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This is Info file, produced by Makeinfo version 1.68 from the
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This file documents the use and the internals of the GNU compiler.
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original English.

File:, Node: External Bugs, Next: Incompatibilities, Prev: Interoperation, Up: Trouble
Problems Compiling Certain Programs
Certain programs have problems compiling.
* Parse errors may occur compiling X11 on a Decstation running
Ultrix 4.2 because of problems in DEC's versions of the X11 header
files `X11/Xlib.h' and `X11/Xutil.h'. People recommend adding
`-I/usr/include/mit' to use the MIT versions of the header files,
using the `-traditional' switch to turn off ANSI C, or fixing the
header files by adding this:
#ifdef __STDC__
#define NeedFunctionPrototypes 0
* If you have trouble compiling Perl on a SunOS 4 system, it may be
because Perl specifies `-I/usr/ucbinclude'. This accesses the
unfixed header files. Perl specifies the options
-traditional -Dvolatile=__volatile__
-I/usr/include/sun -I/usr/ucbinclude
most of which are unnecessary with GCC 2.4.5 and newer versions.
You can make a properly working Perl by setting `ccflags' to
`-fwritable-strings' (implied by the `-traditional' in the
original options) and `cppflags' to empty in `', then
typing `./doSH; make depend; make'.
* On various 386 Unix systems derived from System V, including SCO,
ISC, and ESIX, you may get error messages about running out of
virtual memory while compiling certain programs.
You can prevent this problem by linking GNU CC with the GNU malloc
(which thus replaces the malloc that comes with the system). GNU
malloc is available as a separate package, and also in the file
`src/gmalloc.c' in the GNU Emacs 19 distribution.
If you have installed GNU malloc as a separate library package,
use this option when you relink GNU CC:
Alternatively, if you have compiled `gmalloc.c' from Emacs 19, copy
the object file to `gmalloc.o' and use this option when you relink

File:, Node: Incompatibilities, Next: Fixed Headers, Prev: External Bugs, Up: Trouble
Incompatibilities of GNU CC
There are several noteworthy incompatibilities between GNU C and most
existing (non-ANSI) versions of C. The `-traditional' option
eliminates many of these incompatibilities, *but not all*, by telling
GNU C to behave like the other C compilers.
* GNU CC normally makes string constants read-only. If several
identical-looking string constants are used, GNU CC stores only one
copy of the string.
One consequence is that you cannot call `mktemp' with a string
constant argument. The function `mktemp' always alters the string
its argument points to.
Another consequence is that `sscanf' does not work on some systems
when passed a string constant as its format control string or
input. This is because `sscanf' incorrectly tries to write into
the string constant. Likewise `fscanf' and `scanf'.
The best solution to these problems is to change the program to use
`char'-array variables with initialization strings for these
purposes instead of string constants. But if this is not possible,
you can use the `-fwritable-strings' flag, which directs GNU CC to
handle string constants the same way most C compilers do.
`-traditional' also has this effect, among others.
* `-2147483648' is positive.
This is because 2147483648 cannot fit in the type `int', so
(following the ANSI C rules) its data type is `unsigned long int'.
Negating this value yields 2147483648 again.
* GNU CC does not substitute macro arguments when they appear inside
of string constants. For example, the following macro in GNU CC
#define foo(a) "a"
will produce output `"a"' regardless of what the argument A is.
The `-traditional' option directs GNU CC to handle such cases
(among others) in the old-fashioned (non-ANSI) fashion.
* When you use `setjmp' and `longjmp', the only automatic variables
guaranteed to remain valid are those declared `volatile'. This is
a consequence of automatic register allocation. Consider this
jmp_buf j;
foo ()
int a, b;
a = fun1 ();
if (setjmp (j))
return a;
a = fun2 ();
/* `longjmp (j)' may occur in `fun3'. */
return a + fun3 ();
Here `a' may or may not be restored to its first value when the
`longjmp' occurs. If `a' is allocated in a register, then its
first value is restored; otherwise, it keeps the last value stored
in it.
If you use the `-W' option with the `-O' option, you will get a
warning when GNU CC thinks such a problem might be possible.
The `-traditional' option directs GNU C to put variables in the
stack by default, rather than in registers, in functions that call
`setjmp'. This results in the behavior found in traditional C
* Programs that use preprocessing directives in the middle of macro
arguments do not work with GNU CC. For example, a program like
this will not work:
foobar (
#define luser
ANSI C does not permit such a construct. It would make sense to
support it when `-traditional' is used, but it is too much work to
* Declarations of external variables and functions within a block
apply only to the block containing the declaration. In other
words, they have the same scope as any other declaration in the
same place.
In some other C compilers, a `extern' declaration affects all the
rest of the file even if it happens within a block.
The `-traditional' option directs GNU C to treat all `extern'
declarations as global, like traditional compilers.
* In traditional C, you can combine `long', etc., with a typedef
name, as shown here:
typedef int foo;
typedef long foo bar;
In ANSI C, this is not allowed: `long' and other type modifiers
require an explicit `int'. Because this criterion is expressed by
Bison grammar rules rather than C code, the `-traditional' flag
cannot alter it.
* PCC allows typedef names to be used as function parameters. The
difficulty described immediately above applies here too.
* PCC allows whitespace in the middle of compound assignment
operators such as `+='. GNU CC, following the ANSI standard, does
not allow this. The difficulty described immediately above
applies here too.
* GNU CC complains about unterminated character constants inside of
preprocessing conditionals that fail. Some programs have English
comments enclosed in conditionals that are guaranteed to fail; if
these comments contain apostrophes, GNU CC will probably report an
error. For example, this code would produce an error:
#if 0
You can't expect this to work.
The best solution to such a problem is to put the text into an
actual C comment delimited by `/*...*/'. However, `-traditional'
suppresses these error messages.
* Many user programs contain the declaration `long time ();'. In the
past, the system header files on many systems did not actually
declare `time', so it did not matter what type your program
declared it to return. But in systems with ANSI C headers, `time'
is declared to return `time_t', and if that is not the same as
`long', then `long time ();' is erroneous.
The solution is to change your program to use `time_t' as the
return type of `time'.
* When compiling functions that return `float', PCC converts it to a
double. GNU CC actually returns a `float'. If you are concerned
with PCC compatibility, you should declare your functions to return
`double'; you might as well say what you mean.
* When compiling functions that return structures or unions, GNU CC
output code normally uses a method different from that used on most
versions of Unix. As a result, code compiled with GNU CC cannot
call a structure-returning function compiled with PCC, and vice
The method used by GNU CC is as follows: a structure or union
which is 1, 2, 4 or 8 bytes long is returned like a scalar. A
structure or union with any other size is stored into an address
supplied by the caller (usually in a special, fixed register, but
on some machines it is passed on the stack). The
machine-description macros `STRUCT_VALUE' and
`STRUCT_INCOMING_VALUE' tell GNU CC where to pass this address.
By contrast, PCC on most target machines returns structures and
unions of any size by copying the data into an area of static
storage, and then returning the address of that storage as if it
were a pointer value. The caller must copy the data from that
memory area to the place where the value is wanted. GNU CC does
not use this method because it is slower and nonreentrant.
On some newer machines, PCC uses a reentrant convention for all
structure and union returning. GNU CC on most of these machines
uses a compatible convention when returning structures and unions
in memory, but still returns small structures and unions in
You can tell GNU CC to use a compatible convention for all
structure and union returning with the option
* GNU C complains about program fragments such as `0x74ae-0x4000'
which appear to be two hexadecimal constants separated by the minus
operator. Actually, this string is a single "preprocessing token".
Each such token must correspond to one token in C. Since this
does not, GNU C prints an error message. Although it may appear
obvious that what is meant is an operator and two values, the ANSI
C standard specifically requires that this be treated as erroneous.
A "preprocessing token" is a "preprocessing number" if it begins
with a digit and is followed by letters, underscores, digits,
periods and `e+', `e-', `E+', or `E-' character sequences.
To make the above program fragment valid, place whitespace in
front of the minus sign. This whitespace will end the
preprocessing number.

File:, Node: Fixed Headers, Next: Standard Libraries, Prev: Incompatibilities, Up: Trouble
Fixed Header Files
GNU CC needs to install corrected versions of some system header
files. This is because most target systems have some header files that
won't work with GNU CC unless they are changed. Some have bugs, some
are incompatible with ANSI C, and some depend on special features of
other compilers.
Installing GNU CC automatically creates and installs the fixed header
files, by running a program called `fixincludes' (or for certain
targets an alternative such as `fixinc.svr4'). Normally, you don't
need to pay attention to this. But there are cases where it doesn't do
the right thing automatically.
* If you update the system's header files, such as by installing a
new system version, the fixed header files of GNU CC are not
automatically updated. The easiest way to update them is to
reinstall GNU CC. (If you want to be clever, look in the makefile
and you can find a shortcut.)
* On some systems, in particular SunOS 4, header file directories
contain machine-specific symbolic links in certain places. This
makes it possible to share most of the header files among hosts
running the same version of SunOS 4 on different machine models.
The programs that fix the header files do not understand this
special way of using symbolic links; therefore, the directory of
fixed header files is good only for the machine model used to
build it.
In SunOS 4, only programs that look inside the kernel will notice
the difference between machine models. Therefore, for most
purposes, you need not be concerned about this.
It is possible to make separate sets of fixed header files for the
different machine models, and arrange a structure of symbolic
links so as to use the proper set, but you'll have to do this by
* On Lynxos, GNU CC by default does not fix the header files. This
is because bugs in the shell cause the `fixincludes' script to
This means you will encounter problems due to bugs in the system
header files. It may be no comfort that they aren't GNU CC's
fault, but it does mean that there's nothing for us to do about

File:, Node: Standard Libraries, Next: Disappointments, Prev: Fixed Headers, Up: Trouble
Standard Libraries
GNU CC by itself attempts to be what the ISO/ANSI C standard calls a
"conforming freestanding implementation". This means all ANSI C
language features are available, as well as the contents of `float.h',
`limits.h', `stdarg.h', and `stddef.h'. The rest of the C library is
supplied by the vendor of the operating system. If that C library
doesn't conform to the C standards, then your programs might get
warnings (especially when using `-Wall') that you don't expect.
For example, the `sprintf' function on SunOS 4.1.3 returns `char *'
while the C standard says that `sprintf' returns an `int'. The
`fixincludes' program could make the prototype for this function match
the Standard, but that would be wrong, since the function will still
return `char *'.
If you need a Standard compliant library, then you need to find one,
as GNU CC does not provide one. The GNU C library (called `glibc') has
been ported to a number of operating systems, and provides ANSI/ISO,
POSIX, BSD and SystemV compatibility. You could also ask your operating
system vendor if newer libraries are available.

File:, Node: Disappointments, Next: C++ Misunderstandings, Prev: Standard Libraries, Up: Trouble
Disappointments and Misunderstandings
These problems are perhaps regrettable, but we don't know any
practical way around them.
* Certain local variables aren't recognized by debuggers when you
compile with optimization.
This occurs because sometimes GNU CC optimizes the variable out of
existence. There is no way to tell the debugger how to compute the
value such a variable "would have had", and it is not clear that
would be desirable anyway. So GNU CC simply does not mention the
eliminated variable when it writes debugging information.
You have to expect a certain amount of disagreement between the
executable and your source code, when you use optimization.
* Users often think it is a bug when GNU CC reports an error for code
like this:
int foo (struct mumble *);
struct mumble { ... };
int foo (struct mumble *x)
{ ... }
This code really is erroneous, because the scope of `struct
mumble' in the prototype is limited to the argument list
containing it. It does not refer to the `struct mumble' defined
with file scope immediately below--they are two unrelated types
with similar names in different scopes.
But in the definition of `foo', the file-scope type is used
because that is available to be inherited. Thus, the definition
and the prototype do not match, and you get an error.
This behavior may seem silly, but it's what the ANSI standard
specifies. It is easy enough for you to make your code work by
moving the definition of `struct mumble' above the prototype.
It's not worth being incompatible with ANSI C just to avoid an
error for the example shown above.
* Accesses to bitfields even in volatile objects works by accessing
larger objects, such as a byte or a word. You cannot rely on what
size of object is accessed in order to read or write the bitfield;
it may even vary for a given bitfield according to the precise
If you care about controlling the amount of memory that is
accessed, use volatile but do not use bitfields.
* GNU CC comes with shell scripts to fix certain known problems in
system header files. They install corrected copies of various
header files in a special directory where only GNU CC will
normally look for them. The scripts adapt to various systems by
searching all the system header files for the problem cases that
we know about.
If new system header files are installed, nothing automatically
arranges to update the corrected header files. You will have to
reinstall GNU CC to fix the new header files. More specifically,
go to the build directory and delete the files `stmp-fixinc' and
`stmp-headers', and the subdirectory `include'; then do `make
install' again.
* On 68000 and x86 systems, for instance, you can get paradoxical
results if you test the precise values of floating point numbers.
For example, you can find that a floating point value which is not
a NaN is not equal to itself. This results from the fact that the
floating point registers hold a few more bits of precision than
fit in a `double' in memory. Compiled code moves values between
memory and floating point registers at its convenience, and moving
them into memory truncates them.
You can partially avoid this problem by using the `-ffloat-store'
option (*note Optimize Options::.).
* On the MIPS, variable argument functions using `varargs.h' cannot
have a floating point value for the first argument. The reason
for this is that in the absence of a prototype in scope, if the
first argument is a floating point, it is passed in a floating
point register, rather than an integer register.
If the code is rewritten to use the ANSI standard `stdarg.h'
method of variable arguments, and the prototype is in scope at the
time of the call, everything will work fine.
* On the H8/300 and H8/300H, variable argument functions must be
implemented using the ANSI standard `stdarg.h' method of variable
arguments. Furthermore, calls to functions using `stdarg.h'
variable arguments must have a prototype for the called function
in scope at the time of the call.

File:, Node: C++ Misunderstandings, Next: Protoize Caveats, Prev: Disappointments, Up: Trouble
Common Misunderstandings with GNU C++
C++ is a complex language and an evolving one, and its standard
definition (the ANSI C++ draft standard) is also evolving. As a result,
your C++ compiler may occasionally surprise you, even when its behavior
is correct. This section discusses some areas that frequently give
rise to questions of this sort.
* Menu:
* Static Definitions:: Static member declarations are not definitions
* Temporaries:: Temporaries may vanish before you expect

File:, Node: Static Definitions, Next: Temporaries, Up: C++ Misunderstandings
Declare *and* Define Static Members
When a class has static data members, it is not enough to *declare*
the static member; you must also *define* it. For example:
class Foo
void method();
static int bar;
This declaration only establishes that the class `Foo' has an `int'
named `Foo::bar', and a member function named `Foo::method'. But you
still need to define *both* `method' and `bar' elsewhere. According to
the draft ANSI standard, you must supply an initializer in one (and
only one) source file, such as:
int Foo::bar = 0;
Other C++ compilers may not correctly implement the standard
behavior. As a result, when you switch to `g++' from one of these
compilers, you may discover that a program that appeared to work
correctly in fact does not conform to the standard: `g++' reports as
undefined symbols any static data members that lack definitions.

File:, Node: Temporaries, Prev: Static Definitions, Up: C++ Misunderstandings
Temporaries May Vanish Before You Expect
It is dangerous to use pointers or references to *portions* of a
temporary object. The compiler may very well delete the object before
you expect it to, leaving a pointer to garbage. The most common place
where this problem crops up is in classes like the libg++ `String'
class, that define a conversion function to type `char *' or `const
char *'. However, any class that returns a pointer to some internal
structure is potentially subject to this problem.
For example, a program may use a function `strfunc' that returns
`String' objects, and another function `charfunc' that operates on
pointers to `char':
String strfunc ();
void charfunc (const char *);
In this situation, it may seem natural to write
`charfunc (strfunc ());' based on the knowledge that class `String' has
an explicit conversion to `char' pointers. However, what really
happens is akin to `charfunc (strfunc ().convert ());', where the
`convert' method is a function to do the same data conversion normally
performed by a cast. Since the last use of the temporary `String'
object is the call to the conversion function, the compiler may delete
that object before actually calling `charfunc'. The compiler has no
way of knowing that deleting the `String' object will invalidate the
pointer. The pointer then points to garbage, so that by the time
`charfunc' is called, it gets an invalid argument.
Code like this may run successfully under some other compilers,
especially those that delete temporaries relatively late. However, the
GNU C++ behavior is also standard-conforming, so if your program depends
on late destruction of temporaries it is not portable.
If you think this is surprising, you should be aware that the ANSI
C++ committee continues to debate the lifetime-of-temporaries problem.
For now, at least, the safe way to write such code is to give the
temporary a name, which forces it to remain until the end of the scope
of the name. For example:
String& tmp = strfunc ();
charfunc (tmp);

File:, Node: Protoize Caveats, Next: Non-bugs, Prev: C++ Misunderstandings, Up: Trouble
Caveats of using `protoize'
The conversion programs `protoize' and `unprotoize' can sometimes
change a source file in a way that won't work unless you rearrange it.
* `protoize' can insert references to a type name or type tag before
the definition, or in a file where they are not defined.
If this happens, compiler error messages should show you where the
new references are, so fixing the file by hand is straightforward.
* There are some C constructs which `protoize' cannot figure out.
For example, it can't determine argument types for declaring a
pointer-to-function variable; this you must do by hand. `protoize'
inserts a comment containing `???' each time it finds such a
variable; so you can find all such variables by searching for this
string. ANSI C does not require declaring the argument types of
pointer-to-function types.
* Using `unprotoize' can easily introduce bugs. If the program
relied on prototypes to bring about conversion of arguments, these
conversions will not take place in the program without prototypes.
One case in which you can be sure `unprotoize' is safe is when you
are removing prototypes that were made with `protoize'; if the
program worked before without any prototypes, it will work again
without them.
You can find all the places where this problem might occur by
compiling the program with the `-Wconversion' option. It prints a
warning whenever an argument is converted.
* Both conversion programs can be confused if there are macro calls
in and around the text to be converted. In other words, the
standard syntax for a declaration or definition must not result
from expanding a macro. This problem is inherent in the design of
C and cannot be fixed. If only a few functions have confusing
macro calls, you can easily convert them manually.
* `protoize' cannot get the argument types for a function whose
definition was not actually compiled due to preprocessing
conditionals. When this happens, `protoize' changes nothing in
regard to such a function. `protoize' tries to detect such
instances and warn about them.
You can generally work around this problem by using `protoize' step
by step, each time specifying a different set of `-D' options for
compilation, until all of the functions have been converted.
There is no automatic way to verify that you have got them all,
* Confusion may result if there is an occasion to convert a function
declaration or definition in a region of source code where there
is more than one formal parameter list present. Thus, attempts to
convert code containing multiple (conditionally compiled) versions
of a single function header (in the same vicinity) may not produce
the desired (or expected) results.
If you plan on converting source files which contain such code, it
is recommended that you first make sure that each conditionally
compiled region of source code which contains an alternative
function header also contains at least one additional follower
token (past the final right parenthesis of the function header).
This should circumvent the problem.
* `unprotoize' can become confused when trying to convert a function
definition or declaration which contains a declaration for a
pointer-to-function formal argument which has the same name as the
function being defined or declared. We recommand you avoid such
choices of formal parameter names.
* You might also want to correct some of the indentation by hand and
break long lines. (The conversion programs don't write lines
longer than eighty characters in any case.)

File:, Node: Non-bugs, Next: Warnings and Errors, Prev: Protoize Caveats, Up: Trouble
Certain Changes We Don't Want to Make
This section lists changes that people frequently request, but which
we do not make because we think GNU CC is better without them.
* Checking the number and type of arguments to a function which has
an old-fashioned definition and no prototype.
Such a feature would work only occasionally--only for calls that
appear in the same file as the called function, following the
definition. The only way to check all calls reliably is to add a
prototype for the function. But adding a prototype eliminates the
motivation for this feature. So the feature is not worthwhile.
* Warning about using an expression whose type is signed as a shift
Shift count operands are probably signed more often than unsigned.
Warning about this would cause far more annoyance than good.
* Warning about assigning a signed value to an unsigned variable.
Such assignments must be very common; warning about them would
cause more annoyance than good.
* Warning about unreachable code.
It's very common to have unreachable code in machine-generated
programs. For example, this happens normally in some files of GNU
C itself.
* Warning when a non-void function value is ignored.
Coming as I do from a Lisp background, I balk at the idea that
there is something dangerous about discarding a value. There are
functions that return values which some callers may find useful;
it makes no sense to clutter the program with a cast to `void'
whenever the value isn't useful.
* Assuming (for optimization) that the address of an external symbol
is never zero.
This assumption is false on certain systems when `#pragma weak' is
* Making `-fshort-enums' the default.
This would cause storage layout to be incompatible with most other
C compilers. And it doesn't seem very important, given that you
can get the same result in other ways. The case where it matters
most is when the enumeration-valued object is inside a structure,
and in that case you can specify a field width explicitly.
* Making bitfields unsigned by default on particular machines where
"the ABI standard" says to do so.
The ANSI C standard leaves it up to the implementation whether a
bitfield declared plain `int' is signed or not. This in effect
creates two alternative dialects of C.
The GNU C compiler supports both dialects; you can specify the
signed dialect with `-fsigned-bitfields' and the unsigned dialect
with `-funsigned-bitfields'. However, this leaves open the
question of which dialect to use by default.
Currently, the preferred dialect makes plain bitfields signed,
because this is simplest. Since `int' is the same as `signed int'
in every other context, it is cleanest for them to be the same in
bitfields as well.
Some computer manufacturers have published Application Binary
Interface standards which specify that plain bitfields should be
unsigned. It is a mistake, however, to say anything about this
issue in an ABI. This is because the handling of plain bitfields
distinguishes two dialects of C. Both dialects are meaningful on
every type of machine. Whether a particular object file was
compiled using signed bitfields or unsigned is of no concern to
other object files, even if they access the same bitfields in the
same data structures.
A given program is written in one or the other of these two
dialects. The program stands a chance to work on most any machine
if it is compiled with the proper dialect. It is unlikely to work
at all if compiled with the wrong dialect.
Many users appreciate the GNU C compiler because it provides an
environment that is uniform across machines. These users would be
inconvenienced if the compiler treated plain bitfields differently
on certain machines.
Occasionally users write programs intended only for a particular
machine type. On these occasions, the users would benefit if the
GNU C compiler were to support by default the same dialect as the
other compilers on that machine. But such applications are rare.
And users writing a program to run on more than one type of
machine cannot possibly benefit from this kind of compatibility.
This is why GNU CC does and will treat plain bitfields in the same
fashion on all types of machines (by default).
There are some arguments for making bitfields unsigned by default
on all machines. If, for example, this becomes a universal de
facto standard, it would make sense for GNU CC to go along with
it. This is something to be considered in the future.
(Of course, users strongly concerned about portability should
indicate explicitly in each bitfield whether it is signed or not.
In this way, they write programs which have the same meaning in
both C dialects.)
* Undefining `__STDC__' when `-ansi' is not used.
Currently, GNU CC defines `__STDC__' as long as you don't use
`-traditional'. This provides good results in practice.
Programmers normally use conditionals on `__STDC__' to ask whether
it is safe to use certain features of ANSI C, such as function
prototypes or ANSI token concatenation. Since plain `gcc' supports
all the features of ANSI C, the correct answer to these questions
is "yes".
Some users try to use `__STDC__' to check for the availability of
certain library facilities. This is actually incorrect usage in
an ANSI C program, because the ANSI C standard says that a
conforming freestanding implementation should define `__STDC__'
even though it does not have the library facilities. `gcc -ansi
-pedantic' is a conforming freestanding implementation, and it is
therefore required to define `__STDC__', even though it does not
come with an ANSI C library.
Sometimes people say that defining `__STDC__' in a compiler that
does not completely conform to the ANSI C standard somehow
violates the standard. This is illogical. The standard is a
standard for compilers that claim to support ANSI C, such as `gcc
-ansi'--not for other compilers such as plain `gcc'. Whatever the
ANSI C standard says is relevant to the design of plain `gcc'
without `-ansi' only for pragmatic reasons, not as a requirement.
GNU CC normally defines `__STDC__' to be 1, and in addition
defines `__STRICT_ANSI__' if you specify the `-ansi' option. On
some hosts, system include files use a different convention, where
`__STDC__' is normally 0, but is 1 if the user specifies strict
conformance to the C Standard. GNU CC follows the host convention
when processing system include files, but when processing user
files it follows the usual GNU C convention.
* Undefining `__STDC__' in C++.
Programs written to compile with C++-to-C translators get the
value of `__STDC__' that goes with the C compiler that is
subsequently used. These programs must test `__STDC__' to
determine what kind of C preprocessor that compiler uses: whether
they should concatenate tokens in the ANSI C fashion or in the
traditional fashion.
These programs work properly with GNU C++ if `__STDC__' is defined.
They would not work otherwise.
In addition, many header files are written to provide prototypes
in ANSI C but not in traditional C. Many of these header files
can work without change in C++ provided `__STDC__' is defined. If
`__STDC__' is not defined, they will all fail, and will all need
to be changed to test explicitly for C++ as well.
* Deleting "empty" loops.
GNU CC does not delete "empty" loops because the most likely reason
you would put one in a program is to have a delay. Deleting them
will not make real programs run any faster, so it would be
It would be different if optimization of a nonempty loop could
produce an empty one. But this generally can't happen.
* Making side effects happen in the same order as in some other
It is never safe to depend on the order of evaluation of side
effects. For example, a function call like this may very well
behave differently from one compiler to another:
void func (int, int);
int i = 2;
func (i++, i++);
There is no guarantee (in either the C or the C++ standard language
definitions) that the increments will be evaluated in any
particular order. Either increment might happen first. `func'
might get the arguments `2, 3', or it might get `3, 2', or even
`2, 2'.
* Not allowing structures with volatile fields in registers.
Strictly speaking, there is no prohibition in the ANSI C standard
against allowing structures with volatile fields in registers, but
it does not seem to make any sense and is probably not what you
wanted to do. So the compiler will give an error message in this

File:, Node: Warnings and Errors, Prev: Non-bugs, Up: Trouble
Warning Messages and Error Messages
The GNU compiler can produce two kinds of diagnostics: errors and
warnings. Each kind has a different purpose:
*Errors* report problems that make it impossible to compile your
program. GNU CC reports errors with the source file name and line
number where the problem is apparent.
*Warnings* report other unusual conditions in your code that *may*
indicate a problem, although compilation can (and does) proceed.
Warning messages also report the source file name and line number,
but include the text `warning:' to distinguish them from error
Warnings may indicate danger points where you should check to make
sure that your program really does what you intend; or the use of
obsolete features; or the use of nonstandard features of GNU C or C++.
Many warnings are issued only if you ask for them, with one of the `-W'
options (for instance, `-Wall' requests a variety of useful warnings).
GNU CC always tries to compile your program if possible; it never
gratuitously rejects a program whose meaning is clear merely because
(for instance) it fails to conform to a standard. In some cases,
however, the C and C++ standards specify that certain extensions are
forbidden, and a diagnostic *must* be issued by a conforming compiler.
The `-pedantic' option tells GNU CC to issue warnings in such cases;
`-pedantic-errors' says to make them errors instead. This does not
mean that *all* non-ANSI constructs get warnings or errors.
*Note Options to Request or Suppress Warnings: Warning Options, for
more detail on these and related command-line options.

File:, Node: Bugs, Next: Service, Prev: Trouble, Up: Top
Reporting Bugs
Your bug reports play an essential role in making GNU CC reliable.
When you encounter a problem, the first thing to do is to see if it
is already known. *Note Trouble::. If it isn't known, then you should
report the problem.
Reporting a bug may help you by bringing a solution to your problem,
or it may not. (If it does not, look in the service directory; see
*Note Service::.) In any case, the principal function of a bug report
is to help the entire community by making the next version of GNU CC
work better. Bug reports are your contribution to the maintenance of
Since the maintainers are very overloaded, we cannot respond to every
bug report. However, if the bug has not been fixed, we are likely to
send you a patch and ask you to tell us whether it works.
In order for a bug report to serve its purpose, you must include the
information that makes for fixing the bug.
* Menu:
* Criteria: Bug Criteria. Have you really found a bug?
* Where: Bug Lists. Where to send your bug report.
* Reporting: Bug Reporting. How to report a bug effectively.
* Patches: Sending Patches. How to send a patch for GNU CC.
* Known: Trouble. Known problems.
* Help: Service. Where to ask for help.

File:, Node: Bug Criteria, Next: Bug Lists, Up: Bugs
Have You Found a Bug?
If you are not sure whether you have found a bug, here are some
* If the compiler gets a fatal signal, for any input whatever, that
is a compiler bug. Reliable compilers never crash.
* If the compiler produces invalid assembly code, for any input
whatever (except an `asm' statement), that is a compiler bug,
unless the compiler reports errors (not just warnings) which would
ordinarily prevent the assembler from being run.
* If the compiler produces valid assembly code that does not
correctly execute the input source code, that is a compiler bug.
However, you must double-check to make sure, because you may have
run into an incompatibility between GNU C and traditional C (*note
Incompatibilities::.). These incompatibilities might be considered
bugs, but they are inescapable consequences of valuable features.
Or you may have a program whose behavior is undefined, which
happened by chance to give the desired results with another C or
C++ compiler.
For example, in many nonoptimizing compilers, you can write `x;'
at the end of a function instead of `return x;', with the same
results. But the value of the function is undefined if `return'
is omitted; it is not a bug when GNU CC produces different results.
Problems often result from expressions with two increment
operators, as in `f (*p++, *p++)'. Your previous compiler might
have interpreted that expression the way you intended; GNU CC might
interpret it another way. Neither compiler is wrong. The bug is
in your code.
After you have localized the error to a single source line, it
should be easy to check for these things. If your program is
correct and well defined, you have found a compiler bug.
* If the compiler produces an error message for valid input, that is
a compiler bug.
* If the compiler does not produce an error message for invalid
input, that is a compiler bug. However, you should note that your
idea of "invalid input" might be my idea of "an extension" or
"support for traditional practice".
* If you are an experienced user of C or C++ compilers, your
suggestions for improvement of GNU CC or GNU C++ are welcome in
any case.

File:, Node: Bug Lists, Next: Bug Reporting, Prev: Bug Criteria, Up: Bugs
Where to Report Bugs
Send bug reports for GNU C to `'.
Send bug reports for GNU C++ to `'. If your
bug involves the C++ class library libg++, send mail instead to the
address `'. If you're not sure, you can
send the bug report to both lists.
*Do not send bug reports to `' or to the
newsgroup `'.* Most users of GNU CC do not want to receive
bug reports. Those that do, have asked to be on `bug-gcc' and/or
The mailing lists `bug-gcc' and `bug-g++' both have newsgroups which
serve as repeaters: `gnu.gcc.bug' and `gnu.g++.bug'. Each mailing list
and its newsgroup carry exactly the same messages.
Often people think of posting bug reports to the newsgroup instead of
mailing them. This appears to work, but it has one problem which can be
crucial: a newsgroup posting does not contain a mail path back to the
sender. Thus, if maintainers need more information, they may be unable
to reach you. For this reason, you should always send bug reports by
mail to the proper mailing list.
As a last resort, send bug reports on paper to:
GNU Compiler Bugs
Free Software Foundation
59 Temple Place - Suite 330
Boston, MA 02111-1307, USA