out of
@c date.  See PR other/928.

@c The only difference between the two versions of this menu is that the
@c version for clear INTERNALS has an extra node, "Constraints" (which
@c appears in a separate chapter in the other version of the manual).
@ifset INTERNALS
@menu
* Statement Exprs::     Putting statements and declarations inside expressions.
* Local Labels::        Labels local to a statement-expression.
* Labels as Values::    Getting pointers to labels, and computed gotos.
* Nested Functions::    As in Algol and Pascal, lexical scoping of functions.
* Constructing Calls::	Dispatching a call to another function.
* Naming Types::        Giving a name to the type of some expression.
* Typeof::              @code{typeof}: referring to the type of an expression.
* Lvalues::             Using @samp{?:}, @samp{,} and casts in lvalues.
* Conditionals::        Omitting the middle operand of a @samp{?:} expression.
* Long Long::		Double-word integers---@code{long long int}.
* Complex::             Data types for complex numbers.
* Hex Floats::          Hexadecimal floating-point constants.
* Zero Length::         Zero-length arrays.
* Variable Length::     Arrays whose length is computed at run time.
* Variadic Macros::	Macros with a variable number of arguments.
* Escaped Newlines::    Slightly looser rules for escaped newlines.
* Multi-line Strings::  String literals with embedded newlines.
* Subscripting::        Any array can be subscripted, even if not an lvalue.
* Pointer Arith::       Arithmetic on @code{void}-pointers and function pointers.
* Initializers::        Non-constant initializers.
* Constructors::        Constructor expressions give structures, unions
                         or arrays as values.
* Labeled Elements::	Labeling elements of initializers.
* Cast to Union::       Casting to union type from any member of the union.
* Case Ranges::		`case 1 ... 9' and such.
* Function Attributes:: Declaring that functions have no side effects,
                         or that they can never return.
* Attribute Syntax::    Formal syntax for attributes.
* Function Prototypes:: Prototype declarations and old-style definitions.
* C++ Comments::        C++ comments are recognized.
* Dollar Signs::        Dollar sign is allowed in identifiers.
* Character Escapes::   @samp{\e} stands for the character @key{ESC}.
* Variable Attributes::	Specifying attributes of variables.
* Type Attributes::	Specifying attributes of types.
* Alignment::           Inquiring about the alignment of a type or variable.
* Inline::              Defining inline functions (as fast as macros).
* Extended Asm::        Assembler instructions with C expressions as operands.
                         (With them you can define ``built-in'' functions.)
* Asm Labels::          Specifying the assembler name to use for a C symbol.
* Explicit Reg Vars::   Defining variables residing in specified registers.
* Alternate Keywords::  @code{__const__}, @code{__asm__}, etc., for header files.
* Incomplete Enums::    @code{enum foo;}, with details to follow.
* Function Names::	Printable strings which are the name of the current
			 function.
* Return Address::      Getting the return or frame address of a function.
* Other Builtins::      Other built-in functions.
@end menu
@end ifset
@ifclear INTERNALS
@menu
* Statement Exprs::     Putting statements and declarations inside expressions.
* Local Labels::        Labels local to a statement-expression.
* Labels as Values::    Getting pointers to labels, and computed gotos.
* Nested Functions::    As in Algol and Pascal, lexical scoping of functions.
* Constructing Calls::	Dispatching a call to another function.
* Naming Types::        Giving a name to the type of some expression.
* Typeof::              @code{typeof}: referring to the type of an expression.
* Lvalues::             Using @samp{?:}, @samp{,} and casts in lvalues.
* Conditionals::        Omitting the middle operand of a @samp{?:} expression.
* Long Long::		Double-word integers---@code{long long int}.
* Complex::             Data types for complex numbers.
* Hex Floats::          Hexadecimal floating-point constants.
* Zero Length::         Zero-length arrays.
* Variable Length::     Arrays whose length is computed at run time.
* Variadic Macros::	Macros with a variable number of arguments.
* Escaped Newlines::    Slightly looser rules for escaped newlines.
* Multi-line Strings::  String literals with embedded newlines.
* Subscripting::        Any array can be subscripted, even if not an lvalue.
* Pointer Arith::       Arithmetic on @code{void}-pointers and function pointers.
* Initializers::        Non-constant initializers.
* Constructors::        Constructor expressions give structures, unions
                         or arrays as values.
* Labeled Elements::	Labeling elements of initializers.
* Cast to Union::       Casting to union type from any member of the union.
* Case Ranges::		`case 1 ... 9' and such.
* Function Attributes:: Declaring that functions have no side effects,
                         or that they can never return.
* Attribute Syntax::    Formal syntax for attributes.
* Function Prototypes:: Prototype declarations and old-style definitions.
* C++ Comments::        C++ comments are recognized.
* Dollar Signs::        Dollar sign is allowed in identifiers.
* Character Escapes::   @samp{\e} stands for the character @key{ESC}.
* Variable Attributes::	Specifying attributes of variables.
* Type Attributes::	Specifying attributes of types.
* Alignment::           Inquiring about the alignment of a type or variable.
* Inline::              Defining inline functions (as fast as macros).
* Extended Asm::        Assembler instructions with C expressions as operands.
                         (With them you can define ``built-in'' functions.)
* Constraints::         Constraints for asm operands
* Asm Labels::          Specifying the assembler name to use for a C symbol.
* Explicit Reg Vars::   Defining variables residing in specified registers.
* Alternate Keywords::  @code{__const__}, @code{__asm__}, etc., for header files.
* Incomplete Enums::    @code{enum foo;}, with details to follow.
* Function Names::	Printable strings which are the name of the current
			 function.
* Return Address::      Getting the return or frame address of a function.
* Other Builtins::      Other built-in functions.
@end menu
@end ifclear

@node Statement Exprs
@section Statements and Declarations in Expressions
@cindex statements inside expressions
@cindex declarations inside expressions
@cindex expressions containing statements
@cindex macros, statements in expressions

@c the above section title wrapped and causes an underfull hbox.. i
@c changed it from "within" to "in". --mew 4feb93

A compound statement enclosed in parentheses may appear as an expression
in GNU C.  This allows you to use loops, switches, and local variables
within an expression.

Recall that a compound statement is a sequence of statements surrounded
by braces; in this construct, parentheses go around the braces.  For
example:

@example
(@{ int y = foo (); int z;
   if (y > 0) z = y;
   else z = - y;
   z; @})
@end example

@noindent
is a valid (though slightly more complex than necessary) expression
for the absolute value of @code{foo ()}.

The last thing in the compound statement should be an expression
followed by a semicolon; the value of this subexpression serves as the
value of the entire construct.  (If you use some other kind of statement
last within the braces, the construct has type @code{void}, and thus
effectively no value.)

This feature is especially useful in making macro definitions ``safe'' (so
that they evaluate each operand exactly once).  For example, the
``maximum'' function is commonly defined as a macro in standard C as
follows:

@example
#define max(a,b) ((a) > (b) ? (a) : (b))
@end example

@noindent
@cindex side effects, macro argument
But this definition computes either @var{a} or @var{b} twice, with bad
results if the operand has side effects.  In GNU C, if you know the
type of the operands (here let's assume @code{int}), you can define
the macro safely as follows:

@example
#define maxint(a,b) \
  (@{int _a = (a), _b = (b); _a > _b ? _a : _b; @})
@end example

Embedded statements are not allowed in constant expressions, such as
the value of an enumeration constant, the width of a bit field, or
the initial value of a static variable.

If you don't know the type of the operand, you can still do this, but you
must use @code{typeof} (@pxref{Typeof}) or type naming (@pxref{Naming
Types}).

Statement expressions are not supported fully in G++, and their fate
there is unclear.  (It is possible that they will become fully supported
at some point, or that they will be deprecated, or that the bugs that
are present will continue to exist indefinitely.)  Presently, statement
expressions do not work well as default arguments. 

In addition, there are semantic issues with statement-expressions in
C++.  If you try to use statement-expressions instead of inline
functions in C++, you may be surprised at the way object destruction is
handled.  For example:

@example
#define foo(a)  (@{int b = (a); b + 3; @})
@end example

@noindent
does not work the same way as:

@example
inline int foo(int a) @{ int b = a; return b + 3; @}
@end example

@noindent
In particular, if the expression passed into @code{foo} involves the
creation of temporaries, the destructors for those temporaries will be
run earlier in the case of the macro than in the case of the function.

These considerations mean that it is probably a bad idea to use
statement-expressions of this form in header files that are designed to
work with C++.  (Note that some versions of the GNU C Library contained
header files using statement-expression that lead to precisely this
bug.)

@node Local Labels
@section Locally Declared Labels
@cindex local labels
@cindex macros, local labels

Each statement expression is a scope in which @dfn{local labels} can be
declared.  A local label is simply an identifier; you can jump to it
with an ordinary @code{goto} statement, but only from within the
statement expression it belongs to.

A local label declaration looks like this:

@example
__label__ @var{label};
@end example

@noindent
or

@example
__label__ @var{label1}, @var{label2}, @dots{};
@end example

Local label declarations must come at the beginning of the statement
expression, right after the @samp{(@{}, before any ordinary
declarations.

The label declaration defines the label @emph{name}, but does not define
the label itself.  You must do this in the usual way, with
@code{@var{label}:}, within the statements of the statement expression.

The local label feature is useful because statement expressions are
often used in macros.  If the macro contains nested loops, a @code{goto}
can be useful for breaking out of them.  However, an ordinary label
whose scope is the whole function cannot be used: if the macro can be
expanded several times in one function, the label will be multiply
defined in that function.  A local label avoids this problem.  For
example:

@example
#define SEARCH(array, target)                     \
(@{                                               \
  __label__ found;                                \
  typeof (target) _SEARCH_target = (target);      \
  typeof (*(array)) *_SEARCH_array = (array);     \
  int i, j;                                       \
  int value;                                      \
  for (i = 0; i < max; i++)                       \
    for (j = 0; j < max; j++)                     \
      if (_SEARCH_array[i][j] == _SEARCH_target)  \
        @{ value = i; goto found; @}              \
  value = -1;                                     \
 found:                                           \
  value;                                          \
@})
@end example

@node Labels as Values
@section Labels as Values
@cindex labels as values
@cindex computed gotos
@cindex goto with computed label
@cindex address of a label

You can get the address of a label defined in the current function
(or a containing function) with the unary operator @samp{&&}.  The
value has type @code{void *}.  This value is a constant and can be used
wherever a constant of that type is valid.  For example:

@example
void *ptr;
@dots{}
ptr = &&foo;
@end example

To use these values, you need to be able to jump to one.  This is done
with the computed goto statement@footnote{The analogous feature in
Fortran is called an assigned goto, but that name seems inappropriate in
C, where one can do more than simply store label addresses in label
variables.}, @code{goto *@var{exp};}.  For example,

@example
goto *ptr;
@end example

@noindent
Any expression of type @code{void *} is allowed.

One way of using these constants is in initializing a static array that
will serve as a jump table:

@example
static void *array[] = @{ &&foo, &&bar, &&hack @};
@end example

Then you can select a label with indexing, like this:

@example
goto *array[i];
@end example

@noindent
Note that this does not check whether the subscript is in bounds---array
indexing in C never does that.

Such an array of label values serves a purpose much like that of the
@code{switch} statement.  The @code{switch} statement is cleaner, so
use that rather than an array unless the problem does not fit a
@code{switch} statement very well.

Another use of label values is in an interpreter for threaded code.
The labels within the interpreter function can be stored in the
threaded code for super-fast dispatching.

You may not use this mechanism to jump to code in a different function. 
If you do that, totally unpredictable things will happen.  The best way to
avoid this is to store the label address only in automatic variables and
never pass it as an argument.

An alternate way to write the above example is

@example
static const int array[] = @{ &&foo - &&foo, &&bar - &&foo, &&hack - &&foo @};
goto *(&&foo + array[i]);
@end example

@noindent
This is more friendly to code living in shared libraries, as it reduces
the number of dynamic relocations that are needed, and by consequence,
allows the data to be read-only.

@node Nested Functions
@section Nested Functions
@cindex nested functions
@cindex downward funargs
@cindex thunks

A @dfn{nested function} is a function defined inside another function.
(Nested functions are not supported for GNU C++.)  The nested function's
name is local to the block where it is defined.  For example, here we
define a nested function named @code{square}, and call it twice:

@example
@group
foo (double a, double b)
@{
  double square (double z) @{ return z * z; @}

  return square (a) + square (b);
@}
@end group
@end example

The nested function can access all the variables of the containing
function that are visible at the point of its definition.  This is
called @dfn{lexical scoping}.  For example, here we show a nested
function which uses an inherited variable named @code{offset}:

@example
bar (int *array, int offset, int size)
@{
  int access (int *array, int index)
    @{ return array[index + offset]; @}
  int i;
  @dots{}
  for (i = 0; i < size; i++)
    @dots{} access (array, i) @dots{}
@}
@end example

Nested function definitions are permitted within functions in the places
where variable definitions are allowed; that is, in any block, before
the first statement in the block.

It is possible to call the nested function from outside the scope of its
name by storing its address or passing the address to another function:

@example
hack (int *array, int size)
@{
  void store (int index, int value)
    @{ array[index] = value; @}

  intermediate (store, size);
@}
@end example

Here, the function @code{intermediate} receives the address of
@code{store} as an argument.  If @code{intermediate} calls @code{store},
the arguments given to @code{store} are used to store into @code{array}.
But this technique works only so long as the containing function
(@code{hack}, in this example) does not exit.

If you try to call the nested function through its address after the
containing function has exited, all hell will break loose.  If you try
to call it after a containing scope level has exited, and if it refers
to some of the variables that are no longer in scope, you may be lucky,
but it's not wise to take the risk.  If, however, the nested function
does not refer to anything that has gone out of scope, you should be
safe.

GNU CC implements taking the address of a nested function using a
technique called @dfn{trampolines}.   A paper describing them is
available as @uref{http://people.debian.org/~karlheg/Usenix88-lexic.pdf}.

A nested function can jump to a label inherited from a containing
function, provided the label was explicitly declared in the containing
function (@pxref{Local Labels}).  Such a jump returns instantly to the
containing function, exiting the nested function which did the
@code{goto} and any intermediate functions as well.  Here is an example:

@example
@group
bar (int *array, int offset, int size)
@{
  __label__ failure;
  int access (int *array, int index)
    @{
      if (index > size)
        goto failure;
      return array[index + offset];
    @}
  int i;
  @dots{}
  for (i = 0; i < size; i++)
    @dots{} access (array, i) @dots{}
  @dots{}
  return 0;

 /* @r{Control comes here from @code{access}
    if it detects an error.}  */
 failure:
  return -1;
@}
@end group
@end example

A nested function always has internal linkage.  Declaring one with
@code{extern} is erroneous.  If you need to declare the nested function
before its definition, use @code{auto} (which is otherwise meaningless
for function declarations).

@example
bar (int *array, int offset, int size)
@{
  __label__ failure;
  auto int access (int *, int);
  @dots{}
  int access (int *array, int index)
    @{
      if (index > size)
        goto failure;
      return array[index + offset];
    @}
  @dots{}
@}
@end example

@node Constructing Calls
@section Constructing Function Calls
@cindex constructing calls
@cindex forwarding calls

Using the built-in functions described below, you can record
the arguments a function received, and call another function
with the same arguments, without knowing the number or types
of the arguments.

You can also record the return value of that function call,
and later return that value, without knowing what data type
the function tried to return (as long as your caller expects
that data type).

@table @code
@findex __builtin_apply_args
@item __builtin_apply_args ()
This built-in function returns a pointer of type @code{void *} to data
describing how to perform a call with the same arguments as were passed
to the current function.

The function saves the arg pointer register, structure value address,
and all registers that might be used to pass arguments to a function
into a block of memory allocated on the stack.  Then it returns the
address of that block.

@findex __builtin_apply
@item __builtin_apply (@var{function}, @var{arguments}, @var{size})
This built-in function invokes @var{function} (type @code{void (*)()})
with a copy of the parameters described by @var{arguments} (type
@code{void *}) and @var{size} (type @code{int}).

The value of @var{arguments} should be the value returned by
@code{__builtin_apply_args}.  The argument @var{size} specifies the size
of the stack argument data, in bytes.

This function returns a pointer of type @code{void *} to data describing
how to return whatever value was returned by @var{function}.  The data
is saved in a block of memory allocated on the stack.

It is not always simple to compute the proper value for @var{size}.  The
value is used by @code{__builtin_apply} to compute the amount of data
that should be pushed on the stack and copied from the incoming argument
area.

@findex __builtin_return
@item __builtin_return (@var{result})
This built-in function returns the value described by @var{result} from
the containing function.  You should specify, for @var{result}, a value
returned by @code{__builtin_apply}.
@end table

@node Naming Types
@section Naming an Expression's Type
@cindex naming types

You can give a name to the type of an expression using a @code{typedef}
declaration with an initializer.  Here is how to define @var{name} as a
type name for the type of @var{exp}:

@example
typedef @var{name} = @var{exp};
@end example

This is useful in conjunction with the statements-within-expressions
feature.  Here is how the two together can be used to define a safe
``maximum'' macro that operates on any arithmetic type:

@example
#define max(a,b) \
  (@{typedef _ta = (a), _tb = (b);  \
    _ta _a = (a); _tb _b = (b);     \
    _a > _b ? _a : _b; @})
@end example

@cindex underscores in variables in macros
@cindex @samp{_} in variables in macros
@cindex local variables in macros
@cindex variables, local, in macros
@cindex macros, local variables in

The reason for using names that start with underscores for the local
variables is to avoid conflicts with variable names that occur within the
expressions that are substituted for @code{a} and @code{b}.  Eventually we
hope to design a new form of declaration syntax that allows you to declare
variables whose scopes start only after their initializers; this will be a
more reliable way to prevent such conflicts.

@node Typeof
@section Referring to a Type with @code{typeof}
@findex typeof
@findex sizeof
@cindex macros, types of arguments

Another way to refer to the type of an expression is with @code{typeof}.
The syntax of using of this keyword looks like @code{sizeof}, but the
construct acts semantically like a type name defined with @code{typedef}.

There are two ways of writing the argument to @code{typeof}: with an
expression or with a type.  Here is an example with an expression:

@example
typeof (x[0](1))
@end example

@noindent
This assumes that @code{x} is an array of pointers to functions;
the type described is that of the values of the functions.

Here is an example with a typename as the argument:

@example
typeof (int *)
@end example

@noindent
Here the type described is that of pointers to @code{int}.

If you are writing a header file that must work when included in ISO C
programs, write @code{__typeof__} instead of @code{typeof}.
@xref{Alternate Keywords}.

A @code{typeof}-construct can be used anywhere a typedef name could be
used.  For example, you can use it in a declaration, in a cast, or inside
of @code{sizeof} or @code{typeof}.

@itemize @bullet
@item
This declares @code{y} with the type of what @code{x} points to.

@example
typeof (*x) y;
@end example

@item
This declares @code{y} as an array of such values.

@example
typeof (*x) y[4];
@end example

@item
This declares @code{y} as an array of pointers to characters:

@example
typeof (typeof (char *)[4]) y;
@end example

@noindent
It is equivalent to the following traditional C declaration:

@example
char *y[4];
@end example

To see the meaning of the declaration using @code{typeof}, and why it
might be a useful way to write, let's rewrite it with these macros:

@example
#define pointer(T)  typeof(T *)
#define array(T, N) typeof(T [N])
@end example

@noindent
Now the declaration can be rewritten this way:

@example
array (pointer (char), 4) y;
@end example

@noindent
Thus, @code{array (pointer (char), 4)} is the type of arrays of 4
pointers to @code{char}.
@end itemize

@node Lvalues
@section Generalized Lvalues
@cindex compound expressions as lvalues
@cindex expressions, compound, as lvalues
@cindex conditional expressions as lvalues
@cindex expressions, conditional, as lvalues
@cindex casts as lvalues
@cindex generalized lvalues
@cindex lvalues, generalized
@cindex extensions, @code{?:}
@cindex @code{?:} extensions
Compound expressions, conditional expressions and casts are allowed as
lvalues provided their operands are lvalues.  This means that you can take
their addresses or store values into them.

Standard C++ allows compound expressions and conditional expressions as
lvalues, and permits casts to reference type, so use of this extension
is deprecated for C++ code.

For example, a compound expression can be assigned, provided the last
expression in the sequence is an lvalue.  These two expressions are
equivalent:

@example
(a, b) += 5
a, (b += 5)
@end example

Similarly, the address of the compound expression can be taken.  These two
expressions are equivalent:

@example
&(a, b)
a, &b
@end example

A conditional expression is a valid lvalue if its type is not void and the
true and false branches are both valid lvalues.  For example, these two
expressions are equivalent:

@example
(a ? b : c) = 5
(a ? b = 5 : (c = 5))
@end example

A cast is a valid lvalue if its operand is an lvalue.  A simple
assignment whose left-hand side is a cast works by converting the
right-hand side first to the specified type, then to the type of the
inner left-hand side expression.  After this is stored, the value is
converted back to the specified type to become the value of the
assignment.  Thus, if @code{a} has type @code{char *}, the following two
expressions are equivalent:

@example
(int)a = 5
(int)(a = (char *)(int)5)
@end example

An assignment-with-arithmetic operation such as @samp{+=} applied to a cast
performs the arithmetic using the type resulting from the cast, and then
continues as in the previous case.  Therefore, these two expressions are
equivalent:

@example
(int)a += 5
(int)(a = (char *)(int) ((int)a + 5))
@end example

You cannot take the address of an lvalue cast, because the use of its
address would not work out coherently.  Suppose that @code{&(int)f} were
permitted, where @code{f} has type @code{float}.  Then the following
statement would try to store an integer bit-pattern where a floating
point number belongs:

@example
*&(int)f = 1;
@end example

This is quite different from what @code{(int)f = 1} would do---that
would convert 1 to floating point and store it.  Rather than cause this
inconsistency, we think it is better to prohibit use of @samp{&} on a cast.

If you really do want an @code{int *} pointer with the address of
@code{f}, you can simply write @code{(int *)&f}.

@node Conditionals
@section Conditionals with Omitted Operands
@cindex conditional expressions, extensions
@cindex omitted middle-operands
@cindex middle-operands, omitted
@cindex extensions, @code{?:}
@cindex @code{?:} extensions

The middle operand in a conditional expression may be omitted.  Then
if the first operand is nonzero, its value is the value of the conditional
expression.

Therefore, the expression

@example
x ? : y
@end example

@noindent
has the value of @code{x} if that is nonzero; otherwise, the value of
@code{y}.

This example is perfectly equivalent to

@example
x ? x : y
@end example

@cindex side effect in ?:
@cindex ?: side effect
@noindent
In this simple case, the ability to omit the middle operand is not
especially useful.  When it becomes useful is when the first operand does,
or may (if it is a macro argument), contain a side effect.  Then repeating
the operand in the middle would perform the side effect twice.  Omitting
the middle operand uses the value already computed without the undesirable
effects of recomputing it.

@node Long Long
@section Double-Word Integers
@cindex @code{long long} data types
@cindex double-word arithmetic
@cindex multiprecision arithmetic

GNU C supports data types for integers that are twice as long as
@code{int}.  Simply write @code{long long int} for a signed integer, or
@code{unsigned long long int} for an unsigned integer.  To make an
integer constant of type @code{long long int}, add the suffix @code{LL}
to the integer.  To make an integer constant of type @code{unsigned long
long int}, add the suffix @code{ULL} to the integer.

You can use these types in arithmetic like any other integer types.
Addition, subtraction, and bitwise boolean operations on these types
are open-coded on all types of machines.  Multiplication is open-coded
if the machine supports fullword-to-doubleword a widening multiply
instruction.  Division and shifts are open-coded only on machines that
provide special support.  The operations that are not open-coded use
special library routines that come with GNU CC.

There may be pitfalls when you use @code{long long} types for function
arguments, unless you declare function prototypes.  If a function
expects type @code{int} for its argument, and you pass a value of type
@code{long long int}, confusion will result because the caller and the
subroutine will disagree about the number of bytes for the argument.
Likewise, if the function expects @code{long long int} and you pass
@code{int}.  The best way to avoid such problems is to use prototypes.

@node Complex
@section Complex Numbers
@cindex complex numbers

GNU C supports complex data types.  You can declare both complex integer
types and complex floating types, using the keyword @code{__complex__}.

For example, @samp{__complex__ double x;} declares @code{x} as a
variable whose real part and imaginary part are both of type
@code{double}.  @samp{__complex__ short int y;} declares @code{y} to
have real and imaginary parts of type @code{short int}; this is not
likely to be useful, but it shows that the set of complex types is
complete.

To write a constant with a complex data type, use the suffix @samp{i} or
@samp{j} (either one; they are equivalent).  For example, @code{2.5fi}
has type @code{__complex__ float} and @code{3i} has type
@code{__complex__ int}.  Such a constant always has a pure imaginary
value, but you can form any complex value you like by adding one to a
real constant.

To extract the real part of a complex-valued expression @var{exp}, write
@code{__real__ @var{exp}}.  Likewise, use @code{__imag__} to
extract the imaginary part.

The operator @samp{~} performs complex conjugation when used on a value
with a complex type.

GNU CC can allocate complex automatic variables in a noncontiguous
fashion; it's even possible for the real part to be in a register while
the imaginary part is on the stack (or vice-versa).  None of the
supported debugging info formats has a way to represent noncontiguous
allocation like this, so GNU CC describes a noncontiguous complex
variable as if it were two separate variables of noncomplex type.
If the variable's actual name is @code{foo}, the two fictitious
variables are named @code{foo$real} and @code{foo$imag}.  You can
examine and set these two fictitious variables with your debugger.

A future version of GDB will know how to recognize such pairs and treat
them as a single variable with a complex type.

@node Hex Floats
@section Hex Floats
@cindex hex floats

GNU CC recognizes floating-point numbers written not only in the usual
decimal notation, such as @code{1.55e1}, but also numbers such as
@code{0x1.fp3} written in hexadecimal format.  In that format the
@code{0x} hex introducer and the @code{p} or @code{P} exponent field are
mandatory.  The exponent is a decimal number that indicates the power of
2 by which the significant part will be multiplied.  Thus @code{0x1.f} is
1 15/16, @code{p3} multiplies it by 8, and the value of @code{0x1.fp3}
is the same as @code{1.55e1}.

Unlike for floating-point numbers in the decimal notation the exponent
is always required in the hexadecimal notation.  Otherwise the compiler
would not be able to resolve the ambiguity of, e.g., @code{0x1.f}.  This
could mean @code{1.0f} or @code{1.9375} since @code{f} is also the
extension for floating-point constants of type @code{float}.

@node Zero Length
@section Arrays of Length Zero
@cindex arrays of length zero
@cindex zero-length arrays
@cindex length-zero arrays
@cindex flexible array members

Zero-length arrays are allowed in GNU C.  They are very useful as the
last element of a structure which is really a header for a variable-length
object:

@example
struct line @{
  int length;
  char contents[0];
@};

struct line *thisline = (struct line *)
  malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
@end example

In ISO C89, you would have to give @code{contents} a length of 1, which
means either you waste space or complicate the argument to @code{malloc}.

In ISO C99, you would use a @dfn{flexible array member}, which is 
slightly different in syntax and semantics:

@itemize @bullet
@item
Flexible array members are written as @code{contents[]} without
the @code{0}.

@item
Flexible array members have incomplete type, and so the @code{sizeof}
operator may not be applied.  As a quirk of the original implementation
of zero-length arrays, @code{sizeof} evaluates to zero.

@item
Flexible array members may only appear as the last member of a
@code{struct} that is otherwise non-empty.  GCC currently allows 
zero-length arrays anywhere.  You may encounter problems, however,
defining structures containing only a zero-length array.  Such usage
is deprecated, and we recommend using zero-length arrays only in
places in which flexible array members would be allowed.
@end itemize

GCC versions before 3.0 allowed zero-length arrays to be statically
initialized.  In addition to those cases that were useful, it also
allowed initializations in situations that would corrupt later data.
Non-empty initialization of zero-length arrays is now deprecated.

Instead GCC allows static initialization of flexible array members.
This is equivalent to defining a new structure containing the original
structure followed by an array of sufficient size to contain the data.
I.e. in the following, @code{f1} is constructed as if it were declared
like @code{f2}.

@example
struct f1 @{
  int x; int y[];
@} f1 = @{ 1, @{ 2, 3, 4 @} @};

struct f2 @{
  struct f1 f1; int data[3];
@} f2 = @{ @{ 1 @}, @{ 2, 3, 4 @} @};
@end example

@noindent
The convenience of this extension is that @code{f1} has the desired
type, eliminating the need to consistently refer to @code{f2.f1}.

This has symmetry with normal static arrays, in that an array of
unknown size is also written with @code{[]}.

Of course, this extension only makes sense if the extra data comes at
the end of a top-level object, as otherwise we would be overwriting
data at subsequent offsets.  To avoid undue complication and confusion
with initialization of deeply nested arrays, we simply disallow any
non-empty initialization except when the structure is the top-level
object.  For example:

@example
struct foo @{ int x; int y[]; @};
struct bar @{ struct foo z; @};

struct foo a = @{ 1, @{ 2, 3, 4 @} @};        // Legal.
struct bar b = @{ @{ 1, @{ 2, 3, 4 @} @} @};    // Illegal.
struct bar c = @{ @{ 1, @{ @} @} @};            // Legal.
struct foo d[1] = @{ @{ 1 @{ 2, 3, 4 @} @} @};  // Illegal.
@end example

@node Variable Length
@section Arrays of Variable Length
@cindex variable-length arrays
@cindex arrays of variable length

Variable-length automatic arrays are allowed in GNU C.  These arrays are
declared like any other automatic arrays, but with a length that is not
a constant expression.  The storage is allocated at the point of
declaration and deallocated when the brace-level is exited.  For
example:

@example
FILE *
concat_fopen (char *s1, char *s2, char *mode)
@{
  char str[strlen (s1) + strlen (s2) + 1];
  strcpy (str, s1);
  strcat (str, s2);
  return fopen (str, mode);
@}
@end example

@cindex scope of a variable length array
@cindex variable-length array scope
@cindex deallocating variable length arrays
Jumping or breaking out of the scope of the array name deallocates the
storage.  Jumping into the scope is not allowed; you get an error
message for it.

@cindex @code{alloca} vs variable-length arrays
You can use the function @code{alloca} to get an effect much like
variable-length arrays.  The function @code{alloca} is available in
many other C implementations (but not in all).  On the other hand,
variable-length arrays are more elegant.

There are other differences between these two methods.  Space allocated
with @code{alloca} exists until the containing @emph{function} returns.
The space for a variable-length array is deallocated as soon as the array
name's scope ends.  (If you use both variable-length arrays and
@code{alloca} in the same function, deallocation of a variable-length array
will also deallocate anything more recently allocated with @code{alloca}.)

You can also use variable-length arrays as arguments to functions:

@example
struct entry
tester (int len, char data[len][len])
@{
  @dots{}
@}
@end example

The length of an array is computed once when the storage is allocated
and is remembered for the scope of the array in case you access it with
@code{sizeof}.

If you want to pass the array first and the length afterward, you can
use a forward declaration in the parameter list---another GNU extension.

@example
struct entry
tester (int len; char data[len][len], int len)
@{
  @dots{}
@}
@end example

@cindex parameter forward declaration
The @samp{int len} before the semicolon is a @dfn{parameter forward
declaration}, and it serves the purpose of making the name @code{len}
known when the declaration of @code{data} is parsed.

You can write any number of such parameter forward declarations in the
parameter list.  They can be separated by commas or semicolons, but the
last one must end with a semicolon, which is followed by the ``real''
parameter declarations.  Each forward declaration must match a ``real''
declaration in parameter name and data type.

@node Variadic Macros
@section Macros with a Variable Number of Arguments.
@cindex variable number of arguments
@cindex macro with variable arguments
@cindex rest argument (in macro)
@cindex variadic macros

In the ISO C standard of 1999, a macro can be declared to accept a
variable number of arguments much as a function can.  The syntax for
defining the macro is similar to that of a function.  Here is an
example:

@example
#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)
@end example

Here @samp{@dots{}} is a @dfn{variable argument}.  In the invocation of
such a macro, it represents the zero or more tokens until the closing
parenthesis that ends the invocation, including any commas.  This set of
tokens replaces the identifier @code{__VA_ARGS__} in the macro body
wherever it appears.  See the CPP manual for more information.

GCC has long supported variadic macros, and used a different syntax that
allowed you to give a name to the variable arguments just like any other
argument.  Here is an example:

@example
#define debug(format, args...) fprintf (stderr, format, args)
@end example

This is in all ways equivalent to the ISO C example above, but arguably
more readable and descriptive.

GNU CPP has two further variadic macro extensions, and permits them to
be used with either of the above forms of macro definition.

In standard C, you are not allowed to leave the variable argument out
entirely; but you are allowed to pass an empty argument.  For example,
this invocation is invalid in ISO C, because there is no comma after
the string:

@example
debug ("A message")
@end example

GNU CPP permits you to completely omit the variable arguments in this
way.  In the above examples, the compiler would complain, though since
the expansion of the macro still has the extra comma after the format
string.

To help solve this problem, CPP behaves specially for variable arguments
used with the token paste operator, @samp{##}.  If instead you write

@example
#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)
@end example

and if the variable arguments are omitted or empty, the @samp{##}
operator causes the preprocessor to remove the comma before it.  If you
do provide some variable arguments in your macro invocation, GNU CPP
does not complain about the paste operation and instead places the
variable arguments after the comma.  Just like any other pasted macro
argument, these arguments are not macro expanded.

@node Escaped Newlines
@section Slightly Looser Rules for Escaped Newlines
@cindex escaped newlines
@cindex newlines (escaped)

Recently, the non-traditional preprocessor has relaxed its treatment of
escaped newlines.  Previously, the newline had to immediately follow a
backslash.  The current implementation allows whitespace in the form of
spaces, horizontal and vertical tabs, and form feeds between the
backslash and the subsequent newline.  The preprocessor issues a
warning, but treats it as a valid escaped newline and combines the two
lines to form a single logical line.  This works within comments and
tokens, including multi-line strings, as well as between tokens.
Comments are @emph{not} treated as whitespace for the purposes of this
relaxation, since they have not yet been replaced with spaces.

@node Multi-line Strings
@section String Literals with Embedded Newlines
@cindex multi-line string literals

As an extension, GNU CPP permits string literals to cross multiple lines
without escaping the embedded newlines.  Each embedded newline is
replaced with a single @samp{\n} character in the resulting string
literal, regardless of what form the newline took originally.

CPP currently allows such strings in directives as well (other than the
@samp{#include} family).  This is deprecated and will eventually be
removed.

@node Subscripting
@section Non-Lvalue Arrays May Have Subscripts
@cindex subscripting
@cindex arrays, non-lvalue

@cindex subscripting and function values
Subscripting is allowed on arrays that are not lvalues, even though the
unary @samp{&} operator is not.  For example, this is valid in GNU C though
not valid in other C dialects:

@example
@group
struct foo @{int a[4];@};

struct foo f();

bar (int index)
@{
  return f().a[index];
@}
@end group
@end example

@node Pointer Arith
@section Arithmetic on @code{void}- and Function-Pointers
@cindex void pointers, arithmetic
@cindex void, size of pointer to
@cindex function pointers, arithmetic
@cindex function, size of pointer to

In GNU C, addition and subtraction operations are supported on pointers to
@code{void} and on pointers to functions.  This is done by treating the
size of a @code{void} or of a function as 1.

A consequence of this is that @code{sizeof} is also allowed on @code{void}
and on function types, and returns 1.

The option @samp{-Wpointer-arith} requests a warning if these extensions
are used.

@node Initializers
@section Non-Constant Initializers
@cindex initializers, non-constant
@cindex non-constant initializers

As in standard C++, the elements of an aggregate initializer for an
automatic variable are not required to be constant expressions in GNU C.
Here is an example of an initializer with run-time varying elements:

@example
foo (float f, float g)
@{
  float beat_freqs[2] = @{ f-g, f+g @};
  @dots{}
@}
@end example

@node Constructors
@section Constructor Expressions
@cindex constructor expressions
@cindex initializations in expressions
@cindex structures, constructor expression
@cindex expressions, constructor

GNU C supports constructor expressions.  A constructor looks like
a cast containing an initializer.  Its value is an object of the
type specified in the cast, containing the elements specified in
the initializer.

Usually, the specified type is a structure.  Assume that
@code{struct foo} and @code{structure} are declared as shown:

@example
struct foo @{int a; char b[2];@} structure;
@end example

@noindent
Here is an example of constructing a @code{struct foo} with a constructor:

@example
structure = ((struct foo) @{x + y, 'a', 0@});
@end example

@noindent
This is equivalent to writing the following:

@example
@{
  struct foo temp = @{x + y, 'a', 0@};
  structure = temp;
@}
@end example

You can also construct an array.  If all the elements of the constructor
are (made up of) simple constant expressions, suitable for use in
initializers, then the constructor is an lvalue and can be coerced to a
pointer to its first element, as shown here:

@example
char **foo = (char *[]) @{ "x", "y", "z" @};
@end example

Array constructors whose elements are not simple constants are
not very useful, because the constructor is not an lvalue.  There
are only two valid ways to use it: to subscript it, or initialize
an array variable with it.  The former is probably slower than a
@code{switch} statement, while the latter does the same thing an
ordinary C initializer would do.  Here is an example of
subscripting an array constructor:

@example
output = ((int[]) @{ 2, x, 28 @}) [input];
@end example

Constructor expressions for scalar types and union types are is
also allowed, but then the constructor expression is equivalent
to a cast.

@node Labeled Elements
@section Labeled Elements in Initializers
@cindex initializers with labeled elements
@cindex labeled elements in initializers
@cindex case labels in initializers

Standard C89 requires the elements of an initializer to appear in a fixed
order, the same as the order of the elements in the array or structure
being initialized.

In ISO C99 you can give the elements in any order, specifying the array
indices or structure field names they apply to, and GNU C allows this as
an extension in C89 mode as well.  This extension is not
implemented in GNU C++.

To specify an array index, write
@samp{[@var{index}] =} before the element value.  For example,

@example
int a[6] = @{ [4] = 29, [2] = 15 @};
@end example

@noindent
is equivalent to

@example
int a[6] = @{ 0, 0, 15, 0, 29, 0 @};
@end example

@noindent
The index values must be constant expressions, even if the array being
initialized is automatic.

An alternative syntax for this which has been obsolete since GCC 2.5 but
GCC still accepts is to write @samp{[@var{index}]} before the element
value, with no @samp{=}.

To initialize a range of elements to the same value, write
@samp{[@var{first} ... @var{last}] = @var{value}}.  This is a GNU
extension.  For example,

@example
int widths[] = @{ [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 @};
@end example

@noindent
If the value in it has side-effects, the side-effects will happen only once,
not for each initialized field by the range initializer.

@noindent
Note that the length of the array is the highest value specified
plus one.

In a structure initializer, specify the name of a field to initialize
with @samp{.@var{fieldname} =} before the element value.  For example,
given the following structure,

@example
struct point @{ int x, y; @};
@end example

@noindent
the following initialization

@example
struct point p = @{ .y = yvalue, .x = xvalue @};
@end example

@noindent
is equivalent to

@example
struct point p = @{ xvalue, yvalue @};
@end example

Another syntax which has the same meaning, obsolete since GCC 2.5, is
@samp{@var{fieldname}:}, as shown here:

@example
struct point p = @{ y: yvalue, x: xvalue @};
@end example

You can also use an element label (with either the colon syntax or the
period-equal syntax) when initializing a union, to specify which element
of the union should be used.  For example,

@example
union foo @{ int i; double d; @};

union foo f = @{ .d = 4 @};
@end example

@noindent
will convert 4 to a @code{double} to store it in the union using
the second element.  By contrast, casting 4 to type @code{union foo}
would store it into the union as the integer @code{i}, since it is
an integer.  (@xref{Cast to Union}.)

You can combine this technique of naming elements with ordinary C
initialization of successive elements.  Each initializer element that
does not have a label applies to the next consecutive element of the
array or structure.  For example,

@example
int a[6] = @{ [1] = v1, v2, [4] = v4 @};
@end example

@noindent
is equivalent to

@example
int a[6] = @{ 0, v1, v2, 0, v4, 0 @};
@end example

Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an @code{enum} type.
For example:

@example
int whitespace[256]
  = @{ [' '] = 1, ['\t'] = 1, ['\h'] = 1,
      ['\f'] = 1, ['\n'] = 1, ['\r'] = 1 @};
@end example

You can also write a series of @samp{.@var{fieldname}} and
@samp{[@var{index}]} element labels before an @samp{=} to specify a
nested subobject to initialize; the list is taken relative to the
subobject corresponding to the closest surrounding brace pair.  For
example, with the @samp{struct point} declaration above:

@example
struct point ptarray[10] = @{ [2].y = yv2, [2].x = xv2, [0].x = xv0 @};
@end example

@noindent
If the same field is initialized multiple times, it will have value from
the last initialization.  If any such overridden initialization has
side-effect, it is unspecified whether the side-effect happens or not.
Currently, gcc will discard them and issue a warning.

@node Case Ranges
@section Case Ranges
@cindex case ranges
@cindex ranges in case statements

You can specify a range of consecutive values in a single @code{case} label,
like this:

@example
case @var{low} ... @var{high}:
@end example

@noindent
This has the same effect as the proper number of individual @code{case}
labels, one for each integer value from @var{low} to @var{high}, inclusive.

This feature is especially useful for ranges of ASCII character codes:

@example
case 'A' ... 'Z':
@end example

@strong{Be careful:} Write spaces around the @code{...}, for otherwise
it may be parsed wrong when you use it with integer values.  For example,
write this:

@example
case 1 ... 5:
@end example

@noindent
rather than this:

@example
case 1...5:
@end example

@node Cast to Union
@section Cast to a Union Type
@cindex cast to a union
@cindex union, casting to a

A cast to union type is similar to other casts, except that the type
specified is a union type.  You can specify the type either with
@code{union @var{tag}} or with a typedef name.  A cast to union is actually
a constructor though, not a cast, and hence does not yield an lvalue like
normal casts.  (@xref{Constructors}.)

The types that may be cast to the union type are those of the members
of the union.  Thus, given the following union and variables:

@example
union foo @{ int i; double d; @};
int x;
double y;
@end example

@noindent
both @code{x} and @code{y} can be cast to type @code{union} foo.

Using the cast as the right-hand side of an assignment to a variable of
union type is equivalent to storing in a member of the union:

@example
union foo u;
@dots{}
u = (union foo) x  @equiv{}  u.i = x
u = (union foo) y  @equiv{}  u.d = y
@end example

You can also use the union cast as a function argument:

@example
void hack (union foo);
@dots{}
hack ((union foo) x);
@end example

@node Function Attributes
@section Declaring Attributes of Functions
@cindex function attributes
@cindex declaring attributes of functions
@cindex functions that never return
@cindex functions that have no side effects
@cindex functions in arbitrary sections
@cindex functions that behave like malloc
@cindex @code{volatile} applied to function
@cindex @code{const} applied to function
@cindex functions with @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon} style arguments
@cindex functions that are passed arguments in registers on the 386
@cindex functions that pop the argument stack on the 386
@cindex functions that do not pop the argument stack on the 386

In GNU C, you declare certain things about functions called in your program
which help the compiler optimize function calls and check your code more
carefully.

The keyword @code{__attribute__} allows you to specify special
attributes when making a declaration.  This keyword is followed by an
attribute specification inside double parentheses.  Fourteen attributes,
@code{noreturn}, @code{pure}, @code{const}, @code{format},
@code{format_arg}, @code{no_instrument_function}, @code{section},
@code{constructor}, @code{destructor}, @code{unused}, @code{weak},
@code{malloc}, @code{alias} and @code{no_check_memory_usage} are
currently defined for functions.  Several other attributes are defined
for functions on particular target systems.  Other attributes, including
@code{section} are supported for variables declarations (@pxref{Variable
Attributes}) and for types (@pxref{Type Attributes}).

You may also specify attributes with @samp{__} preceding and following
each keyword.  This allows you to use them in header files without
being concerned about a possible macro of the same name.  For example,
you may use @code{__noreturn__} instead of @code{noreturn}.

@xref{Attribute Syntax}, for details of the exact syntax for using
attributes.

@table @code
@cindex @code{noreturn} function attribute
@item noreturn
A few standard library functions, such as @code{abort} and @code{exit},
cannot return.  GNU CC knows this automatically.  Some programs define
their own functions that never return.  You can declare them
@code{noreturn} to tell the compiler this fact.  For example,

@smallexample
void fatal () __attribute__ ((noreturn));

void
fatal (@dots{})
@{
  @dots{} /* @r{Print error message.} */ @dots{}
  exit (1);
@}
@end smallexample

The @code{noreturn} keyword tells the compiler to assume that
@code{fatal} cannot return.  It can then optimize without regard to what
would happen if @code{fatal} ever did return.  This makes slightly
better code.  More importantly, it helps avoid spurious warnings of
uninitialized variables.

Do not assume that registers saved by the calling function are
restored before calling the @code{noreturn} function.

It does not make sense for a @code{noreturn} function to have a return
type other than @code{void}.

The attribute @code{noreturn} is not implemented in GNU C versions
earlier than 2.5.  An alternative way to declare that a function does
not return, which works in the current version and in some older
versions, is as follows:

@smallexample
typedef void voidfn ();

volatile voidfn fatal;
@end smallexample

@cindex @code{pure} function attribute
@item pure
Many functions have no effects except the return value and their
return value depends only on the parameters and/or global variables.
Such a function can be subject
to common subexpression elimination and loop optimization just as an
arithmetic operator would be.  These functions should be declared
with the attribute @code{pure}.  For example,

@smallexample
int square (int) __attribute__ ((pure));
@end smallexample

@noindent
says that the hypothetical function @code{square} is safe to call
fewer times than the program says.

Some of common examples of pure functions are @code{strlen} or @code{memcmp}.
Interesting non-pure functions are functions with infinite loops or those
depending on volatile memory or other system resource, that may change between
two consecutive calls (such as @code{feof} in a multithreading environment).

The attribute @code{pure} is not implemented in GNU C versions earlier
than 2.96.
@cindex @code{const} function attribute
@item const
Many functions do not examine any values except their arguments, and
have no effects except the return value.  Basically this is just slightly
more strict class than the "pure" attribute above, since function is not
allowed to read global memory.

@cindex pointer arguments
Note that a function that has pointer arguments and examines the data
pointed to must @emph{not} be declared @code{const}.  Likewise, a
function that calls a non-@code{const} function usually must not be
@code{const}.  It does not make sense for a @code{const} function to
return @code{void}.

The attribute @code{const} is not implemented in GNU C versions earlier
than 2.5.  An alternative way to declare that a function has no side
effects, which works in the current version and in some older versions,
is as follows:

@smallexample
typedef int intfn ();

extern const intfn square;
@end smallexample

This approach does not work in GNU C++ from 2.6.0 on, since the language
specifies that the @samp{const} must be attached to the return value.


@item format (@var{archetype}, @var{string-index}, @var{first-to-check})
@cindex @code{format} function attribute
The @code{format} attribute specifies that a function takes @code{printf},
@code{scanf}, @code{strftime} or @code{strfmon} style arguments which
should be type-checked against a format string.  For example, the
declaration:

@smallexample
extern int
my_printf (void *my_object, const char *my_format, ...)
      __attribute__ ((format (printf, 2, 3)));
@end smallexample

@noindent
causes the compiler to check the arguments in calls to @code{my_printf}
for consistency with the @code{printf} style format string argument
@code{my_format}.

The parameter @var{archetype} determines how the format string is
interpreted, and should be @code{printf}, @code{scanf}, @code{strftime}
or @code{strfmon}.  (You can also use @code{__printf__},
@code{__scanf__}, @code{__strftime__} or @code{__strfmon__}.)  The
parameter @var{string-index} specifies which argument is the format
string argument (starting from 1), while @var{first-to-check} is the
number of the first argument to check against the format string.  For
functions where the arguments are not available to be checked (such as
@code{vprintf}), specify the third parameter as zero.  In this case the
compiler only checks the format string for consistency.  For
@code{strftime} formats, the third parameter is required to be zero.

In the example above, the format string (@code{my_format}) is the second
argument of the function @code{my_print}, and the arguments to check
start with the third argument, so the correct parameters for the format
attribute are 2 and 3.

The @code{format} attribute allows you to identify your own functions
which take format strings as arguments, so that GNU CC can check the
calls to these functions for errors.  The compiler always (unless
@samp{-ffreestanding} is used) checks formats
for the standard library functions @code{printf}, @code{fprintf},
@code{sprintf}, @code{scanf}, @code{fscanf}, @code{sscanf}, @code{strftime},
@code{vprintf}, @code{vfprintf} and @code{vsprintf} whenever such
warnings are requested (using @samp{-Wformat}), so there is no need to
modify the header file @file{stdio.h}.  In C99 mode, the functions
@code{snprintf}, @code{vsnprintf}, @code{vscanf}, @code{vfscanf} and
@code{vsscanf} are also checked.  Except in strictly conforming C
standard modes, the X/Open function @code{strfmon} is also checked.
@xref{C Dialect Options,,Options Controlling C Dialect}.

@item format_arg (@var{string-index})
@cindex @code{format_arg} function attribute
The @code{format_arg} attribute specifies that a function takes a format
string for a @code{printf}, @code{scanf}, @code{strftime} or
@code{strfmon} style function and modifies it (for example, to translate
it into another language), so the result can be passed to a
@code{printf}, @code{scanf}, @code{strftime} or @code{strfmon} style
function (with the remaining arguments to the format function the same
as they would have been for the unmodified string).  For example, the
declaration:

@smallexample
extern char *
my_dgettext (char *my_domain, const char *my_format)
      __attribute__ ((format_arg (2)));
@end smallexample

@noindent
causes the compiler to check the arguments in calls to a @code{printf},
@code{scanf}, @code{strftime} or @code{strfmon} type function, whose
format string argument is a call to the @code{my_dgettext} function, for
consistency with the format string argument @code{my_format}.  If the
@code{format_arg} attribute had not been specified, all the compiler
could tell in such calls to format functions would be that the format
string argument is not constant; this would generate a warning when
@code{-Wformat-nonliteral} is used, but the calls could not be checked
without the attribute.

The parameter @var{string-index} specifies which argument is the format
string argument (starting from 1).

The @code{format-arg} attribute allows you to identify your own
functions which modify format strings, so that GNU CC can check the
calls to @code{printf}, @code{scanf}, @code{strftime} or @code{strfmon}
type function whose operands are a call to one of your own function.
The compiler always treats @code{gettext}, @code{dgettext}, and
@code{dcgettext} in this manner except when strict ISO C support is
requested by @samp{-ansi} or an appropriate @samp{-std} option, or
@samp{-ffreestanding} is used.  @xref{C Dialect Options,,Options
Controlling C Dialect}.

@item no_instrument_function
@cindex @code{no_instrument_function} function attribute
If @samp{-finstrument-functions} is given, profiling function calls will
be generated at entry and exit of most user-compiled functions.
Functions with this attribute will not be so instrumented.

@item section ("section-name")
@cindex @code{section} function attribute
Normally, the compiler places the code it generates in the @code{text} section.
Sometimes, however, you need additional sections, or you need certain
particular functions to appear in special sections.  The @code{section}
attribute specifies that a function lives in a particular section.
For example, the declaration:

@smallexample
extern void foobar (void) __attribute__ ((section ("bar")));
@end smallexample

@noindent
puts the function @code{foobar} in the @code{bar} section.

Some file formats do not support arbitrary sections so the @code{section}
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.

@item constructor
@itemx destructor
@cindex @code{constructor} function attribute
@cindex @code{destructor} function attribute
The @code{constructor} attribute causes the function to be called
automatically before execution enters @code{main ()}.  Similarly, the
@code{destructor} attribute causes the function to be called
automatically after @code{main ()} has completed or @code{exit ()} has
been called.  Functions with these attributes are useful for
initializing data that will be used implicitly during the execution of
the program.

These attributes are not currently implemented for Objective C.

@item unused
This attribute, attached to a function, means that the function is meant
to be possibly unused.  GNU CC will not produce a warning for this
function.  GNU C++ does not currently support this attribute as
definitions without parameters are valid in C++.

@item weak
@cindex @code{weak} attribute
The @code{weak} attribute causes the declaration to be emitted as a weak
symbol rather than a global.  This is primarily useful in defining
library functions which can be overridden in user code, though it can
also be used with non-function declarations.  Weak symbols are supported
for ELF targets, and also for a.out targets when using the GNU assembler
and linker.

@item malloc
@cindex @code{malloc} attribute
The @code{malloc} attribute is used to tell the compiler that a function
may be treated as if it were the malloc function.  The compiler assumes
that calls to malloc result in a pointers that cannot alias anything.
This will often improve optimization.

@item alias ("target")
@cindex @code{alias} attribute
The @code{alias} attribute causes the declaration to be emitted as an
alias for another symbol, which must be specified.  For instance,

@smallexample
void __f () @{ /* do something */; @}
void f () __attribute__ ((weak, alias ("__f")));
@end smallexample

declares @samp{f} to be a weak alias for @samp{__f}.  In C++, the
mangled name for the target must be used.

Not all target machines support this attribute.

@item no_check_memory_usage
@cindex @code{no_check_memory_usage} function attribute
The @code{no_check_memory_usage} attribute causes GNU CC to omit checks
of memory references when it generates code for that function.  Normally
if you specify @samp{-fcheck-memory-usage} (see @pxref{Code Gen
Options}), GNU CC generates calls to support routines before most memory
accesses to permit support code to record usage and detect uses of
uninitialized or unallocated storage.  Since GNU CC cannot handle
@code{asm} statements properly they are not allowed in such functions.
If you declare a function with this attribute, GNU CC will not generate
memory checking code for that function, permitting the use of @code{asm}
statements without having to compile that function with different
options.  This also allows you to write support routines of your own if
you wish, without getting infinite recursion if they get compiled with
@code{-fcheck-memory-usage}.

@item regparm (@var{number})
@cindex functions that are passed arguments in registers on the 386
On the Intel 386, the @code{regparm} attribute causes the compiler to
pass up to @var{number} integer arguments in registers @var{EAX},
@var{EDX}, and @var{ECX} instead of on the stack.  Functions that take a
variable number of arguments will continue to be passed all of their
arguments on the stack.

@item stdcall
@cindex functions that pop the argument stack on the 386
On the Intel 386, the @code{stdcall} attribute causes the compiler to
assume that the called function will pop off the stack space used to
pass arguments, unless it takes a variable number of arguments.

The PowerPC compiler for Windows NT currently ignores the @code{stdcall}
attribute.

@item cdecl
@cindex functions that do pop the argument stack on the 386
On the Intel 386, the @code{cdecl} attribute causes the compiler to
assume that the calling function will pop off the stack space used to
pass arguments.  This is
useful to override the effects of the @samp{-mrtd} switch.

The PowerPC compiler for Windows NT currently ignores the @code{cdecl}
attribute.

@item longcall
@cindex functions called via pointer on the RS/6000 and PowerPC
On the RS/6000 and PowerPC, the @code{longcall} attribute causes the
compiler to always call the function via a pointer, so that functions
which reside further than 64 megabytes (67,108,864 bytes) from the
current location can be called.

@item long_call/short_call
@cindex indirect calls on ARM
This attribute allows to specify how to call a particular function on
ARM.  Both attributes override the @code{-mlong-calls} (@pxref{ARM Options})
command line switch and @code{#pragma long_calls} settings.  The
@code{long_call} attribute causes the compiler to always call the
function by first loading its address into a register and then using the
contents of that register.   The @code{short_call} attribute always places
the offset to the function from the call site into the @samp{BL}
instruction directly.

@item dllimport
@cindex functions which are imported from a dll on PowerPC Windows NT
On the PowerPC running Windows NT, the @code{dllimport} attribute causes
the compiler to call the function via a global pointer to the function
pointer that is set up by the Windows NT dll library.  The pointer name
is formed by combining @code{__imp_} and the function name.

@item dllexport
@cindex functions which are exported from a dll on PowerPC Windows NT
On the PowerPC running Windows NT, the @code{dllexport} attribute causes
the compiler to provide a global pointer to the function pointer, so
that it can be called with the @code{dllimport} attribute.  The pointer
name is formed by combining @code{__imp_} and the function name.

@item exception (@var{except-func} [, @var{except-arg}])
@cindex functions which specify exception handling on PowerPC Windows NT
On the PowerPC running Windows NT, the @code{exception} attribute causes
the compiler to modify the structured exception table entry it emits for
the declared function.  The string or identifier @var{except-func} is
placed in the third entry of the structured exception table.  It
represents a function, which is called by the exception handling
mechanism if an exception occurs.  If it was specified, the string or
identifier @var{except-arg} is placed in the fourth entry of the
structured exception table.

@item function_vector
@cindex calling functions through the function vector on the H8/300 processors
Use this option on the H8/300 and H8/300H to indicate that the specified
function should be called through the function vector.  Calling a
function through the function vector will reduce code size, however;
the function vector has a limited size (maximum 128 entries on the H8/300
and 64 entries on the H8/300H) and shares space with the interrupt vector.

You must use GAS and GLD from GNU binutils version 2.7 or later for
this option to work correctly.

@item interrupt_handler
@cindex interrupt handler functions on the H8/300 processors
Use this option on the H8/300 and H8/300H to indicate that the specified
function is an interrupt handler.  The compiler will generate function
entry and exit sequences suitable for use in an interrupt handler when this
attribute is present.

@item interrupt
@cindex interrupt handler functions
Use this option on the ARM, AVR and M32R/D ports to indicate that the
specified function is an interrupt handler.  The compiler will generate
function entry and exit sequences suitable for use in an interrupt
handler when this attribute is present.

Note, interrupt handlers for the H8/300 and H8/300H processors can be
specified via the @code{interrupt_handler} attribute.

Note, on the AVR interrupts will be enabled inside the function.

Note, for the ARM you can specify the kind of interrupt to be handled by
adding an optional parameter to the interrupt attribute like this:

@smallexample
void f () __attribute__ ((interrupt ("IRQ")));
@end smallexample

Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF.

@item eightbit_data
@cindex eight bit data on the H8/300 and H8/300H
Use this option on the H8/300 and H8/300H to indicate that the specified
variable should be placed into the eight bit data section.
The compiler will generate more efficient code for certain operations
on data in the eight bit data area.  Note the eight bit data area is limited to
256 bytes of data.

You must use GAS and GLD from GNU binutils version 2.7 or later for
this option to work correctly.

@item tiny_data
@cindex tiny data section on the H8/300H
Use this option on the H8/300H to indicate that the specified
variable should be placed into the tiny data section.
The compiler will generate more efficient code for loads and stores
on data in the tiny data section.  Note the tiny data area is limited to
slightly under 32kbytes of data.

@item signal
@cindex signal handler functions on the AVR processors
Use this option on the AVR to indicate that the specified
function is an signal handler.  The compiler will generate function
entry and exit sequences suitable for use in an signal handler when this
attribute is present. Interrupts will be disabled inside function.

@item naked
@cindex function without a prologue/epilogue code
Use this option on the ARM or AVR ports to indicate that the specified
function do not need prologue/epilogue sequences generated by the
compiler.  It is up to the programmer to provide these sequences.

@item model (@var{model-name})
@cindex function addressability on the M32R/D
Use this attribute on the M32R/D to set the addressability of an object,
and the code generated for a function.
The identifier @var{model-name} is one of @code{small}, @code{medium},
or @code{large}, representing each of the code models.

Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the @code{ld24} instruction), and are
callable with the @code{bl} instruction.

Medium model objects may live anywhere in the 32 bit address space (the
compiler will generate @code{seth/add3} instructions to load their addresses),
and are callable with the @code{bl} instruction.

Large model objects may live anywhere in the 32 bit address space (the
compiler will generate @code{seth/add3} instructions to load their addresses),
and may not be reachable with the @code{bl} instruction (the compiler will
generate the much slower @code{seth/add3/jl} instruction sequence).

@end table

You can specify multiple attributes in a declaration by separating them
by commas within the double parentheses or by immediately following an
attribute declaration with another attribute declaration.

@cindex @code{#pragma}, reason for not using
@cindex pragma, reason for not using
Some people object to the @code{__attribute__} feature, suggesting that
ISO C's @code{#pragma} should be used instead.  At the time
@code{__attribute__} was designed, there were two reasons for not doing
this.

@enumerate
@item
It is impossible to generate @code{#pragma} commands from a macro.

@item
There is no telling what the same @code{#pragma} might mean in another
compiler.
@end enumerate

These two reasons applied to almost any application that might have been
proposed for @code{#pragma}.  It was basically a mistake to use
@code{#pragma} for @emph{anything}.

The ISO C99 standard includes @code{_Pragma}, which now allows pragmas
to be generated from macros.  In addition, a @code{#pragma GCC}
namespace is now in use for GCC-specific pragmas.  However, it has been
found convenient to use @code{__attribute__} to achieve a natural
attachment of attributes to their corresponding declarations, whereas
@code{#pragma GCC} is of use for constructs that do not naturally form
part of the grammar.  @xref{Other Directives,,Miscellaneous
Preprocessing Directives, cpp, The C Preprocessor}.

@node Attribute Syntax
@section Attribute Syntax
@cindex attribute syntax

This section describes the syntax with which @code{__attribute__} may be
used, and the constructs to which attribute specifiers bind, for the C
language.  Some details may vary for C++ and Objective C.  Because of
infelicities in the grammar for attributes, some forms described here
may not be successfully parsed in all cases.

@xref{Function Attributes}, for details of the semantics of attributes
applying to functions.  @xref{Variable Attributes}, for details of the
semantics of attributes applying to variables.  @xref{Type Attributes},
for details of the semantics of attributes applying to structure, union
and enumerated types.

An @dfn{attribute specifier} is of the form
@code{__attribute__ ((@var{attribute-list}))}.  An @dfn{attribute list}
is a possibly empty comma-separated sequence of @dfn{attributes}, where
each attribute is one of the following:

@itemize @bullet
@item
Empty.  Empty attributes are ignored.

@item
A word (which may be an identifier such as @code{unused}, or a reserved
word such as @code{const}).

@item
A word, followed by, in parentheses, parameters for the attribute.
These parameters take one of the following forms:

@itemize @bullet
@item
An identifier.  For example, @code{mode} attributes use this form.

@item
An identifier followed by a comma and a non-empty comma-separated list
of expressions.  For example, @code{format} attributes use this form.

@item
A possibly empty comma-separated list of expressions.  For example,
@code{format_arg} attributes use this form with the list being a single
integer constant expression, and @code{alias} attributes use this form
with the list being a single string constant.
@end itemize
@end itemize

An @dfn{attribute specifier list} is a sequence of one or more attribute
specifiers, not separated by any other tokens.

An attribute specifier list may appear after the colon following a
label, other than a @code{case} or @code{default} label.  The only
attribute it makes sense to use after a label is @code{unused}.  This
feature is intended for code generated by programs which contains labels
that may be unused but which is compiled with @option{-Wall}.  It would
not normally be appropriate to use in it human-written code, though it
could be useful in cases where the code that jumps to the label is
contained within an @code{#ifdef} conditional.

An attribute specifier list may appear as part of a @code{struct},
@code{union} or @code{enum} specifier.  It may go either immediately
after the @code{struct}, @code{union} or @code{enum} keyword, or after
the closing brace.  It is ignored if the content of the structure, union
or enumerated type is not defined in the specifier in which the
attribute specifier list is used---that is, in usages such as
@code{struct __attribute__((foo)) bar} with no following opening brace.
Where attribute specifiers follow the closing brace, they are considered
to relate to the structure, union or enumerated type defined, not to any
enclosing declaration the type specifier appears in, and the type
defined is not complete until after the attribute specifiers.
@c Otherwise, there would be the following problems: a shift/reduce
@c conflict between attributes binding the the struct/union/enum and
@c binding to the list of specifiers/qualifiers; and "aligned"
@c attributes could use sizeof for the structure, but the size could be
@c changed later by "packed" attributes.

Otherwise, an attribute specifier appears as part of a declaration,
counting declarations of unnamed parameters and type names, and relates
to that declaration (which may be nested in another declaration, for
example in the case of a parameter declaration).  In future, attribute
specifiers in some places may however apply to a particular declarator
within a declaration instead; these cases are noted below.

Any list of specifiers and qualifiers at the start of a declaration may
contain attribute specifiers, whether or not such a list may in that
context contain storage class specifiers.  (Some attributes, however,
are essentially in the nature of storage class specifiers, and only make
sense where storage class specifiers may be used; for example,
@code{section}.)  There is one necessary limitation to this syntax: the
first old-style parameter declaration in a function definition cannot
begin with an attribute specifier, because such an attribute applies to
the function instead by syntax described below (which, however, is not
yet implemented in this case).  In some other cases, attribute
specifiers are permitted by this grammar but not yet supported by the
compiler.  All attribute specifiers in this place relate to the
declaration as a whole.  In the obsolencent usage where a type of
@code{int} is implied by the absence of type specifiers, such a list of
specifiers and qualifiers may be an attribute specifier list with no
other specifiers or qualifiers.

An attribute specifier list may appear immediately before a declarator
(other than the first) in a comma-separated list of declarators in a
declaration of more than one identifier using a single list of
specifiers and qualifiers.  At present, such attribute specifiers apply
not only to the identifier before whose declarator they appear, but to
all subsequent identifiers declared in that declaration, but in future
they may apply only to that single identifier.  For example, in
@code{__attribute__((noreturn)) void d0 (void),
__attribute__((format(printf, 1, 2))) d1 (const char *, ...), d2
(void)}, the @code{noreturn} attribute applies to all the functions
declared; the @code{format} attribute should only apply to @code{d1},
but at present applies to @code{d2} as well (and so causes an error).

An attribute specifier list may appear immediately before the comma,
@code{=} or semicolon terminating the declaration of an identifier other
than a function definition.  At present, such attribute specifiers apply
to the declared object or function, but in future they may attach to the
outermost adjacent declarator.  In simple cases there is no difference,
but, for example, in @code{void (****f)(void)
__attribute__((noreturn));}, at present the @code{noreturn} attribute
applies to @code{f}, which causes a warning since @code{f} is not a
function, but in future it may apply to the function @code{****f}.  The
precise semantics of what attributes in such cases will apply to are not
yet specified.  Where an assembler name for an object or function is
specified (@pxref{Asm Labels}), at present the attribute must follow the
@code{asm} specification; in future, attributes before the @code{asm}
specification may apply to the adjacent declarator, and those after it
to the declared object or function.

An attribute specifier list may, in future, be permitted to appear after
the declarator in a function definition (before any old-style parameter
declarations or the function body).

An attribute specifier list may appear at the start of a nested
declarator.  At present, there are some limitations in this usage: the
attributes apply to the identifer declared, and to all subsequent
identifiers declared in that declaration (if it includes a
comma-separated list of declarators), rather than to a specific
declarator.  When attribute specifiers follow the @code{*} of a pointer
declarator, they must presently follow any type qualifiers present, and
cannot be mixed with them.  The following describes intended future
semantics which make this syntax more useful only.  It will make the
most sense if you are familiar with the formal specification of
declarators in the ISO C standard.

Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration @code{T
D1}, where @code{T} contains declaration specifiers that specify a type
@var{Type} (such as @code{int}) and @code{D1} is a declarator that
contains an identifier @var{ident}.  The type specified for @var{ident}
for derived declarators whose type does not include an attribute
specifier is as in the ISO C standard.

If @code{D1} has the form @code{( @var{attribute-specifier-list} D )},
and the declaration @code{T D} specifies the type
``@var{derived-declarator-type-list} @var{Type}'' for @var{ident}, then
@code{T D1} specifies the type ``@var{derived-declarator-type-list}
@var{attribute-specifier-list} @var{Type}'' for @var{ident}.

If @code{D1} has the form @code{*
@var{type-qualifier-and-attribute-specifier-list} D}, and the
declaration @code{T D} specifies the type
``@var{derived-declarator-type-list} @var{Type}'' for @var{ident}, then
@code{T D1} specifies the type ``@var{derived-declarator-type-list}
@var{type-qualifier-and-attribute-specifier-list} @var{Type}'' for
@var{ident}.

For example, @code{void (__attribute__((noreturn)) ****f)();} specifies
the type ``pointer to pointer to pointer to pointer to non-returning
function returning @code{void}''.  As another example, @code{char
*__attribute__((aligned(8))) *f;} specifies the type ``pointer to
8-byte-aligned pointer to @code{char}''.  Note again that this describes
intended future semantics, not current implementation.

@node Function Prototypes
@section Prototypes and Old-Style Function Definitions
@cindex function prototype declarations
@cindex old-style function definitions
@cindex promotion of formal parameters

GNU C extends ISO C to allow a function prototype to override a later
old-style non-prototype definition.  Consider the following example:

@example
/* @r{Use prototypes unless the compiler is old-fashioned.}  */
#ifdef __STDC__
#define P(x) x
#else
#define P(x) ()
#endif

/* @r{Prototype function declaration.}  */
int isroot P((uid_t));

/* @r{Old-style function definition.}  */
int
isroot (x)   /* ??? lossage here ??? */
     uid_t x;
@{
  return x == 0;
@}
@end example

Suppose the type @code{uid_t} happens to be @code{short}.  ISO C does
not allow this example, because subword arguments in old-style
non-prototype definitions are promoted.  Therefore in this example the
function definition's argument is really an @code{int}, which does not
match the prototype argument type of @code{short}.

This restriction of ISO C makes it hard to write code that is portable
to traditional C compilers, because the programmer does not know
whether the @code{uid_t} type is @code{short}, @code{int}, or
@code{long}.  Therefore, in cases like these GNU C allows a prototype
to override a later old-style definition.  More precisely, in GNU C, a
function prototype argument type overrides the argument type specified
by a later old-style definition if the former type is the same as the
latter type before promotion.  Thus in GNU C the above example is
equivalent to the following:

@example
int isroot (uid_t);

int
isroot (uid_t x)
@{
  return x == 0;
@}
@end example

GNU C++ does not support old-style function definitions, so this
extension is irrelevant.

@node C++ Comments
@section C++ Style Comments
@cindex //
@cindex C++ comments
@cindex comments, C++ style

In GNU C, you may use C++ style comments, which start with @samp{//} and
continue until the end of the line.  Many other C implementations allow
such comments, and they are likely to be in a future C standard.
However, C++ style comments are not recognized if you specify
@w{@samp{-ansi}}, a @option{-std} option specifying a version of ISO C
before C99, or @w{@samp{-traditional}}, since they are incompatible
with traditional constructs like @code{dividend//*comment*/divisor}.

@node Dollar Signs
@section Dollar Signs in Identifier Names
@cindex $
@cindex dollar signs in identifier names
@cindex identifier names, dollar signs in

In GNU C, you may normally use dollar signs in identifier names.
This is because many traditional C implementations allow such identifiers.
However, dollar signs in identifiers are not supported on a few target
machines, typically because the target assembler does not allow them.

@node Character Escapes
@section The Character @key{ESC} in Constants

You can use the sequence @samp{\e} in a string or character constant to
stand for the ASCII character @key{ESC}.

@node Alignment
@section Inquiring on Alignment of Types or Variables
@cindex alignment
@cindex type alignment
@cindex variable alignment

The keyword @code{__alignof__} allows you to inquire about how an object
is aligned, or the minimum alignment usually required by a type.  Its
syntax is just like @code{sizeof}.

For example, if the target machine requires a @code{double} value to be
aligned on an 8-byte boundary, then @code{__alignof__ (double)} is 8.
This is true on many RISC machines.  On more traditional machine
designs, @code{__alignof__ (double)} is 4 or even 2.

Some machines never actually require alignment; they allow reference to any
data type even at an odd addresses.  For these machines, @code{__alignof__}
reports the @emph{recommended} alignment of a type.

When the operand of @code{__alignof__} is an lvalue rather than a type, the
value is the largest alignment that the lvalue is known to have.  It may
have this alignment as a result of its data type, or because it is part of
a structure and inherits alignment from that structure.  For example, after
this declaration:

@example
struct foo @{ int x; char y; @} foo1;
@end example

@noindent
the value of @code{__alignof__ (foo1.y)} is probably 2 or 4, the same as
@code{__alignof__ (int)}, even though the data type of @code{foo1.y}
does not itself demand any alignment.@refill

It is an error to ask for the alignment of an incomplete type.

A related feature which lets you specify the alignment of an object is
@code{__attribute__ ((aligned (@var{alignment})))}; see the following
section.

@node Variable Attributes
@section Specifying Attributes of Variables
@cindex attribute of variables
@cindex variable attributes

The keyword @code{__attribute__} allows you to specify special
attributes of variables or structure fields.  This keyword is followed
by an attribute specification inside double parentheses.  Eight
attributes are currently defined for variables: @code{aligned},
@code{mode}, @code{nocommon}, @code{packed}, @code{section},
@code{transparent_union}, @code{unused}, and @code{weak}.  Some other
attributes are defined for variables on particular target systems.  Other
attributes are available for functions (@pxref{Function Attributes}) and
for types (@pxref{Type Attributes}).  Other front-ends might define more
attributes (@pxref{C++ Extensions,,Extensions to the C++ Language}).

You may also specify attributes with @samp{__} preceding and following
each keyword.  This allows you to use them in header files without
being concerned about a possible macro of the same name.  For example,
you may use @code{__aligned__} instead of @code{aligned}.

@xref{Attribute Syntax}, for details of the exact syntax for using
attributes.

@table @code
@cindex @code{aligned} attribute
@item aligned (@var{alignment})
This attribute specifies a minimum alignment for the variable or
structure field, measured in bytes.  For example, the declaration:

@smallexample
int x __attribute__ ((aligned (16))) = 0;
@end smallexample

@noindent
causes the compiler to allocate the global variable @code{x} on a
16-byte boundary.  On a 68040, this could be used in conjunction with
an @code{asm} expression to access the @code{move16} instruction which
requires 16-byte aligned operands.

You can also specify the alignment of structure fields.  For example, to
create a double-word aligned @code{int} pair, you could write:

@smallexample
struct foo @{ int x[2] __attribute__ ((aligned (8))); @};
@end smallexample

@noindent
This is an alternative to creating a union with a @code{double} member
that forces the union to be double-word aligned.

It is not possible to specify the alignment of functions; the alignment
of functions is determined by the machine's requirements and cannot be
changed.  You cannot specify alignment for a typedef name because such a
name is just an alias, not a distinct type.

As in the preceding examples, you can explicitly specify the alignment
(in bytes) that you wish the compiler to use for a given variable or
structure field.  Alternatively, you can leave out the alignment factor
and just ask the compiler to align a variable or field to the maximum
useful alignment for the target machine you are compiling for.  For
example, you could write:

@smallexample
short array[3] __attribute__ ((aligned));
@end smallexample

Whenever you leave out the alignment factor in an @code{aligned} attribute
specification, the compiler automatically sets the alignment for the declared
variable or field to the largest alignment which is ever used for any data
type on the target machine you are compiling for.  Doing this can often make
copy operations more efficient, because the compiler can use whatever
instructions copy the biggest chunks of memory when performing copies to
or from the variables or fields that you have aligned this way.

The @code{aligned} attribute can only increase the alignment; but you
can decrease it by specifying @code{packed} as well.  See below.

Note that the effectiveness of @code{aligned} attributes may be limited
by inherent limitations in your linker.  On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment.  (For some linkers, the maximum supported alignment may
be very very small.)  If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying @code{aligned(16)}
in an @code{__attribute__} will still only provide you with 8 byte
alignment.  See your linker documentation for further information.

@item mode (@var{mode})
@cindex @code{mode} attribute
This attribute specifies the data type for the declaration---whichever
type corresponds to the mode @var{mode}.  This in effect lets you
request an integer or floating point type according to its width.

You may also specify a mode of @samp{byte} or @samp{__byte__} to
indicate the mode corresponding to a one-byte integer, @samp{word} or
@samp{__word__} for the mode of a one-word integer, and @samp{pointer}
or @samp{__pointer__} for the mode used to represent pointers.

@item nocommon
@cindex @code{nocommon} attribute
This attribute specifies requests GNU CC not to place a variable
``common'' but instead to allocate space for it directly.  If you
specify the @samp{-fno-common} flag, GNU CC will do this for all
variables.

Specifying the @code{nocommon} attribute for a variable provides an
initialization of zeros.  A variable may only be initialized in one
source file.

@item packed
@cindex @code{packed} attribute
The @code{packed} attribute specifies that a variable or structure field
should have the smallest possible alignment---one byte for a variable,
and one bit for a field, unless you specify a larger value with the
@code{aligned} attribute.

Here is a structure in which the field @code{x} is packed, so that it
immediately follows @code{a}:

@example
struct foo
@{
  char a;
  int x[2] __attribute__ ((packed));
@};
@end example

@item section ("section-name")
@cindex @code{section} variable attribute
Normally, the compiler places the objects it generates in sections like
@code{data} and @code{bss}.  Sometimes, however, you need additional sections,
or you need certain particular variables to appear in special sections,
for example to map to special hardware.  The @code{section}
attribute specifies that a variable (or function) lives in a particular
section.  For example, this small program uses several specific section names:

@smallexample
struct duart a __attribute__ ((section ("DUART_A"))) = @{ 0 @};
struct duart b __attribute__ ((section ("DUART_B"))) = @{ 0 @};
char stack[10000] __attribute__ ((section ("STACK"))) = @{ 0 @};
int init_data __attribute__ ((section ("INITDATA"))) = 0;

main()
@{
  /* Initialize stack pointer */
  init_sp (stack + sizeof (stack));

  /* Initialize initialized data */
  memcpy (&init_data, &data, &edata - &data);

  /* Turn on the serial ports */
  init_duart (&a);
  init_duart (&b);
@}
@end smallexample

@noindent
Use the @code{section} attribute with an @emph{initialized} definition
of a @emph{global} variable, as shown in the example.  GNU CC issues
a warning and otherwise ignores the @code{section} attribute in
uninitialized variable declarations.

You may only use the @code{section} attribute with a fully initialized
global definition because of the way linkers work.  The linker requires
each object be defined once, with the exception that uninitialized
variables tentatively go in the @code{common} (or @code{bss}) section
and can be multiply "defined".  You can force a variable to be
initialized with the @samp{-fno-common} flag or the @code{nocommon}
attribute.

Some file formats do not support arbitrary sections so the @code{section}
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.

@item shared
@cindex @code{shared} variable attribute
On Windows NT, in addition to putting variable definitions in a named 
section, the section can also be shared among all running copies of an 
executable or DLL. For example, this small program defines shared data 
by putting it in a named section "shared" and marking the section 
shareable:

@smallexample
int foo __attribute__((section ("shared"), shared)) = 0;

int
main()
@{
  /* Read and write foo. All running copies see the same value. */
  return 0;
@}
@end smallexample

@noindent
You may only use the @code{shared} attribute along with @code{section}
attribute with a fully initialized global definition because of the way 
linkers work.  See @code{section} attribute for more information.

The @code{shared} attribute is only available on Windows NT.

@item transparent_union
This attribute, attached to a function parameter which is a union, means
that the corresponding argument may have the type of any union member,
but the argument is passed as if its type were that of the first union
member.  For more details see @xref{Type Attributes}.  You can also use
this attribute on a @code{typedef} for a union data type; then it
applies to all function parameters with that type.

@item unused
This attribute, attached to a variable, means that the variable is meant
to be possibly unused.  GNU CC will not produce a warning for this
variable.

@item weak
The @code{weak} attribute is described in @xref{Function Attributes}.

@item model (@var{model-name})
@cindex variable addressability on the M32R/D
Use this attribute on the M32R/D to set the addressability of an object.
The identifier @var{model-name} is one of @code{small}, @code{medium},
or @code{large}, representing each of the code models.

Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the @code{ld24} instruction).

Medium and large model objects may live anywhere in the 32 bit address space
(the compiler will generate @code{seth/add3} instructions to load their
addresses).

@end table

To specify multiple attributes, separate them by commas within the
double parentheses: for example, @samp{__attribute__ ((aligned (16),
packed))}.

@node Type Attributes
@section Specifying Attributes of Types
@cindex attribute of types
@cindex type attributes

The keyword @code{__attribute__} allows you to specify special
attributes of @code{struct} and @code{union} types when you define such
types.  This keyword is followed by an attribute specification inside
double parentheses.  Four attributes are currently defined for types:
@code{aligned}, @code{packed}, @code{transparent_union}, and @code{unused}.
Other attributes are defined for functions (@pxref{Function Attributes}) and
for variables (@pxref{Variable Attributes}).

You may also specify any one of these attributes with @samp{__}
preceding and following its keyword.  This allows you to use these
attributes in header files without being concerned about a possible
macro of the same name.  For example, you may use @code{__aligned__}
instead of @code{aligned}.

You may specify the @code{aligned} and @code{transparent_union}
attributes either in a @code{typedef} declaration or just past the
closing curly brace of a complete enum, struct or union type
@emph{definition} and the @code{packed} attribute only past the closing
brace of a definition.

You may also specify attributes between the enum, struct or union
tag and the name of the type rather than after the closing brace.

@xref{Attribute Syntax}, for details of the exact syntax for using
attributes.

@table @code
@cindex @code{aligned} attribute
@item aligned (@var{alignment})
This attribute specifies a minimum alignment (in bytes) for variables
of the specified type.  For example, the declarations:

@smallexample
struct S @{ short f[3]; @} __attribute__ ((aligned (8)));
typedef int more_aligned_int __attribute__ ((aligned (8)));
@end smallexample

@noindent
force the compiler to insure (as far as it can) that each variable whose
type is @code{struct S} or @code{more_aligned_int} will be allocated and
aligned @emph{at least} on a 8-byte boundary.  On a Sparc, having all
variables of type @code{struct S} aligned to 8-byte boundaries allows
the compiler to use the @code{ldd} and @code{std} (doubleword load and
store) instructions when copying one variable of type @code{struct S} to
another, thus improving run-time efficiency.

Note that the alignment of any given @code{struct} or @code{union} type
is required by the ISO C standard to be at least a perfect multiple of
the lowest common multiple of the alignments of all of the members of
the @code{struct} or @code{union} in question.  This means that you @emph{can}
effectively adjust the alignment of a @code{struct} or @code{union}
type by attaching an @code{aligned} attribute to any one of the members
of such a type, but the notation illustrated in the example above is a
more obvious, intuitive, and readable way to request the compiler to
adjust the alignment of an entire @code{struct} or @code{union} type.

As in the preceding example, you can explicitly specify the alignment
(in bytes) that you wish the compiler to use for a given @code{struct}
or @code{union} type.  Alternatively, you can leave out the alignment factor
and just ask the compiler to align a type to the maximum
useful alignment for the target machine you are compiling for.  For
example, you could write:

@smallexample
struct S @{ short f[3]; @} __attribute__ ((aligned));
@end smallexample

Whenever you leave out the alignment factor in an @code{aligned}
attribute specification, the compiler automatically sets the alignment
for the type to the largest alignment which is ever used for any data
type on the target machine you are compiling for.  Doing this can often
make copy operations more efficient, because the compiler can use
whatever instructions copy the biggest chunks of memory when performing
copies to or from the variables which have types that you have aligned
this way.

In the example above, if the size of each @code{short} is 2 bytes, then
the size of the entire @code{struct S} type is 6 bytes.  The smallest
power of two which is greater than or equal to that is 8, so the
compiler sets the alignment for the entire @code{struct S} type to 8
bytes.

Note that although you can ask the compiler to select a time-efficient
alignment for a given type and then declare only individual stand-alone
objects of that type, the compiler's ability to select a time-efficient
alignment is primarily useful only when you plan to create arrays of
variables having the relevant (efficiently aligned) type.  If you
declare or use arrays of variables of an efficiently-aligned type, then
it is likely that your program will also be doing pointer arithmetic (or
subscripting, which amounts to the same thing) on pointers to the
relevant type, and the code that the compiler generates for these
pointer arithmetic operations will often be more efficient for
efficiently-aligned types than for other types.

The @code{aligned} attribute can only increase the alignment; but you
can decrease it by specifying @code{packed} as well.  See below.

Note that the effectiveness of @code{aligned} attributes may be limited
by inherent limitations in your linker.  On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment.  (For some linkers, the maximum supported alignment may
be very very small.)  If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying @code{aligned(16)}
in an @code{__attribute__} will still only provide you with 8 byte
alignment.  See your linker documentation for further information.

@item packed
This attribute, attached to an @code{enum}, @code{struct}, or
@code{union} type definition, specified that the minimum required memory
be used to represent the type.

Specifying this attribute for @code{struct} and @code{union} types is
equivalent to specifying the @code{packed} attribute on each of the
structure or union members.  Specifying the @samp{-fshort-enums}
flag on the line is equivalent to specifying the @code{packed}
attribute on all @code{enum} definitions.

You may only specify this attribute after a closing curly brace on an
@code{enum} definition, not in a @code{typedef} declaration, unless that
declaration also contains the definition of the @code{enum}.

@item transparent_union
This attribute, attached to a @code{union} type definition, indicates
that any function parameter having that union type causes calls to that
function to be treated in a special way.

First, the argument corresponding to a transparent union type can be of
any type in the union; no cast is required.  Also, if the union contains
a pointer type, the corresponding argument can be a null pointer
constant or a void pointer expression; and if the union contains a void
pointer type, the corresponding argument can be any pointer expression.
If the union member type is a pointer, qualifiers like @code{const} on
the referenced type must be respected, just as with normal pointer
conversions.

Second, the argument is passed to the function using the calling
conventions of first member of the transparent union, not the calling
conventions of the union itself.  All members of the union must have the
same machine representation; this is necessary for this argument passing
to work properly.

Transparent unions are designed for library functions that have multiple
interfaces for compatibility reasons.  For example, suppose the
@code{wait} function must accept either a value of type @code{int *} to
comply with Posix, or a value of type @code{union wait *} to comply with
the 4.1BSD interface.  If @code{wait}'s parameter were @code{void *},
@code{wait} would accept both kinds of arguments, but it would also
accept any other pointer type and this would make argument type checking
less useful.  Instead, @code{<sys/wait.h>} might define the interface
as follows:

@smallexample
typedef union
  @{
    int *__ip;
    union wait *__up;
  @} wait_status_ptr_t __attribute__ ((__transparent_union__));

pid_t wait (wait_status_ptr_t);
@end smallexample

This interface allows either @code{int *} or @code{union wait *}
arguments to be passed, using the @code{int *} calling convention.
The program can call @code{wait} with arguments of either type:

@example
int w1 () @{ int w; return wait (&w); @}
int w2 () @{ union wait w; return wait (&w); @}
@end example

With this interface, @code{wait}'s implementation might look like this:

@example
pid_t wait (wait_status_ptr_t p)
@{
  return waitpid (-1, p.__ip, 0);
@}
@end example

@item unused
When attached to a type (including a @code{union} or a @code{struct}),
this attribute means that variables of that type are meant to appear
possibly unused.  GNU CC will not produce a warning for any variables of
that type, even if the variable appears to do nothing.  This is often
the case with lock or thread classes, which are usually defined and then
not referenced, but contain constructors and destructors that have
nontrivial bookkeeping functions.

@end table

To specify multiple attributes, separate them by commas within the
double parentheses: for example, @samp{__attribute__ ((aligned (16),
packed))}.

@node Inline
@section An Inline Function is As Fast As a Macro
@cindex inline functions
@cindex integrating function code
@cindex open coding
@cindex macros, inline alternative

By declaring a function @code{inline}, you can direct GNU CC to
integrate that function's code into the code for its callers.  This
makes execution faster by eliminating the function-call overhead; in
addition, if any of the actual argument values are constant, their known
values may permit simplifications at compile time so that not all of the
inline function's code needs to be included.  The effect on code size is
less predictable; object code may be larger or smaller with function
inlining, depending on the particular case.  Inlining of functions is an
optimization and it really ``works'' only in optimizing compilation.  If
you don't use @samp{-O}, no function is really inline.

To declare a function inline, use the @code{inline} keyword in its
declaration, like this:

@example
inline int
inc (int *a)
@{
  (*a)++;
@}
@end example

(If you are writing a header file to be included in ISO C programs, write
@code{__inline__} instead of @code{inline}.  @xref{Alternate Keywords}.)
You can also make all ``simple enough'' functions inline with the option
@samp{-finline-functions}. 

Note that certain usages in a function definition can make it unsuitable
for inline substitution.  Among these usages are: use of varargs, use of
alloca, use of variable sized data types (@pxref{Variable Length}),
use of computed goto (@pxref{Labels as Values}), use of nonlocal goto,
and nested functions (@pxref{Nested Functions}).  Using @samp{-Winline}
will warn when a function marked @code{inline} could not be substituted,
and will give the reason for the failure.

Note that in C and Objective C, unlike C++, the @code{inline} keyword
does not affect the linkage of the function.

@cindex automatic @code{inline} for C++ member fns
@cindex @code{inline} automatic for C++ member fns
@cindex member fns, automatically @code{inline}
@cindex C++ member fns, automatically @code{inline}
GNU CC automatically inlines member functions defined within the class
body of C++ programs even if they are not explicitly declared
@code{inline}.  (You can override this with @samp{-fno-default-inline};
@pxref{C++ Dialect Options,,Options Controlling C++ Dialect}.)

@cindex inline functions, omission of
When a function is both inline and @code{static}, if all calls to the
function are integrated into the caller, and the function's address is
never used, then the function's own assembler code is never referenced.
In this case, GNU CC does not actually output assembler code for the
function, unless you specify the option @samp{-fkeep-inline-functions}.
Some calls cannot be integrated for various reasons (in particular,
calls that precede the function's definition cannot be integrated, and
neither can recursive calls within the definition).  If there is a
nonintegrated call, then the function is compiled to assembler code as
usual.  The function must also be compiled as usual if the program
refers to its address, because that can't be inlined.

@cindex non-static inline function
When an inline function is not @code{static}, then the compiler must assume
that there may be calls from other source files; since a global symbol can
be defined only once in any program, the function must not be defined in
the other source files, so the calls therein cannot be integrated.
Therefore, a non-@code{static} inline function is always compiled on its
own in the usual fashion.

If you specify both @code{inline} and @code{extern} in the function
definition, then the definition is used only for inlining.  In no case
is the function compiled on its own, not even if you refer to its
address explicitly.  Such an address becomes an external reference, as
if you had only declared the function, and had not defined it.

This combination of @code{inline} and @code{extern} has almost the
effect of a macro.  The way to use it is to put a function definition in
a header file with these keywords, and put another copy of the
definition (lacking @code{inline} and @code{extern}) in a library file.
The definition in the header file will cause most calls to the function
to be inlined.  If any uses of the function remain, they will refer to
the single copy in the library.

GNU C does not inline any functions when not optimizing.  It is not
clear whether it is better to inline or not, in this case, but we found
that a correct implementation when not optimizing was difficult.  So we
did the easy thing, and turned it off.

@node Extended Asm
@section Assembler Instructions with C Expression Operands
@cindex extended @code{asm}
@cindex @code{asm} expressions
@cindex assembler instructions
@cindex registers

In an assembler instruction using @code{asm}, you can specify the
operands of the instruction using C expressions.  This means you need not
guess which registers or memory locations will contain the data you want
to use.

You must specify an assembler instruction template much like what
appears in a machine description, plus an operand constraint string for
each operand.

For example, here is how to use the 68881's @code{fsinx} instruction:

@example
asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
@end example

@noindent
Here @code{angle} is the C expression for the input operand while
@code{result} is that of the output operand.  Each has @samp{"f"} as its
operand constraint, saying that a floating point register is required.
The @samp{=} in @samp{=f} indicates that the operand is an output; all
output operands' constraints must use @samp{=}.  The constraints use the
same language used in the machine description (@pxref{Constraints}).

Each operand is described by an operand-constraint string followed by
the C expression in parentheses.  A colon separates the assembler
template from the first output operand and another separates the last
output operand from the first input, if any.  Commas separate the
operands within each group.  The total number of operands is limited to
ten or to the maximum number of operands in any instruction pattern in
the machine description, whichever is greater.

If there are no output operands but there are input operands, you must
place two consecutive colons surrounding the place where the output
operands would go.

Output operand expressions must be lvalues; the compiler can check this.
The input operands need not be lvalues.  The compiler cannot check
whether the operands have data types that are reasonable for the
instruction being executed.  It does not parse the assembler instruction
template and does not know what it means or even whether it is valid
assembler input.  The extended @code{asm} feature is most often used for
machine instructions the compiler itself does not know exist.  If
the output expression cannot be directly addressed (for example, it is a
bit field), your constraint must allow a register.  In that case, GNU CC
will use the register as the output of the @code{asm}, and then store
that register into the output.

The ordinary output operands must be write-only; GNU CC will assume that
the values in these operands before the instruction are dead and need
not be generated.  Extended asm supports input-output or read-write
operands.  Use the constraint character @samp{+} to indicate such an
operand and list it with the output operands.

When the constraints for the read-write operand (or the operand in which
only some of the bits are to be changed) allows a register, you may, as
an alternative, logically split its function into two separate operands,
one input operand and one write-only output operand.  The connection
between them is expressed by constraints which say they need to be in
the same location when the instruction executes.  You can use the same C
expression for both operands, or different expressions.  For example,
here we write the (fictitious) @samp{combine} instruction with
@code{bar} as its read-only source operand and @code{foo} as its
read-write destination:

@example
asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));
@end example

@noindent
The constraint @samp{"0"} for operand 1 says that it must occupy the
same location as operand 0.  A digit in constraint is allowed only in an
input operand and it must refer to an output operand.

Only a digit in the constraint can guarantee that one operand will be in
the same place as another.  The mere fact that @code{foo} is the value
of both operands is not enough to guarantee that they will be in the
same place in the generated assembler code.  The following would not
work reliably:

@example
asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));
@end example

Various optimizations or reloading could cause operands 0 and 1 to be in
different registers; GNU CC knows no reason not to do so.  For example, the
compiler might find a copy of the value of @code{foo} in one register and
use it for operand 1, but generate the output operand 0 in a different
register (copying it afterward to @code{foo}'s own address).  Of course,
since the register for operand 1 is not even mentioned in the assembler
code, the result will not work, but GNU CC can't tell that.

Some instructions clobber specific hard registers.  To describe this,
write a third colon after the input operands, followed by the names of
the clobbered hard registers (given as strings).  Here is a realistic
example for the VAX:

@example
asm volatile ("movc3 %0,%1,%2"
              : /* no outputs */
              : "g" (from), "g" (to), "g" (count)
              : "r0", "r1", "r2", "r3", "r4", "r5");
@end example

You may not write a clobber description in a way that overlaps with an
input or output operand.  For example, you may not have an operand
describing a register class with one member if you mention that register
in the clobber list.  There is no way for you to specify that an input
operand is modified without also specifying it as an output
operand.  Note that if all the output operands you specify are for this
purpose (and hence unused), you will then also need to specify
@code{volatile} for the @code{asm} construct, as described below, to
prevent GNU CC from deleting the @code{asm} statement as unused.

If you refer to a particular hardware register from the assembler code,
you will probably have to list the register after the third colon to
tell the compiler the register's value is modified.  In some assemblers,
the register names begin with @samp{%}; to produce one @samp{%} in the
assembler code, you must write @samp{%%} in the input.

If your assembler instruction can alter the condition code register, add
@samp{cc} to the list of clobbered registers.  GNU CC on some machines
represents the condition codes as a specific hardware register;
@samp{cc} serves to name this register.  On other machines, the
condition code is handled differently, and specifying @samp{cc} has no
effect.  But it is valid no matter what the machine.

If your assembler instruction modifies memory in an unpredictable
fashion, add @samp{memory} to the list of clobbered registers.  This
will cause GNU CC to not keep memory values cached in registers across
the assembler instruction.  You will also want to add the
@code{volatile} keyword if the memory affected is not listed in the
inputs or outputs of the @code{asm}, as the @samp{memory} clobber does
not count as a side-effect of the @code{asm}.

You can put multiple assembler instructions together in a single
@code{asm} template, separated by the characters normally used in assembly
code for the system.  A combination that works in most places is a newline
to break the line, plus a tab character to move to the instruction field
(written as @samp{\n\t}).  Sometimes semicolons can be used, if the
assembler allows semicolons as a line-breaking character.  Note that some
assembler dialects use semicolons to start a comment.
The input operands are guaranteed not to use any of the clobbered
registers, and neither will the output operands' addresses, so you can
read and write the clobbered registers as many times as you like.  Here
is an example of multiple instructions in a template; it assumes the
subroutine @code{_foo} accepts arguments in registers 9 and 10:

@example
asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo"
     : /* no outputs */
     : "g" (from), "g" (to)
     : "r9", "r10");
@end example

Unless an output operand has the @samp{&} constraint modifier, GNU CC
may allocate it in the same register as an unrelated input operand, on
the assumption the inputs are consumed before the outputs are produced.
This assumption may be false if the assembler code actually consists of
more than one instruction.  In such a case, use @samp{&} for each output
operand that may not overlap an input.  @xref{Modifiers}.

If you want to test the condition code produced by an assembler
instruction, you must include a branch and a label in the @code{asm}
construct, as follows:

@example
asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:"
     : "g" (result)
     : "g" (input));
@end example

@noindent
This assumes your assembler supports local labels, as the GNU assembler
and most Unix assemblers do.

Speaking of labels, jumps from one @code{asm} to another are not
supported.  The compiler's optimizers do not know about these jumps, and
therefore they cannot take account of them when deciding how to
optimize.

@cindex macros containing @code{asm}
Usually the most convenient way to use these @code{asm} instructions is to
encapsulate them in macros that look like functions.  For example,

@example
#define sin(x)       \
(@{ double __value, __arg = (x);   \
   asm ("fsinx %1,%0": "=f" (__value): "f" (__arg));  \
   __value; @})
@end example

@noindent
Here the variable @code{__arg} is used to make sure that the instruction
operates on a proper @code{double} value, and to accept only those
arguments @code{x} which can convert automatically to a @code{double}.

Another way to make sure the instruction operates on the correct data
type is to use a cast in the @code{asm}.  This is different from using a
variable @code{__arg} in that it converts more different types.  For
example, if the desired type were @code{int}, casting the argument to
@code{int} would accept a pointer with no complaint, while assigning the
argument to an @code{int} variable named @code{__arg} would warn about
using a pointer unless the caller explicitly casts it.

If an @code{asm} has output operands, GNU CC assumes for optimization
purposes the instruction has no side effects except to change the output
operands.  This does not mean instructions with a side effect cannot be
used, but you must be careful, because the compiler may eliminate them
if the output operands aren't used, or move them out of loops, or
replace two with one if they constitute a common subexpression.  Also,
if your instruction does have a side effect on a variable that otherwise
appears not to change, the old value of the variable may be reused later
if it happens to be found in a register.

You can prevent an @code{asm} instruction from being deleted, moved
significantly, or combined, by writing the keyword @code{volatile} after
the @code{asm}.  For example:

@example
#define get_and_set_priority(new)  \
(@{ int __old; \
   asm volatile ("get_and_set_priority %0, %1": "=g" (__old) : "g" (new)); \
   __old; @})
@end example

@noindent
If you write an @code{asm} instruction with no outputs, GNU CC will know
the instruction has side-effects and will not delete the instruction or
move it outside of loops.  If the side-effects of your instruction are
not purely external, but will affect variables in your program in ways
other than reading the inputs and clobbering the specified registers or
memory, you should write the @code{volatile} keyword to prevent future
versions of GNU CC from moving the instruction around within a core
region.

An @code{asm} instruction without any operands or clobbers (and ``old
style'' @code{asm}) will not be deleted or moved significantly,
regardless, unless it is unreachable, the same way as if you had
written a @code{volatile} keyword.

Note that even a volatile @code{asm} instruction can be moved in ways
that appear insignificant to the compiler, such as across jump
instructions.  You can't expect a sequence of volatile @code{asm}
instructions to remain perfectly consecutive.  If you want consecutive
output, use a single @code{asm}.

It is a natural idea to look for a way to give access to the condition
code left by the assembler instruction.  However, when we attempted to
implement this, we found no way to make it work reliably.  The problem
is that output operands might need reloading, which would result in
additional following ``store'' instructions.  On most machines, these
instructions would alter the condition code before there was time to
test it.  This problem doesn't arise for ordinary ``test'' and
``compare'' instructions because they don't have any output operands.

For reasons similar to those described above, it is not possible to give
an assembler instruction access to the condition code left by previous
instructions.

If you are writing a header file that should be includable in ISO C
programs, write @code{__asm__} instead of @code{asm}.  @xref{Alternate
Keywords}.

@subsection i386 floating point asm operands

There are several rules on the usage of stack-like regs in
asm_operands insns.  These rules apply only to the operands that are
stack-like regs:

@enumerate
@item
Given a set of input regs that die in an asm_operands, it is
necessary to know which are implicitly popped by the asm, and
which must be explicitly popped by gcc.

An input reg that is implicitly popped by the asm must be
explicitly clobbered, unless it is constrained to match an
output operand.

@item
For any input reg that is implicitly popped by an asm, it is
necessary to know how to adjust the stack to compensate for the pop.
If any non-popped input is closer to the top of the reg-stack than
the implicitly popped reg, it would not be possible to know what the
stack looked like --- it's not clear how the rest of the stack ``slides
up''.

All implicitly popped input regs must be closer to the top of
the reg-stack than any input that is not implicitly popped.

It is possible that if an input dies in an insn, reload might
use the input reg for an output reload.  Consider this example:

@example
asm ("foo" : "=t" (a) : "f" (b));
@end example

This asm says that input B is not popped by the asm, and that
the asm pushes a result onto the reg-stack, ie, the stack is one
deeper after the asm than it was before.  But, it is possible that
reload will think that it can use the same reg for both the input and
the output, if input B dies in this insn.

If any input operand uses the @code{f} constraint, all output reg
constraints must use the @code{&} earlyclobber.

The asm above would be written as

@example
asm ("foo" : "=&t" (a) : "f" (b));
@end example

@item
Some operands need to be in particular places on the stack.  All
output operands fall in this category --- there is no other way to
know which regs the outputs appear in unless the user indicates
this in the constraints.

Output operands must specifically indicate which reg an output
appears in after an asm.  @code{=f} is not allowed: the operand
constraints must select a class with a single reg.

@item
Output operands may not be ``inserted'' between existing stack regs.
Since no 387 opcode uses a read/write operand, all output operands
are dead before the asm_operands, and are pushed by the asm_operands.
It makes no sense to push anywhere but the top of the reg-stack.

Output operands must start at the top of the reg-stack: output
operands may not ``skip'' a reg.

@item
Some asm statements may need extra stack space for internal
calculations.  This can be guaranteed by clobbering stack registers
unrelated to the inputs and outputs.

@end enumerate

Here are a couple of reasonable asms to want to write.  This asm
takes one input, which is internally popped, and produces two outputs.

@example
asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));
@end example

This asm takes two inputs, which are popped by the @code{fyl2xp1} opcode,
and replaces them with one output.  The user must code the @code{st(1)}
clobber for reg-stack.c to know that @code{fyl2xp1} pops both inputs.

@example
asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");
@end example

@ifclear INTERNALS
@c Show the details on constraints if they do not appear elsewhere in
@c the manual
@include md.texi
@end ifclear

@node Asm Labels
@section Controlling Names Used in Assembler Code
@cindex assembler names for identifiers
@cindex names used in assembler code
@cindex identifiers, names in assembler code

You can specify the name to be used in the assembler code for a C
function or variable by writing the @code{asm} (or @code{__asm__})
keyword after the declarator as follows:

@example
int foo asm ("myfoo") = 2;
@end example

@noindent
This specifies that the name to be used for the variable @code{foo} in
the assembler code should be @samp{myfoo} rather than the usual
@samp{_foo}.

On systems where an underscore is normally prepended to the name of a C
function or variable, this feature allows you to define names for the
linker that do not start with an underscore.

You cannot use @code{asm} in this way in a function @emph{definition}; but
you can get the same effect by writing a declaration for the function
before its definition and putting @code{asm} there, like this:

@example
extern func () asm ("FUNC");

func (x, y)
     int x, y;
@dots{}
@end example

It is up to you to make sure that the assembler names you choose do not
conflict with any other assembler symbols.  Also, you must not use a
register name; that would produce completely invalid assembler code.  GNU
CC does not as yet have the ability to store static variables in registers.
Perhaps that will be added.

@node Explicit Reg Vars
@section Variables in Specified Registers
@cindex explicit register variables
@cindex variables in specified registers
@cindex specified registers
@cindex registers, global allocation

GNU C allows you to put a few global variables into specified hardware
registers.  You can also specify the register in which an ordinary
register variable should be allocated.

@itemize @bullet
@item
Global register variables reserve registers throughout the program.
This may be useful in programs such as programming language
interpreters which have a couple of global variables that are accessed
very often.

@item
Local register variables in specific registers do not reserve the
registers.  The compiler's data flow analysis is capable of determining
where the specified registers contain live values, and where they are
available for other uses.  Stores into local register variables may be deleted
when they appear to be dead according to dataflow analysis.  References
to local register variables may be deleted or moved or simplified.

These local variables are sometimes convenient for use with the extended
@code{asm} feature (@pxref{Extended Asm}), if you want to write one
output of the assembler instruction directly into a particular register.
(This will work provided the register you specify fits the constraints
specified for that operand in the @code{asm}.)
@end itemize

@menu
* Global Reg Vars::
* Local Reg Vars::
@end menu

@node Global Reg Vars
@subsection Defining Global Register Variables
@cindex global register variables
@cindex registers, global variables in

You can define a global register variable in GNU C like this:

@example
register int *foo asm ("a5");
@end example

@noindent
Here @code{a5} is the name of the register which should be used.  Choose a
register which is normally saved and restored by function calls on your
machine, so that library routines will not clobber it.

Naturally the register name is cpu-dependent, so you would need to
conditionalize your program according to cpu type.  The register
@code{a5} would be a good choice on a 68000 for a variable of pointer
type.  On machines with register windows, be sure to choose a ``global''
register that is not affected magically by the function call mechanism.

In addition, operating systems on one type of cpu may differ in how they
name the registers; then you would need additional conditionals.  For
example, some 68000 operating systems call this register @code{%a5}.

Eventually there may be a way of asking the compiler to choose a register
automatically, but first we need to figure out how it should choose and
how to enable you to guide the choice.  No solution is evident.

Defining a global register variable in a certain register reserves that
register entirely for this use, at least within the current compilation.
The register will not be allocated for any other purpose in the functions
in the current compilation.  The register will not be saved and restored by
these functions.  Stores into this register are never deleted even if they
would appear to be dead, but references may be deleted or moved or
simplified.

It is not safe to access the global register variables from signal
handlers, or from more than one thread of control, because the system
library routines may temporarily use the register for other things (unless
you recompile them specially for the task at hand).

@cindex @code{qsort}, and global register variables
It is not safe for one function that uses a global register variable to
call another such function @code{foo} by way of a third function
@code{lose} that was compiled without knowledge of this variable (i.e. in a
different source file in which the variable wasn't declared).  This is
because @code{lose} might save the register and put some other value there.
For example, you can't expect a global register variable to be available in
the comparison-function that you pass to @code{qsort}, since @code{qsort}
might have put something else in that register.  (If you are prepared to
recompile @code{qsort} with the same global register variable, you can
solve this problem.)

If you want to recompile @code{qsort} or other source files which do not
actually use your global register variable, so that they will not use that
register for any other purpose, then it suffices to specify the compiler
option @samp{-ffixed-@var{reg}}.  You need not actually add a global
register declaration to their source code.

A function which can alter the value of a global register variable cannot
safely be called from a function compiled without this variable, because it
could clobber the value the caller expects to find there on return.
Therefore, the function which is the entry point into the part of the
program that uses the global register variable must explicitly save and
restore the value which belongs to its caller.

@cindex register variable after @code{longjmp}
@cindex global register after @code{longjmp}
@cindex value after @code{longjmp}
@findex longjmp
@findex setjmp
On most machines, @code{longjmp} will restore to each global register
variable the value it had at the time of the @code{setjmp}.  On some
machines, however, @code{longjmp} will not change the value of global
register variables.  To be portable, the function that called @code{setjmp}
should make other arrangements to save the values of the global register
variables, and to restore them in a @code{longjmp}.  This way, the same
thing will happen regardless of what @code{longjmp} does.

All global register variable declarations must precede all function
definitions.  If such a declaration could appear after function
definitions, the declaration would be too late to prevent the register from
being used for other purposes in the preceding functions.

Global register variables may not have initial values, because an
executable file has no means to supply initial contents for a register.

On the Sparc, there are reports that g3 @dots{} g7 are suitable
registers, but certain library functions, such as @code{getwd}, as well
as the subroutines for division and remainder, modify g3 and g4.  g1 and
g2 are local temporaries.

On the 68000, a2 @dots{} a5 should be suitable, as should d2 @dots{} d7.
Of course, it will not do to use more than a few of those.

@node Local Reg Vars
@subsection Specifying Registers for Local Variables
@cindex local variables, specifying registers
@cindex specifying registers for local variables
@cindex registers for local variables

You can define a local register variable with a specified register
like this:

@example
register int *foo asm ("a5");
@end example

@noindent
Here @code{a5} is the name of the register which should be used.  Note
that this is the same syntax used for defining global register
variables, but for a local variable it would appear within a function.

Naturally the register name is cpu-dependent, but this is not a
problem, since specific registers are most often useful with explicit
assembler instructions (@pxref{Extended Asm}).  Both of these things
generally require that you conditionalize your program according to
cpu type.

In addition, operating systems on one type of cpu may differ in how they
name the registers; then you would need additional conditionals.  For
example, some 68000 operating systems call this register @code{%a5}.

Defining such a register variable does not reserve the register; it
remains available for other uses in places where flow control determines
the variable's value is not live.  However, these registers are made
unavailable for use in the reload pass; excessive use of this feature
leaves the compiler too few available registers to compile certain
functions.

This option does not guarantee that GNU CC will generate code that has
this variable in the register you specify at all times.  You may not
code an explicit reference to this register in an @code{asm} statement
and assume it will always refer to this variable.

Stores into local register variables may be deleted when they appear to be dead
according to dataflow analysis.  References to local register variables may
be deleted or moved or simplified.

@node Alternate Keywords
@section Alternate Keywords
@cindex alternate keywords
@cindex keywords, alternate

The option @option{-traditional} disables certain keywords;
@option{-ansi} and the various @option{-std} options disable certain
others.  This causes trouble when you want to use GNU C extensions, or
ISO C features, in a general-purpose header file that should be usable
by all programs, including ISO C programs and traditional ones.  The
keywords @code{asm}, @code{typeof} and @code{inline} cannot be used
since they won't work in a program compiled with @option{-ansi}
(although @code{inline} can be used in a program compiled with
@option{-std=c99}), while the keywords @code{const}, @code{volatile},
@code{signed}, @code{typeof} and @code{inline} won't work in a program
compiled with @option{-traditional}.  The ISO C99 keyword
@code{restrict} is only available when @option{-std=gnu99} (which will
eventually be the default) or @option{-std=c99} (or the equivalent
@option{-std=iso9899:1999}) is used.@refill

The way to solve these problems is to put @samp{__} at the beginning and
end of each problematical keyword.  For example, use @code{__asm__}
instead of @code{asm}, @code{__const__} instead of @code{const}, and
@code{__inline__} instead of @code{inline}.

Other C compilers won't accept these alternative keywords; if you want to
compile with another compiler, you can define the alternate keywords as
macros to replace them with the customary keywords.  It looks like this:

@example
#ifndef __GNUC__
#define __asm__ asm
#endif
@end example

@findex __extension__
@samp{-pedantic} and other options cause warnings for many GNU C extensions.
You can
prevent such warnings within one expression by writing
@code{__extension__} before the expression.  @code{__extension__} has no
effect aside from this.

@node Incomplete Enums
@section Incomplete @code{enum} Types

You can define an @code{enum} tag without specifying its possible values.
This results in an incomplete type, much like what you get if you write
@code{struct foo} without describing the elements.  A later declaration
which does specify the possible values completes the type.

You can't allocate variables or storage using the type while it is
incomplete.  However, you can work with pointers to that type.

This extension may not be very useful, but it makes the handling of
@code{enum} more consistent with the way @code{struct} and @code{union}
are handled.

This extension is not supported by GNU C++.

@node Function Names
@section Function Names as Strings

GNU CC predefines two magic identifiers to hold the name of the current
function. The identifier @code{__FUNCTION__} holds the name of the function
as it appears in the source. The identifier @code{__PRETTY_FUNCTION__}
holds the name of the function pretty printed in a language specific
fashion.

These names are always the same in a C function, but in a C++ function
they may be different.  For example, this program:

@smallexample
extern "C" @{
extern int printf (char *, ...);
@}

class a @{
 public:
  sub (int i)
    @{
      printf ("__FUNCTION__ = %s\n", __FUNCTION__);
      printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
    @}
@};

int
main (void)
@{
  a ax;
  ax.sub (0);
  return 0;
@}
@end smallexample

@noindent
gives this output:

@smallexample
__FUNCTION__ = sub
__PRETTY_FUNCTION__ = int  a::sub (int)
@end smallexample

The compiler automagically replaces the identifiers with a string
literal containing the appropriate name. Thus, they are neither
preprocessor macros, like @code{__FILE__} and @code{__LINE__}, nor
variables. This means that they catenate with other string literals, and
that they can be used to initialize char arrays. For example

@smallexample
char here[] = "Function " __FUNCTION__ " in " __FILE__;
@end smallexample

On the other hand, @samp{#ifdef __FUNCTION__} does not have any special
meaning inside a function, since the preprocessor does not do anything
special with the identifier @code{__FUNCTION__}.

GNU CC also supports the magic word @code{__func__}, defined by the
ISO standard C-99:

@display
The identifier @code{__func__} is implicitly declared by the translator
as if, immediately following the opening brace of each function
definition, the declaration

@smallexample
static const char __func__[] = "function-name";
@end smallexample

appeared, where function-name is the name of the lexically-enclosing
function. This name is the unadorned name of the function.
@end display

By this definition, @code{__func__} is a variable, not a string literal.
In particular, @code{__func__} does not catenate with other string
literals.

In @code{C++}, @code{__FUNCTION__} and @code{__PRETTY_FUNCTION__} are
variables, declared in the same way as @code{__func__}.

@node Return Address
@section Getting the Return or Frame Address of a Function

These functions may be used to get information about the callers of a
function.

@table @code
@findex __builtin_return_address
@item __builtin_return_address (@var{level})
This function returns the return address of the current function, or of
one of its callers.  The @var{level} argument is number of frames to
scan up the call stack.  A value of @code{0} yields the return address
of the current function, a value of @code{1} yields the return address
of the caller of the current function, and so forth.

The @var{level} argument must be a constant integer.

On some machines it may be impossible to determine the return address of
any function other than the current one; in such cases, or when the top
of the stack has been reached, this function will return @code{0}.

This function should only be used with a non-zero argument for debugging
purposes.

@findex __builtin_frame_address
@item __builtin_frame_address (@var{level})
This function is similar to @code{__builtin_return_address}, but it
returns the address of the function frame rather than the return address
of the function.  Calling @code{__builtin_frame_address} with a value of
@code{0} yields the frame address of the current function, a value of
@code{1} yields the frame address of the caller of the current function,
and so forth.

The frame is the area on the stack which holds local variables and saved
registers.  The frame address is normally the address of the first word
pushed on to the stack by the function.  However, the exact definition
depends upon the processor and the calling convention.  If the processor
has a dedicated frame pointer register, and the function has a frame,
then @code{__builtin_frame_address} will return the value of the frame
pointer register.

The caveats that apply to @code{__builtin_return_address} apply to this
function as well.
@end table

@node Other Builtins
@section Other built-in functions provided by GNU CC
@cindex builtin functions
@findex __builtin_isgreater
@findex __builtin_isgreaterequal
@findex __builtin_isless
@findex __builtin_islessequal
@findex __builtin_islessgreater
@findex __builtin_isunordered
@findex abort
@findex abs
@findex alloca
@findex bcmp
@findex bzero
@findex cimag
@findex cimagf
@findex cimagl
@findex conj
@findex conjf
@findex conjl
@findex cos
@findex cosf
@findex cosl
@findex creal
@findex crealf
@findex creall
@findex exit
@findex _exit
@findex _Exit
@findex fabs
@findex fabsf
@findex fabsl
@findex ffs
@findex fprintf
@findex fputs
@findex imaxabs
@findex index
@findex labs
@findex llabs
@findex memcmp
@findex memcpy
@findex memset
@findex printf
@findex rindex
@findex sin
@findex sinf
@findex sinl
@findex sqrt
@findex sqrtf
@findex sqrtl
@findex strcat
@findex strchr
@findex strcmp
@findex strcpy
@findex strcspn
@findex strlen
@findex strncat
@findex strncmp
@findex strncpy
@findex strpbrk
@findex strrchr
@findex strspn
@findex strstr

GNU CC provides a large number of built-in functions other than the ones
mentioned above.  Some of these are for internal use in the processing
of exceptions or variable-length argument lists and will not be
documented here because they may change from time to time; we do not
recommend general use of these functions.

The remaining functions are provided for optimization purposes.

GNU CC includes builtin versions of many of the functions in the
standard C library.  The versions prefixed with @code{__builtin_} will
always be treated as having the same meaning as the C library function
even if you specify the @samp{-fno-builtin} (@pxref{C Dialect Options})
option.  Many of these functions are only optimized in certain cases; if
not optimized in a particular case, a call to the library function will
be emitted.

The functions @code{abort}, @code{exit}, @code{_Exit} and @code{_exit}
are recognized and presumed not to return, but otherwise are not built
in.  @code{_exit} is not recognized in strict ISO C mode (@samp{-ansi},
@samp{-std=c89} or @samp{-std=c99}).  @code{_Exit} is not recognized in
strict C89 mode (@samp{-ansi} or @samp{-std=c89}).

Outside strict ISO C mode, the functions @code{alloca}, @code{bcmp},
@code{bzero}, @code{index}, @code{rindex} and @code{ffs} may be handled
as builtins.  Corresponding versions @code{__builtin_alloca},
@code{__builtin_bcmp}, @code{__builtin_bzero}, @code{__builtin_index},
@code{__builtin_rindex} and @code{__builtin_ffs} are also recognized in
strict ISO C mode.

The ISO C99 functions @code{conj}, @code{conjf}, @code{conjl},
@code{creal}, @code{crealf}, @code{creall}, @code{cimag}, @code{cimagf},
@code{cimagl}, @code{llabs} and @code{imaxabs} are handled as builtins
except in strict ISO C89 mode.  There are also builtin versions of the ISO C99
functions @code{cosf}, @code{cosl}, @code{fabsf}, @code{fabsl},
@code{sinf}, @code{sinl}, @code{sqrtf}, and @code{sqrtl}, that are
recognized in any mode since ISO C89 reserves these names for the
purpose to which ISO C99 puts them.  All these functions have
corresponding versions prefixed with @code{__builtin_}.

The following ISO C89 functions are recognized as builtins unless
@samp{-fno-builtin} is specified: @code{abs}, @code{cos}, @code{fabs},
@code{fprintf}, @code{fputs}, @code{labs}, @code{memcmp}, @code{memcpy},
@code{memset}, @code{printf}, @code{sin}, @code{sqrt}, @code{strcat},
@code{strchr}, @code{strcmp}, @code{strcpy}, @code{strcspn},
@code{strlen}, @code{strncat}, @code{strncmp}, @code{strncpy},
@code{strpbrk}, @code{strrchr}, @code{strspn}, and @code{strstr}.  All
of these functions have corresponding versions prefixed with
@code{__builtin_}, except that the version for @code{sqrt} is called
@code{__builtin_fsqrt}.

GNU CC provides builtin versions of the ISO C99 floating point
comparison macros (that avoid raising exceptions for unordered
operands): @code{__builtin_isgreater}, @code{__builtin_isgreaterequal},
@code{__builtin_isless}, @code{__builtin_islessequal},
@code{__builtin_islessgreater}, and @code{__builtin_isunordered}.


@table @code
@findex __builtin_constant_p
@item __builtin_constant_p (@var{exp})
You can use the builtin function @code{__builtin_constant_p} to
determine if a value is known to be constant at compile-time and hence
that GNU CC can perform constant-folding on expressions involving that
value.  The argument of the function is the value to test.  The function
returns the integer 1 if the argument is known to be a compile-time
constant and 0 if it is not known to be a compile-time constant.  A
return of 0 does not indicate that the value is @emph{not} a constant,
but merely that GNU CC cannot prove it is a constant with the specified
value of the @samp{-O} option.

You would typically use this function in an embedded application where
memory was a critical resource.  If you have some complex calculation,
you may want it to be folded if it involves constants, but need to call
a function if it does not.  For example:

@smallexample
#define Scale_Value(X)  \
  (__builtin_constant_p (X) ? ((X) * SCALE + OFFSET) : Scale (X))
@end smallexample

You may use this builtin function in either a macro or an inline
function.  However, if you use it in an inlined function and pass an
argument of the function as the argument to the builtin, GNU CC will
never return 1 when you call the inline function with a string constant
or constructor expression (@pxref{Constructors}) and will not return 1
when you pass a constant numeric value to the inline function unless you
specify the @samp{-O} option.

@findex __builtin_expect
@item __builtin_expect(@var{exp}, @var{c})
You may use @code{__builtin_expect} to provide the compiler with 
branch prediction information.  In general, you should prefer to
use actual profile feedback for this (@samp{-fprofile-arcs}), as
programmers are notoriously bad at predicting how their programs
actually perform.  However, there are applications in which this
data is hard to collect.

The return value is the value of @var{exp}, which should be an
integral expression.  The value of @var{c} must be a compile-time
constant.  The semantics of the builtin are that it is expected
that @var{exp} == @var{c}.  For example:

@smallexample
if (__builtin_expect (x, 0))
  foo ();
@end smallexample

@noindent
would indicate that we do not expect to call @code{foo}, since
we expect @code{x} to be zero.  Since you are limited to integral
expressions for @var{exp}, you should use constructions such as

@smallexample
if (__builtin_expect (ptr != NULL, 1))
  error ();
@end smallexample

@noindent
when testing pointer or floating-point values.
@end table

@node C++ Extensions
@chapter Extensions to the C++ Language
@cindex extensions, C++ language
@cindex C++ language extensions

The GNU compiler provides these extensions to the C++ language (and you
can also use most of the C language extensions in your C++ programs).  If you
want to write code that checks whether these features are available, you can
test for the GNU compiler the same way as for C programs: check for a
predefined macro @code{__GNUC__}.  You can also use @code{__GNUG__} to
test specifically for GNU C++ (@pxref{Standard Predefined,,Standard
Predefined Macros,cpp.info,The C Preprocessor}).

@menu
* Min and Max::		C++ Minimum and maximum operators.
* Volatiles::		What constitutes an access to a volatile object.
* Restricted Pointers:: C99 restricted pointers and references.
* C++ Interface::       You can use a single C++ header file for both
                        declarations and definitions.
* Template Instantiation:: Methods for ensuring that exactly one copy of
                        each needed template instantiation is emitted.
* Bound member functions:: You can extract a function pointer to the
                        method denoted by a @samp{->*} or @samp{.*} expression.
* C++ Attributes::      Variable, function, and type attributes for C++ only.
* Deprecated Features:: Things might disappear from g++.
* Backwards Compatibility:: Compatibilities with earlier definitions of C++.
@end menu

@node Min and Max
@section Minimum and Maximum Operators in C++

It is very convenient to have operators which return the ``minimum'' or the
``maximum'' of two arguments.  In GNU C++ (but not in GNU C),

@table @code
@item @var{a} <? @var{b}
@findex <?
@cindex minimum operator
is the @dfn{minimum}, returning the smaller of the numeric values
@var{a} and @var{b};

@item @var{a} >? @var{b}
@findex >?
@cindex maximum operator
is the @dfn{maximum}, returning the larger of the numeric values @var{a}
and @var{b}.
@end table

These operations are not primitive in ordinary C++, since you can
use a macro to return the minimum of two things in C++, as in the
following example.

@example
#define MIN(X,Y) ((X) < (Y) ? : (X) : (Y))
@end example

@noindent
You might then use @w{@samp{int min = MIN (i, j);}} to set @var{min} to
the minimum value of variables @var{i} and @var{j}.

However, side effects in @code{X} or @code{Y} may cause unintended
behavior.  For example, @code{MIN (i++, j++)} will fail, incrementing
the smaller counter twice.  A GNU C extension allows you to write safe
macros that avoid this kind of problem (@pxref{Naming Types,,Naming an
Expression's Type}).  However, writing @code{MIN} and @code{MAX} as
macros also forces you to use function-call notation for a
fundamental arithmetic operation.  Using GNU C++ extensions, you can
write @w{@samp{int min = i <? j;}} instead.

Since @code{<?} and @code{>?} are built into the compiler, they properly
handle expressions with side-effects;  @w{@samp{int min = i++ <? j++;}}
works correctly.

@node Volatiles
@section When is a Volatile Object Accessed?
@cindex accessing volatiles
@cindex volatile read
@cindex volatile write
@cindex volatile access

Both the C and C++ standard have the concept of volatile objects. These
are normally accessed by pointers and used for accessing hardware. The
standards encourage compilers to refrain from optimizations
concerning accesses to volatile objects that it might perform on
non-volatile objects. The C standard leaves it implementation defined
as to what constitutes a volatile access. The C++ standard omits to
specify this, except to say that C++ should behave in a similar manner
to C with respect to volatiles, where possible. The minimum either
standard specifies is that at a sequence point all previous accesses to
volatile objects have stabilized and no subsequent accesses have
occurred. Thus an implementation is free to reorder and combine
volatile accesses which occur between sequence points, but cannot do so
for accesses across a sequence point. The use of volatiles does not
allow you to violate the restriction on updating objects multiple times
within a sequence point.

In most expressions, it is intuitively obvious what is a read and what is
a write. For instance

@example
volatile int *dst = <somevalue>;
volatile int *src = <someothervalue>;
*dst = *src;
@end example

@noindent
will cause a read of the volatile object pointed to by @var{src} and stores the
value into the volatile object pointed to by @var{dst}. There is no
guarantee that these reads and writes are atomic, especially for objects
larger than @code{int}.

Less obvious expressions are where something which looks like an access
is used in a void context. An example would be,

@example
volatile int *src = <somevalue>;
*src;
@end example

With C, such expressions are rvalues, and as rvalues cause a read of
the object, gcc interprets this as a read of the volatile being pointed
to. The C++ standard specifies that such expressions do not undergo
lvalue to rvalue conversion, and that the type of the dereferenced
object may be incomplete. The C++ standard does not specify explicitly
that it is this lvalue to rvalue conversion which is responsible for
causing an access. However, there is reason to believe that it is,
because otherwise certain simple expressions become undefined. However,
because it would surprise most programmers, g++ treats dereferencing a
pointer to volatile object of complete type in a void context as a read
of the object. When the object has incomplete type, g++ issues a
warning.

@example
struct S;
struct T @{int m;@};
volatile S *ptr1 = <somevalue>;
volatile T *ptr2 = <somevalue>;
*ptr1;
*ptr2;
@end example

In this example, a warning is issued for @code{*ptr1}, and @code{*ptr2}
causes a read of the object pointed to. If you wish to force an error on
the first case, you must force a conversion to rvalue with, for instance
a static cast, @code{static_cast<S>(*ptr1)}.

When using a reference to volatile, g++ does not treat equivalent
expressions as accesses to volatiles, but instead issues a warning that
no volatile is accessed. The rationale for this is that otherwise it
becomes difficult to determine where volatile access occur, and not
possible to ignore the return value from functions returning volatile
references. Again, if you wish to force a read, cast the reference to
an rvalue.

@node Restricted Pointers
@section Restricting Pointer Aliasing
@cindex restricted pointers
@cindex restricted references
@cindex restricted this pointer

As with gcc, g++ understands the C99 feature of restricted pointers,
specified with the @code{__restrict__}, or @code{__restrict} type
qualifier. Because you cannot compile C++ by specifying the -std=c99
language flag, @code{restrict} is not a keyword in C++.

In addition to allowing restricted pointers, you can specify restricted
references, which indicate that the reference is not aliased in the local
context.

@example
void fn (int *__restrict__ rptr, int &__restrict__ rref)
@{
  @dots{}
@}
@end example

@noindent
In the body of @code{fn}, @var{rptr} points to an unaliased integer and
@var{rref} refers to a (different) unaliased integer.

You may also specify whether a member function's @var{this} pointer is
unaliased by using @code{__restrict__} as a member function qualifier.

@example
void T::fn () __restrict__
@{
  @dots{}
@}
@end example

@noindent
Within the body of @code{T::fn}, @var{this} will have the effective
definition @code{T *__restrict__ const this}. Notice that the
interpretation of a @code{__restrict__} member function qualifier is
different to that of @code{const} or @code{volatile} qualifier, in that it
is applied to the pointer rather than the object. This is consistent with
other compilers which implement restricted pointers.

As with all outermost parameter qualifiers, @code{__restrict__} is
ignored in function definition matching. This means you only need to
specify @code{__restrict__} in a function definition, rather than
in a function prototype as well.

@node C++ Interface
@section Declarations and Definitions in One Header

@cindex interface and implementation headers, C++
@cindex C++ interface and implementation headers
C++ object definitions can be quite complex.  In principle, your source
code will need two kinds of things for each object that you use across
more than one source file.  First, you need an @dfn{interface}
specification, describing its structure with type declarations and
function prototypes.  Second, you need the @dfn{implementation} itself.
It can be tedious to maintain a separate interface description in a
header file, in parallel to the actual implementation.  It is also
dangerous, since separate interface and implementation definitions may
not remain parallel.

@cindex pragmas, interface and implementation
With GNU C++, you can use a single header file for both purposes.

@quotation
@emph{Warning:} The mechanism to specify this is in transition.  For the
nonce, you must use one of two @code{#pragma} commands; in a future
release of GNU C++, an alternative mechanism will make these
@code{#pragma} commands unnecessary.
@end quotation

The header file contains the full definitions, but is marked with
@samp{#pragma interface} in the source code.  This allows the compiler
to use the header file only as an interface specification when ordinary
source files incorporate it with @code{#include}.  In the single source
file where the full implementation belongs, you can use either a naming
convention or @samp{#pragma implementation} to indicate this alternate
use of the header file.

@table @code
@item #pragma interface
@itemx #pragma interface "@var{subdir}/@var{objects}.h"
@kindex #pragma interface
Use this directive in @emph{header files} that define object classes, to save
space in most of the object files that use those classes.  Normally,
local copies of certain information (backup copies of inline member
functions, debugging information, and the internal tables that implement
virtual functions) must be kept in each object file that includes class
definitions.  You can use this pragma to avoid such duplication.  When a
header file containing @samp{#pragma interface} is included in a
compilation, this auxiliary information will not be generated (unless
the main input source file itself uses @samp{#pragma implementation}).
Instead, the object files will contain references to be resolved at link
time.

The second form of this directive is useful for the case where you have
multiple headers with the same name in different directories.  If you
use this form, you must specify the same string to @samp{#pragma
implementation}.

@item #pragma implementation
@itemx #pragma implementation "@var{objects}.h"
@kindex #pragma implementation
Use this pragma in a @emph{main input file}, when you want full output from
included header files to be generated (and made globally visible).  The
included header file, in turn, should use @samp{#pragma interface}.
Backup copies of inline member functions, debugging information, and the
internal tables used to implement virtual functions are all generated in
implementation files.

@cindex implied @code{#pragma implementation}
@cindex @code{#pragma implementation}, implied
@cindex naming convention, implementation headers
If you use @samp{#pragma implementation} with no argument, it applies to
an include file with the same basename@footnote{A file's @dfn{basename}
was the name stripped of all leading path information and of trailing
suffixes, such as @samp{.h} or @samp{.C} or @samp{.cc}.} as your source
file.  For example, in @file{allclass.cc}, giving just
@samp{#pragma implementation}
by itself is equivalent to @samp{#pragma implementation "allclass.h"}.

In versions of GNU C++ prior to 2.6.0 @file{allclass.h} was treated as
an implementation file whenever you would include it from
@file{allclass.cc} even if you never specified @samp{#pragma
implementation}.  This was deemed to be more trouble than it was worth,
however, and disabled.

If you use an explicit @samp{#pragma implementation}, it must appear in
your source file @emph{before} you include the affected header files.

Use the string argument if you want a single implementation file to
include code from multiple header files.  (You must also use
@samp{#include} to include the header file; @samp{#pragma
implementation} only specifies how to use the file---it doesn't actually
include it.)

There is no way to split up the contents of a single header file into
multiple implementation files.
@end table

@cindex inlining and C++ pragmas
@cindex C++ pragmas, effect on inlining
@cindex pragmas in C++, effect on inlining
@samp{#pragma implementation} and @samp{#pragma interface} also have an
effect on function inlining.

If you define a class in a header file marked with @samp{#pragma
interface}, the effect on a function defined in that class is similar to
an explicit @code{extern} declaration---the compiler emits no code at
all to define an independent version of the function.  Its definition
is used only for inlining with its callers.

Conversely, when you include the same header file in a main source file
that declares it as @samp{#pragma implementation}, the compiler emits
code for the function itself; this defines a version of the function
that can be found via pointers (or by callers compiled without
inlining).  If all calls to the function can be inlined, you can avoid
emitting the function by compiling with @samp{-fno-implement-inlines}.
If any calls were not inlined, you will get linker errors.

@node Template Instantiation
@section Where's the Template?

@cindex template instantiation

C++ templates are the first language feature to require more
intelligence from the environment than one usually finds on a UNIX
system.  Somehow the compiler and linker have to make sure that each
template instance occurs exactly once in the executable if it is needed,
and not at all otherwise.  There are two basic approaches to this
problem, which I will refer to as the Borland model and the Cfront model.

@table @asis
@item Borland model
Borland C++ solved the template instantiation problem by adding the code
equivalent of common blocks to their linker; the compiler emits template
instances in each translation unit that uses them, and the linker
collapses them together.  The advantage of this model is that the linker
only has to consider the object files themselves; there is no external
complexity to worry about.  This disadvantage is that compilation time
is increased because the template code is being compiled repeatedly.
Code written for this model tends to include definitions of all
templates in the header file, since they must be seen to be
instantiated.

@item Cfront model
The AT&T C++ translator, Cfront, solved the template instantiation
problem by creating the notion of a template repository, an
automatically maintained place where template instances are stored.  A
more modern version of the repository works as follows: As individual
object files are built, the compiler places any template definitions and
instantiations encountered in the repository.  At link time, the link
wrapper adds in the objects in the repository and compiles any needed
instances that were not previously emitted.  The advantages of this
model are more optimal compilation speed and the ability to use the
system linker; to implement the Borland model a compiler vendor also
needs to replace the linker.  The disadvantages are vastly increased
complexity, and thus potential for error; for some code this can be
just as transparent, but in practice it can been very difficult to build
multiple programs in one directory and one program in multiple
directories.  Code written for this model tends to separate definitions
of non-inline member templates into a separate file, which should be
compiled separately.
@end table

When used with GNU ld version 2.8 or later on an ELF system such as
Linux/GNU or Solaris 2, or on Microsoft Windows, g++ supports the
Borland model.  On other systems, g++ implements neither automatic
model.

A future version of g++ will support a hybrid model whereby the compiler
will emit any instantiations for which the template definition is
included in the compile, and store template definitions and
instantiation context information into the object file for the rest.
The link wrapper will extract that information as necessary and invoke
the compiler to produce the remaining instantiations.  The linker will
then combine duplicate instantiations.

In the mean time, you have the following options for dealing with
template instantiations:

@enumerate
@item
Compile your template-using code with @samp{-frepo}.  The compiler will
generate files with the extension @samp{.rpo} listing all of the
template instantiations used in the corresponding object files which
could be instantiated there; the link wrapper, @samp{collect2}, will
then update the @samp{.rpo} files to tell the compiler where to place
those instantiations and rebuild any affected object files.  The
link-time overhead is negligible after the first pass, as the compiler
will continue to place the instantiations in the same files.

This is your best option for application code written for the Borland
model, as it will just work.  Code written for the Cfront model will
need to be modified so that the template definitions are available at
one or more points of instantiation; usually this is as simple as adding
@code{#include <tmethods.cc>} to the end of each template header.

For library code, if you want the library to provide all of the template
instantiations it needs, just try to link all of its object files
together; the link will fail, but cause the instantiations to be
generated as a side effect.  Be warned, however, that this may cause
conflicts if multiple libraries try to provide the same instantiations.
For greater control, use explicit instantiation as described in the next
option.

@item
Compile your code with @samp{-fno-implicit-templates} to disable the
implicit generation of template instances, and explicitly instantiate
all the ones you use.  This approach requires more knowledge of exactly
which instances you need than do the others, but it's less
mysterious and allows greater control.  You can scatter the explicit
instantiations throughout your program, perhaps putting them in the
translation units where the instances are used or the translation units
that define the templates themselves; you can put all of the explicit
instantiations you need into one big file; or you can create small files
like

@example
#include "Foo.h"
#include "Foo.cc"

template class Foo<int>;
template ostream& operator <<
                (ostream&, const Foo<int>&);
@end example

for each of the instances you need, and create a template instantiation
library from those.

If you are using Cfront-model code, you can probably get away with not
using @samp{-fno-implicit-templates} when compiling files that don't
@samp{#include} the member template definitions.

If you use one big file to do the instantiations, you may want to
compile it without @samp{-fno-implicit-templates} so you get all of the
instances required by your explicit instantiations (but not by any
other files) without having to specify them as well.

g++ has extended the template instantiation syntax outlined in the
Working Paper to allow forward declaration of explicit instantiations
(with @code{extern}), instantiation of the compiler support data for a
template class (i.e. the vtable) without instantiating any of its
members (with @code{inline}), and instantiation of only the static data
members of a template class, without the support data or member
functions (with (@code{static}):

@example
extern template int max (int, int);
inline template class Foo<int>;
static template class Foo<int>;
@end example

@item
Do nothing.  Pretend g++ does implement automatic instantiation
management.  Code written for the Borland model will work fine, but
each translation unit will contain instances of each of the templates it
uses.  In a large program, this can lead to an unacceptable amount of code
duplication.

@item
Add @samp{#pragma interface} to all files containing template
definitions.  For each of these files, add @samp{#pragma implementation
"@var{filename}"} to the top of some @samp{.C} file which
@samp{#include}s it.  Then compile everything with
@samp{-fexternal-templates}.  The templates will then only be expanded
in the translation unit which implements them (i.e. has a @samp{#pragma
implementation} line for the file where they live); all other files will
use external references.  If you're lucky, everything should work
properly.  If you get undefined symbol errors, you need to make sure
that each template instance which is used in the program is used in the
file which implements that template.  If you don't have any use for a
particular instance in that file, you can just instantiate it
explicitly, using the syntax from the latest C++ working paper:

@example
template class A<int>;
template ostream& operator << (ostream&, const A<int>&);
@end example

This strategy will work with code written for either model.  If you are
using code written for the Cfront model, the file containing a class
template and the file containing its member templates should be
implemented in the same translation unit.

A slight variation on this approach is to instead use the flag
@samp{-falt-external-templates}; this flag causes template
instances to be emitted in the translation unit that implements the
header where they are first instantiated, rather than the one which
implements the file where the templates are defined.  This header must
be the same in all translation units, or things are likely to break.

@xref{C++ Interface,,Declarations and Definitions in One Header}, for
more discussion of these pragmas.
@end enumerate

@node Bound member functions
@section Extracting the function pointer from a bound pointer to member function

@cindex pmf
@cindex pointer to member function
@cindex bound pointer to member function

In C++, pointer to member functions (PMFs) are implemented using a wide
pointer of sorts to handle all the possible call mechanisms; the PMF
needs to store information about how to adjust the @samp{this} pointer,
and if the function pointed to is virtual, where to find the vtable, and
where in the vtable to look for the member function.  If you are using
PMFs in an inner loop, you should really reconsider that decision.  If
that is not an option, you can extract the pointer to the function that
would be called for a given object/PMF pair and call it directly inside
the inner loop, to save a bit of time.

Note that you will still be paying the penalty for the call through a
function pointer; on most modern architectures, such a call defeats the
branch prediction features of the CPU.  This is also true of normal
virtual function calls.

The syntax for this extension is

@example
extern A a;
extern int (A::*fp)();
typedef int (*fptr)(A *);

fptr p = (fptr)(a.*fp);
@end example

For PMF constants (i.e. expressions of the form @samp{&Klasse::Member}),
no object is needed to obtain the address of the function. They can be
converted to function pointers directly:

@example
fptr p1 = (fptr)(&A::foo);
@end example

You must specify @samp{-Wno-pmf-conversions} to use this extension.

@node C++ Attributes
@section C++-Specific Variable, Function, and Type Attributes

Some attributes only make sense for C++ programs.

@table @code
@item init_priority (@var{priority})
@cindex init_priority attribute


In Standard C++, objects defined at namespace scope are guaranteed to be
initialized in an order in strict accordance with that of their definitions
@emph{in a given translation unit}.  No guarantee is made for initializations
across translation units.  However, GNU C++ allows users to control the
order of initialization of objects defined at namespace scope with the
@code{init_priority} attribute by specifying a relative @var{priority},
a constant integral expression currently bounded between 101 and 65535
inclusive.  Lower numbers indicate a higher priority.

In the following example, @code{A} would normally be created before
@code{B}, but the @code{init_priority} attribute has reversed that order:

@example
Some_Class  A  __attribute__ ((init_priority (2000)));
Some_Class  B  __attribute__ ((init_priority (543)));
@end example

@noindent
Note that the particular values of @var{priority} do not matter; only their
relative ordering.


@item com_interface
@cindex com_interface attribute

@c This is based on:  1) grepping the code, 
@c 2) http://gcc.gnu.org/ml/gcc-bugs/1999-08n/msg01212.html
@c 3) http://gcc.gnu.org/ml/gcc-bugs/1999-08n/msg01215.html
@c and 4) a lot of guesswork.  You can tell I don't use COM.  -pme 21Dec00

This type attribute takes no parameters, and marks a class or struct as an
interface for communication via COM; the class will support the COM ABI
rather than the full C++ ABI.  Currently this means that RTTI is not possible
with the resulting class heirarchy.  The virtual pointer table will be
changed to be COM-compliant.  Also, all classes and structs derived from one
marked with this attribute are implicitly marked with the same attribute;
thus, only the base class in a COM hierarchy needs @code{com_interface}.

@item java_interface
@cindex java_interface attribute

This type attribute informs C++ that the class is a Java interface.  It may 
only be applied to classes declared within an @code{extern "Java"} block.
Calls to methods declared in this interface will be dispatched using GCJ's 
interface table mechanism, instead of regular virtual table dispatch.  

@end table

@node Deprecated Features
@section Deprecated Features

In the past, the GNU C++ compiler was extended to experiment with new
features, at a time when the C++ language was still evolving. Now that
the C++ standard is complete, some of those features are superseded by
superior alternatives. Using the old features might cause a warning in
some cases that the feature will be dropped in the future. In other
cases, the feature might be gone already.

While the list below is not exhaustive, it documents some of the options
that are now deprecated:

@table @code
@item -fexternal-templates
@itemx -falt-external-templates
These are two of the many ways for g++ to implement template
instantiation. @xref{Template Instantiation}. The C++ standard clearly
defines how template definitions have to be organized across
implementation units. g++ has an implicit instantiation mechanism that
should work just fine for standard-conforming code.

@item -fstrict-prototype
@itemx -fno-strict-prototype
Previously it was possible to use an empty prototype parameter list to
indicate an unspecified number of parameters (like C), rather than no
parameters, as C++ demands. This feature has been removed, except where
it is required for backwards compatibility @xref{Backwards Compatibility}.
@end table

The named return value extension has been deprecated, and will be
removed from g++ at some point.

The use of initializer lists with new expressions has been deprecated,
and will be removed from g++ at some point.

@node Backwards Compatibility
@section Backwards Compatibility
@cindex Backwards Compatibility
@cindex ARM [Annotated C++ Reference Manual]

Now that there is a definitive ISO standard C++, g++ has a specification
to adhere to. The C++ language evolved over time, and features that
used to be acceptable in previous drafts of the standard, such as the ARM
[Annotated C++ Reference Manual], are no longer accepted. In order to allow
compilation of C++ written to such drafts, g++ contains some backwards
compatibilities. @emph{All such backwards compatibility features are
liable to disappear in future versions of g++.} They should be considered
deprecated @xref{Deprecated Features}.

@table @code
@item For scope
If a variable is declared at for scope, it used to remain in scope until
the end of the scope which contained the for statement (rather than just
within the for scope). g++ retains this, but issues a warning, if such a
variable is accessed outside the for scope.

@item implicit C language
Old C system header files did not contain an @code{extern "C" @{...@}}
scope to set the language. On such systems, all header files are
implicitly scoped inside a C language scope. Also, an empty prototype
@code{()} will be treated as an unspecified number of arguments, rather
than no arguments, as C++ demands.
@end table