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This is Info file, produced by Makeinfo version 1.68 from the
input file gcc.texi.
This file documents the use and the internals of the GNU compiler.
Published by the Free Software Foundation 59 Temple Place - Suite 330
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Free Software Foundation, Inc.
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that the sections entitled "GNU General Public License," "Funding for
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resulting derived work is distributed under the terms of a permission
notice identical to this one.
Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the sections entitled "GNU General Public
License," "Funding for Free Software," and "Protect Your Freedom--Fight
`Look And Feel'", and this permission notice, may be included in
translations approved by the Free Software Foundation instead of in the
original English.

File:, Node: Cross Runtime, Next: Build Cross, Prev: Cross Headers, Up: Cross-Compiler
`libgcc.a' and Cross-Compilers
Code compiled by GNU CC uses certain runtime support functions
implicitly. Some of these functions can be compiled successfully with
GNU CC itself, but a few cannot be. These problem functions are in the
source file `libgcc1.c'; the library made from them is called
When you build a native compiler, these functions are compiled with
some other compiler-the one that you use for bootstrapping GNU CC.
Presumably it knows how to open code these operations, or else knows how
to call the run-time emulation facilities that the machine comes with.
But this approach doesn't work for building a cross-compiler. The
compiler that you use for building knows about the host system, not the
target system.
So, when you build a cross-compiler you have to supply a suitable
library `libgcc1.a' that does the job it is expected to do.
To compile `libgcc1.c' with the cross-compiler itself does not work.
The functions in this file are supposed to implement arithmetic
operations that GNU CC does not know how to open code for your target
machine. If these functions are compiled with GNU CC itself, they will
compile into infinite recursion.
On any given target, most of these functions are not needed. If GNU
CC can open code an arithmetic operation, it will not call these
functions to perform the operation. It is possible that on your target
machine, none of these functions is needed. If so, you can supply an
empty library as `libgcc1.a'.
Many targets need library support only for multiplication and
division. If you are linking with a library that contains functions for
multiplication and division, you can tell GNU CC to call them directly
by defining the macros `MULSI3_LIBCALL', and the like. These macros
need to be defined in the target description macro file. For some
targets, they are defined already. This may be sufficient to avoid the
need for libgcc1.a; if so, you can supply an empty library.
Some targets do not have floating point instructions; they need other
functions in `libgcc1.a', which do floating arithmetic. Recent
versions of GNU CC have a file which emulates floating point. With a
certain amount of work, you should be able to construct a floating
point emulator that can be used as `libgcc1.a'. Perhaps future
versions will contain code to do this automatically and conveniently.
That depends on whether someone wants to implement it.
Some embedded targets come with all the necessary `libgcc1.a'
routines written in C or assembler. These targets build `libgcc1.a'
automatically and you do not need to do anything special for them.
Other embedded targets do not need any `libgcc1.a' routines since all
the necessary operations are supported by the hardware.
If your target system has another C compiler, you can configure GNU
CC as a native compiler on that machine, build just `libgcc1.a' with
`make libgcc1.a' on that machine, and use the resulting file with the
cross-compiler. To do this, execute the following on the target
./configure --host=sparc --target=sun3
make libgcc1.a
And then this on the host machine:
get libgcc1.a
Another way to provide the functions you need in `libgcc1.a' is to
define the appropriate `perform_...' macros for those functions. If
these definitions do not use the C arithmetic operators that they are
meant to implement, you should be able to compile them with the
cross-compiler you are building. (If these definitions already exist
for your target file, then you are all set.)
To build `libgcc1.a' using the perform macros, use
`LIBGCC1=libgcc1.a OLDCC=./xgcc' when building the compiler.
Otherwise, you should place your replacement library under the name
`libgcc1.a' in the directory in which you will build the
cross-compiler, before you run `make'.

File:, Node: Cross Headers, Next: Cross Runtime, Prev: Tools and Libraries, Up: Cross-Compiler
Cross-Compilers and Header Files
If you are cross-compiling a standalone program or a program for an
embedded system, then you may not need any header files except the few
that are part of GNU CC (and those of your program). However, if you
intend to link your program with a standard C library such as `libc.a',
then you probably need to compile with the header files that go with
the library you use.
The GNU C compiler does not come with these files, because (1) they
are system-specific, and (2) they belong in a C library, not in a
If the GNU C library supports your target machine, then you can get
the header files from there (assuming you actually use the GNU library
when you link your program).
If your target machine comes with a C compiler, it probably comes
with suitable header files also. If you make these files accessible
from the host machine, the cross-compiler can use them also.
Otherwise, you're on your own in finding header files to use when
When you have found suitable header files, put them in the directory
`/usr/local/TARGET/include', before building the cross compiler. Then
installation will run fixincludes properly and install the corrected
versions of the header files where the compiler will use them.
Provide the header files before you build the cross-compiler, because
the build stage actually runs the cross-compiler to produce parts of
`libgcc.a'. (These are the parts that *can* be compiled with GNU CC.)
Some of them need suitable header files.
Here's an example showing how to copy the header files from a target
machine. On the target machine, do this:
(cd /usr/include; tar cf - .) > tarfile
Then, on the host machine, do this:
lcd /usr/local/TARGET/include
get tarfile
tar xf tarfile

File:, Node: Build Cross, Prev: Cross Runtime, Up: Cross-Compiler
Actually Building the Cross-Compiler
Now you can proceed just as for compiling a single-machine compiler
through the step of building stage 1. If you have not provided some
sort of `libgcc1.a', then compilation will give up at the point where
it needs that file, printing a suitable error message. If you do
provide `libgcc1.a', then building the compiler will automatically
compile and link a test program called `libgcc1-test'; if you get
errors in the linking, it means that not all of the necessary routines
in `libgcc1.a' are available.
You must provide the header file `float.h'. One way to do this is
to compile `enquire' and run it on your target machine. The job of
`enquire' is to run on the target machine and figure out by experiment
the nature of its floating point representation. `enquire' records its
findings in the header file `float.h'. If you can't produce this file
by running `enquire' on the target machine, then you will need to come
up with a suitable `float.h' in some other way (or else, avoid using it
in your programs).
Do not try to build stage 2 for a cross-compiler. It doesn't work to
rebuild GNU CC as a cross-compiler using the cross-compiler, because
that would produce a program that runs on the target machine, not on the
host. For example, if you compile a 386-to-68030 cross-compiler with
itself, the result will not be right either for the 386 (because it was
compiled into 68030 code) or for the 68030 (because it was configured
for a 386 as the host). If you want to compile GNU CC into 68030 code,
whether you compile it on a 68030 or with a cross-compiler on a 386, you
must specify a 68030 as the host when you configure it.
To install the cross-compiler, use `make install', as usual.

File:, Node: Sun Install, Next: VMS Install, Prev: Cross-Compiler, Up: Installation
Installing GNU CC on the Sun
On Solaris (version 2.1), do not use the linker or other tools in
`/usr/ucb' to build GNU CC. Use `/usr/ccs/bin'.
Make sure the environment variable `FLOAT_OPTION' is not set when
you compile `libgcc.a'. If this option were set to `f68881' when
`libgcc.a' is compiled, the resulting code would demand to be linked
with a special startup file and would not link properly without special
There is a bug in `alloca' in certain versions of the Sun library.
To avoid this bug, install the binaries of GNU CC that were compiled by
GNU CC. They use `alloca' as a built-in function and never the one in
the library.
Some versions of the Sun compiler crash when compiling GNU CC. The
problem is a segmentation fault in cpp. This problem seems to be due to
the bulk of data in the environment variables. You may be able to avoid
it by using the following command to compile GNU CC with Sun CC:
SunOS 4.1.3 and 4.1.3_U1 have bugs that can cause intermittent core
dumps when compiling GNU CC. A common symptom is an internal compiler
error which does not recur if you run it again. To fix the problem,
install Sun recommended patch 100726 (for SunOS 4.1.3) or 101508 (for
SunOS 4.1.3_U1), or upgrade to a later SunOS release.

File:, Node: VMS Install, Next: Collect2, Prev: Sun Install, Up: Installation
Installing GNU CC on VMS
The VMS version of GNU CC is distributed in a backup saveset
containing both source code and precompiled binaries.
To install the `gcc' command so you can use the compiler easily, in
the same manner as you use the VMS C compiler, you must install the VMS
CLD file for GNU CC as follows:
1. Define the VMS logical names `GNU_CC' and `GNU_CC_INCLUDE' to
point to the directories where the GNU CC executables
(`gcc-cpp.exe', `gcc-cc1.exe', etc.) and the C include files are
kept respectively. This should be done with the commands:
$ assign /system /translation=concealed -
disk:[gcc.] gnu_cc
$ assign /system /translation=concealed -
disk:[gcc.include.] gnu_cc_include
with the appropriate disk and directory names. These commands can
be placed in your system startup file so they will be executed
whenever the machine is rebooted. You may, if you choose, do this
via the `GCC_INSTALL.COM' script in the `[GCC]' directory.
2. Install the `GCC' command with the command line:
$ set command /table=sys$common:[syslib]dcltables -
/output=sys$common:[syslib]dcltables gnu_cc:[000000]gcc
$ install replace sys$common:[syslib]dcltables
3. To install the help file, do the following:
$ library/help sys$library:helplib.hlb gcc.hlp
Now you can invoke the compiler with a command like `gcc /verbose
file.c', which is equivalent to the command `gcc -v -c file.c' in
If you wish to use GNU C++ you must first install GNU CC, and then
perform the following steps:
1. Define the VMS logical name `GNU_GXX_INCLUDE' to point to the
directory where the preprocessor will search for the C++ header
files. This can be done with the command:
$ assign /system /translation=concealed -
disk:[gcc.gxx_include.] gnu_gxx_include
with the appropriate disk and directory name. If you are going to
be using a C++ runtime library, this is where its install
procedure will install its header files.
2. Obtain the file `gcc-cc1plus.exe', and place this in the same
directory that `gcc-cc1.exe' is kept.
The GNU C++ compiler can be invoked with a command like `gcc /plus
/verbose', which is equivalent to the command `g++ -v -c' in Unix.
We try to put corresponding binaries and sources on the VMS
distribution tape. But sometimes the binaries will be from an older
version than the sources, because we don't always have time to update
them. (Use the `/version' option to determine the version number of
the binaries and compare it with the source file `version.c' to tell
whether this is so.) In this case, you should use the binaries you get
to recompile the sources. If you must recompile, here is how:
1. Execute the command procedure `' to set up the files
`tm.h', `config.h', `aux-output.c', and `md.', and to create files
`tconfig.h' and `hconfig.h'. This procedure also creates several
linker option files used by `' and a data file used by
2. Setup the logical names and command tables as defined above. In
addition, define the VMS logical name `GNU_BISON' to point at the
to the directories where the Bison executable is kept. This
should be done with the command:
$ assign /system /translation=concealed -
disk:[bison.] gnu_bison
You may, if you choose, use the `INSTALL_BISON.COM' script in the
`[BISON]' directory.
3. Install the `BISON' command with the command line:
$ set command /table=sys$common:[syslib]dcltables -
/output=sys$common:[syslib]dcltables -
$ install replace sys$common:[syslib]dcltables
4. Type `@make-gcc' to recompile everything (alternatively, submit
the file `' to a batch queue). If you wish to build
the GNU C++ compiler as well as the GNU CC compiler, you must
first edit `' and follow the instructions that appear
in the comments.
5. In order to use GCC, you need a library of functions which GCC
compiled code will call to perform certain tasks, and these
functions are defined in the file `libgcc2.c'. To compile this
you should use the command procedure `', which will
generate the library `libgcc2.olb'. `libgcc2.olb' should be built
using the compiler built from the same distribution that
`libgcc2.c' came from, and `' will automatically do
all of this for you.
To install the library, use the following commands:
$ library gnu_cc:[000000]gcclib/delete=(new,eprintf)
$ library gnu_cc:[000000]gcclib/delete=L_*
$ library libgcc2/extract=*/output=libgcc2.obj
$ library gnu_cc:[000000]gcclib libgcc2.obj
The first command simply removes old modules that will be replaced
with modules from `libgcc2' under different module names. The
modules `new' and `eprintf' may not actually be present in your
`gcclib.olb'--if the VMS librarian complains about those modules
not being present, simply ignore the message and continue on with
the next command. The second command removes the modules that
came from the previous version of the library `libgcc2.c'.
Whenever you update the compiler on your system, you should also
update the library with the above procedure.
6. You may wish to build GCC in such a way that no files are written
to the directory where the source files reside. An example would
be the when the source files are on a read-only disk. In these
cases, execute the following DCL commands (substituting your
actual path names):
$ assign dua0:[gcc.build_dir.]/translation=concealed, -
dua1:[gcc.source_dir.]/translation=concealed gcc_build
$ set default gcc_build:[000000]
where the directory `dua1:[gcc.source_dir]' contains the source
code, and the directory `dua0:[gcc.build_dir]' is meant to contain
all of the generated object files and executables. Once you have
done this, you can proceed building GCC as described above. (Keep
in mind that `gcc_build' is a rooted logical name, and thus the
device names in each element of the search list must be an actual
physical device name rather than another rooted logical name).
7. *If you are building GNU CC with a previous version of GNU CC, you
also should check to see that you have the newest version of the
assembler*. In particular, GNU CC version 2 treats global constant
variables slightly differently from GNU CC version 1, and GAS
version 1.38.1 does not have the patches required to work with GCC
version 2. If you use GAS 1.38.1, then `extern const' variables
will not have the read-only bit set, and the linker will generate
warning messages about mismatched psect attributes for these
variables. These warning messages are merely a nuisance, and can
safely be ignored.
If you are compiling with a version of GNU CC older than 1.33,
specify `/DEFINE=("inline=")' as an option in all the
compilations. This requires editing all the `gcc' commands in
`'. (The older versions had problems supporting
`inline'.) Once you have a working 1.33 or newer GNU CC, you can
change this file back.
8. If you want to build GNU CC with the VAX C compiler, you will need
to make minor changes in `' and `' to
choose alternate definitions of `CC', `CFLAGS', and `LIBS'. See
comments in those files. However, you must also have a working
version of the GNU assembler (GNU as, aka GAS) as it is used as
the back-end for GNU CC to produce binary object modules and is
not included in the GNU CC sources. GAS is also needed to compile
`libgcc2' in order to build `gcclib' (see above); `'
expects to be able to find it operational in
To use GNU CC on VMS, you need the VMS driver programs `gcc.exe',
`', and `gcc.cld'. They are distributed with the VMS
binaries (`gcc-vms') rather than the GNU CC sources. GAS is also
included in `gcc-vms', as is Bison.
Once you have successfully built GNU CC with VAX C, you should use
the resulting compiler to rebuild itself. Before doing this, be
sure to restore the `CC', `CFLAGS', and `LIBS' definitions in
`' and `'. The second generation
compiler will be able to take advantage of many optimizations that
must be suppressed when building with other compilers.
Under previous versions of GNU CC, the generated code would
occasionally give strange results when linked with the sharable
`VAXCRTL' library. Now this should work.
Even with this version, however, GNU CC itself should not be linked
with the sharable `VAXCRTL'. The version of `qsort' in `VAXCRTL' has a
bug (known to be present in VMS versions V4.6 through V5.5) which
causes the compiler to fail.
The executables are generated by `' and `'
use the object library version of `VAXCRTL' in order to make use of the
`qsort' routine in `gcclib.olb'. If you wish to link the compiler
executables with the shareable image version of `VAXCRTL', you should
edit the file `tm.h' (created by `') to define the macro
`QSORT_WORKAROUND' is always defined when GNU CC is compiled with
VAX C, to avoid a problem in case `gcclib.olb' is not yet available.

File:, Node: Collect2, Next: Header Dirs, Prev: VMS Install, Up: Installation
Many target systems do not have support in the assembler and linker
for "constructors"--initialization functions to be called before the
official "start" of `main'. On such systems, GNU CC uses a utility
called `collect2' to arrange to call these functions at start time.
The program `collect2' works by linking the program once and looking
through the linker output file for symbols with particular names
indicating they are constructor functions. If it finds any, it creates
a new temporary `.c' file containing a table of them, compiles it, and
links the program a second time including that file.
The actual calls to the constructors are carried out by a subroutine
called `__main', which is called (automatically) at the beginning of
the body of `main' (provided `main' was compiled with GNU CC). Calling
`__main' is necessary, even when compiling C code, to allow linking C
and C++ object code together. (If you use `-nostdlib', you get an
unresolved reference to `__main', since it's defined in the standard
GCC library. Include `-lgcc' at the end of your compiler command line
to resolve this reference.)
The program `collect2' is installed as `ld' in the directory where
the passes of the compiler are installed. When `collect2' needs to
find the *real* `ld', it tries the following file names:
* `real-ld' in the directories listed in the compiler's search
* `real-ld' in the directories listed in the environment variable
* The file specified in the `REAL_LD_FILE_NAME' configuration macro,
if specified.
* `ld' in the compiler's search directories, except that `collect2'
will not execute itself recursively.
* `ld' in `PATH'.
"The compiler's search directories" means all the directories where
`gcc' searches for passes of the compiler. This includes directories
that you specify with `-B'.
Cross-compilers search a little differently:
* `real-ld' in the compiler's search directories.
* `TARGET-real-ld' in `PATH'.
* The file specified in the `REAL_LD_FILE_NAME' configuration macro,
if specified.
* `ld' in the compiler's search directories.
* `TARGET-ld' in `PATH'.
`collect2' explicitly avoids running `ld' using the file name under
which `collect2' itself was invoked. In fact, it remembers up a list
of such names--in case one copy of `collect2' finds another copy (or
version) of `collect2' installed as `ld' in a second place in the
search path.
`collect2' searches for the utilities `nm' and `strip' using the
same algorithm as above for `ld'.

File:, Node: Header Dirs, Prev: Collect2, Up: Installation
Standard Header File Directories
`GCC_INCLUDE_DIR' means the same thing for native and cross. It is
where GNU CC stores its private include files, and also where GNU CC
stores the fixed include files. A cross compiled GNU CC runs
`fixincludes' on the header files in `$(tooldir)/include'. (If the
cross compilation header files need to be fixed, they must be installed
before GNU CC is built. If the cross compilation header files are
already suitable for ANSI C and GNU CC, nothing special need be done).
`GPLUS_INCLUDE_DIR' means the same thing for native and cross. It
is where `g++' looks first for header files. The C++ library installs
only target independent header files in that directory.
`LOCAL_INCLUDE_DIR' is used only for a native compiler. It is
normally `/usr/local/include'. GNU CC searches this directory so that
users can install header files in `/usr/local/include'.
`CROSS_INCLUDE_DIR' is used only for a cross compiler. GNU CC
doesn't install anything there.
`TOOL_INCLUDE_DIR' is used for both native and cross compilers. It
is the place for other packages to install header files that GNU CC will
use. For a cross-compiler, this is the equivalent of `/usr/include'.
When you build a cross-compiler, `fixincludes' processes any header
files in this directory.

File:, Node: C Extensions, Next: C++ Extensions, Prev: Installation, Up: Top
Extensions to the C Language Family
GNU C provides several language features not found in ANSI standard
C. (The `-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
`__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++. *Note Extensions to the C++ Language: C++
Extensions, for extensions that apply *only* to C++.
* 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:: `typeof': referring to the type of an expression.
* Lvalues:: Using `?:', `,' and casts in lvalues.
* Conditionals:: Omitting the middle operand of a `?:' expression.
* Long Long:: Double-word integers--`long long int'.
* Complex:: Data types for complex numbers.
* Zero Length:: Zero-length arrays.
* Variable Length:: Arrays whose length is computed at run time.
* Macro Varargs:: Macros with variable number of arguments.
* Subscripting:: Any array can be subscripted, even if not an lvalue.
* Pointer Arith:: Arithmetic on `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.
* Function Prototypes:: Prototype declarations and old-style definitions.
* C++ Comments:: C++ comments are recognized.
* Dollar Signs:: Dollar sign is allowed in identifiers.
* Character Escapes:: `\e' stands for the character <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:: `__const__', `__asm__', etc., for header files.
* Incomplete Enums:: `enum foo;', with details to follow.
* Function Names:: Printable strings which are the name of the current
* Return Address:: Getting the return or frame address of a function.

File:, Node: Statement Exprs, Next: Local Labels, Up: C Extensions
Statements and Declarations in Expressions
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:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
is a valid (though slightly more complex than necessary) expression for
the absolute value of `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 `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
#define max(a,b) ((a) > (b) ? (a) : (b))
But this definition computes either A or 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 `int'), you can define the macro safely as
#define maxint(a,b) \
({int _a = (a), _b = (b); _a > _b ? _a : _b; })
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 `typeof' (*note Typeof::.) or type naming (*note
Naming Types::.).

File:, Node: Local Labels, Next: Labels as Values, Prev: Statement Exprs, Up: C Extensions
Locally Declared Labels
Each statement expression is a scope in which "local labels" can be
declared. A local label is simply an identifier; you can jump to it
with an ordinary `goto' statement, but only from within the statement
expression it belongs to.
A local label declaration looks like this:
__label__ LABEL;
__label__ LABEL1, LABEL2, ...;
Local label declarations must come at the beginning of the statement
expression, right after the `({', before any ordinary declarations.
The label declaration defines the label *name*, but does not define
the label itself. You must do this in the usual way, with `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 `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
#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; \

File:, Node: Labels as Values, Next: Nested Functions, Prev: Local Labels, Up: C Extensions
Labels as Values
You can get the address of a label defined in the current function
(or a containing function) with the unary operator `&&'. The value has
type `void *'. This value is a constant and can be used wherever a
constant of that type is valid. For example:
void *ptr;
ptr = &&foo;
To use these values, you need to be able to jump to one. This is
done with the computed goto statement(1), `goto *EXP;'. For example,
goto *ptr;
Any expression of type `void *' is allowed.
One way of using these constants is in initializing a static array
that will serve as a jump table:
static void *array[] = { &&foo, &&bar, &&hack };
Then you can select a label with indexing, like this:
goto *array[i];
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
`switch' statement. The `switch' statement is cleaner, so use that
rather than an array unless the problem does not fit a `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 can 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.
---------- Footnotes ----------
(1) 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.

File:, Node: Nested Functions, Next: Constructing Calls, Prev: Labels as Values, Up: C Extensions
Nested Functions
A "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 `square', and call it twice:
foo (double a, double b)
double square (double z) { return z * z; }
return square (a) + square (b);
The nested function can access all the variables of the containing
function that are visible at the point of its definition. This is
called "lexical scoping". For example, here we show a nested function
which uses an inherited variable named `offset':
bar (int *array, int offset, int size)
int access (int *array, int index)
{ return array[index + offset]; }
int i;
for (i = 0; i < size; i++)
... access (array, i) ...
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
hack (int *array, int size)
void store (int index, int value)
{ array[index] = value; }
intermediate (store, size);
Here, the function `intermediate' receives the address of `store' as
an argument. If `intermediate' calls `store', the arguments given to
`store' are used to store into `array'. But this technique works only
so long as the containing function (`hack', in this example) does not
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
GNU CC implements taking the address of a nested function using a
technique called "trampolines".
A nested function can jump to a label inherited from a containing
function, provided the label was explicitly declared in the containing
function (*note Local Labels::.). Such a jump returns instantly to the
containing function, exiting the nested function which did the `goto'
and any intermediate functions as well. Here is an example:
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;
for (i = 0; i < size; i++)
... access (array, i) ...
return 0;
/* Control comes here from `access'
if it detects an error. */
return -1;
A nested function always has internal linkage. Declaring one with
`extern' is erroneous. If you need to declare the nested function
before its definition, use `auto' (which is otherwise meaningless for
function declarations).
bar (int *array, int offset, int size)
__label__ failure;
auto int access (int *, int);
int access (int *array, int index)
if (index > size)
goto failure;
return array[index + offset];

File:, Node: Constructing Calls, Next: Naming Types, Prev: Nested Functions, Up: C Extensions
Constructing Function 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).
`__builtin_apply_args ()'
This built-in function returns a pointer of type `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.
`__builtin_apply (FUNCTION, ARGUMENTS, SIZE)'
This built-in function invokes FUNCTION (type `void (*)()') with a
copy of the parameters described by ARGUMENTS (type `void *') and
SIZE (type `int').
The value of ARGUMENTS should be the value returned by
`__builtin_apply_args'. The argument SIZE specifies the size of
the stack argument data, in bytes.
This function returns a pointer of type `void *' to data describing
how to return whatever value was returned by 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 SIZE. The
value is used by `__builtin_apply' to compute the amount of data
that should be pushed on the stack and copied from the incoming
argument area.
`__builtin_return (RESULT)'
This built-in function returns the value described by RESULT from
the containing function. You should specify, for RESULT, a value
returned by `__builtin_apply'.

File:, Node: Naming Types, Next: Typeof, Prev: Constructing Calls, Up: C Extensions
Naming an Expression's Type
You can give a name to the type of an expression using a `typedef'
declaration with an initializer. Here is how to define NAME as a type
name for the type of EXP:
typedef NAME = EXP;
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:
#define max(a,b) \
({typedef _ta = (a), _tb = (b); \
_ta _a = (a); _tb _b = (b); \
_a > _b ? _a : _b; })
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 `a' and `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.

File:, Node: Typeof, Next: Lvalues, Prev: Naming Types, Up: C Extensions
Referring to a Type with `typeof'
Another way to refer to the type of an expression is with `typeof'.
The syntax of using of this keyword looks like `sizeof', but the
construct acts semantically like a type name defined with `typedef'.
There are two ways of writing the argument to `typeof': with an
expression or with a type. Here is an example with an expression:
typeof (x[0](1))
This assumes that `x' is an array of functions; the type described is
that of the values of the functions.
Here is an example with a typename as the argument:
typeof (int *)
Here the type described is that of pointers to `int'.
If you are writing a header file that must work when included in
ANSI C programs, write `__typeof__' instead of `typeof'. *Note
Alternate Keywords::.
A `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 `sizeof' or `typeof'.
* This declares `y' with the type of what `x' points to.
typeof (*x) y;
* This declares `y' as an array of such values.
typeof (*x) y[4];
* This declares `y' as an array of pointers to characters:
typeof (typeof (char *)[4]) y;
It is equivalent to the following traditional C declaration:
char *y[4];
To see the meaning of the declaration using `typeof', and why it
might be a useful way to write, let's rewrite it with these macros:
#define pointer(T) typeof(T *)
#define array(T, N) typeof(T [N])
Now the declaration can be rewritten this way:
array (pointer (char), 4) y;
Thus, `array (pointer (char), 4)' is the type of arrays of 4
pointers to `char'.

File:, Node: Lvalues, Next: Conditionals, Prev: Typeof, Up: C Extensions
Generalized Lvalues
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
(a, b) += 5
a, (b += 5)
Similarly, the address of the compound expression can be taken.
These two expressions are equivalent:
&(a, b)
a, &b
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:
(a ? b : c) = 5
(a ? b = 5 : (c = 5))
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 `a' has type `char *', the following two
expressions are equivalent:
(int)a = 5
(int)(a = (char *)(int)5)
An assignment-with-arithmetic operation such as `+=' 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:
(int)a += 5
(int)(a = (char *)(int) ((int)a + 5))
You cannot take the address of an lvalue cast, because the use of its
address would not work out coherently. Suppose that `&(int)f' were
permitted, where `f' has type `float'. Then the following statement
would try to store an integer bit-pattern where a floating point number
*&(int)f = 1;
This is quite different from what `(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 `&' on a cast.
If you really do want an `int *' pointer with the address of `f',
you can simply write `(int *)&f'.

File:, Node: Conditionals, Next: Long Long, Prev: Lvalues, Up: C Extensions
Conditionals with Omitted Operands
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
x ? : y
has the value of `x' if that is nonzero; otherwise, the value of `y'.
This example is perfectly equivalent to
x ? x : y
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.

File:, Node: Long Long, Next: Complex, Prev: Conditionals, Up: C Extensions
Double-Word Integers
GNU C supports data types for integers that are twice as long as
`int'. Simply write `long long int' for a signed integer, or `unsigned
long long int' for an unsigned integer. To make an integer constant of
type `long long int', add the suffix `LL' to the integer. To make an
integer constant of type `unsigned long long int', add the suffix `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 `long long' types for function
arguments, unless you declare function prototypes. If a function
expects type `int' for its argument, and you pass a value of type `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 `long long int' and you pass `int'. The best way
to avoid such problems is to use prototypes.

File:, Node: Complex, Next: Zero Length, Prev: Long Long, Up: C Extensions
Complex Numbers
GNU C supports complex data types. You can declare both complex
integer types and complex floating types, using the keyword
For example, `__complex__ double x;' declares `x' as a variable
whose real part and imaginary part are both of type `double'.
`__complex__ short int y;' declares `y' to have real and imaginary
parts of type `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 `i' or
`j' (either one; they are equivalent). For example, `2.5fi' has type
`__complex__ float' and `3i' has type `__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 EXP, write
`__real__ EXP'. Likewise, use `__imag__' to extract the imaginary part.
The operator `~' 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 `foo', the two fictitious variables are
named `foo$real' and `foo$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.

File:, Node: Zero Length, Next: Variable Length, Prev: Complex, Up: C Extensions
Arrays of Length Zero
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:
struct line {
int length;
char contents[0];
struct line *thisline = (struct line *)
malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
In standard C, you would have to give `contents' a length of 1, which
means either you waste space or complicate the argument to `malloc'.