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This is Info file gcc.info, produced by Makeinfo version 1.67 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
Boston, MA 02111-1307 USA
Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998
Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "GNU General Public License," "Funding for
Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are
included exactly as in the original, and provided that the entire
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: gcc.info, Node: Variable Length, Next: Macro Varargs, Prev: Zero Length, Up: C Extensions
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:
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);
}
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.
You can use the function `alloca' to get an effect much like
variable-length arrays. The function `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 `alloca' exists until the containing *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
`alloca' in the same function, deallocation of a variable-length array
will also deallocate anything more recently allocated with `alloca'.)
You can also use variable-length arrays as arguments to functions:
struct entry
tester (int len, char data[len][len])
{
...
}
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
`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.
struct entry
tester (int len; char data[len][len], int len)
{
...
}
The `int len' before the semicolon is a "parameter forward
declaration", and it serves the purpose of making the name `len' known
when the declaration of `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.

File: gcc.info, Node: Macro Varargs, Next: Subscripting, Prev: Variable Length, Up: C Extensions
Macros with Variable Numbers of Arguments
=========================================
In GNU C, a macro can accept a variable number of arguments, much as
a function can. The syntax for defining the macro looks much like that
used for a function. Here is an example:
#define eprintf(format, args...) \
fprintf (stderr, format , ## args)
Here `args' is a "rest argument": it takes in zero or more
arguments, as many as the call contains. All of them plus the commas
between them form the value of `args', which is substituted into the
macro body where `args' is used. Thus, we have this expansion:
eprintf ("%s:%d: ", input_file_name, line_number)
==>
fprintf (stderr, "%s:%d: " , input_file_name, line_number)
Note that the comma after the string constant comes from the definition
of `eprintf', whereas the last comma comes from the value of `args'.
The reason for using `##' is to handle the case when `args' matches
no arguments at all. In this case, `args' has an empty value. In this
case, the second comma in the definition becomes an embarrassment: if
it got through to the expansion of the macro, we would get something
like this:
fprintf (stderr, "success!\n" , )
which is invalid C syntax. `##' gets rid of the comma, so we get the
following instead:
fprintf (stderr, "success!\n")
This is a special feature of the GNU C preprocessor: `##' before a
rest argument that is empty discards the preceding sequence of
non-whitespace characters from the macro definition. (If another macro
argument precedes, none of it is discarded.)
It might be better to discard the last preprocessor token instead of
the last preceding sequence of non-whitespace characters; in fact, we
may someday change this feature to do so. We advise you to write the
macro definition so that the preceding sequence of non-whitespace
characters is just a single token, so that the meaning will not change
if we change the definition of this feature.

File: gcc.info, Node: Subscripting, Next: Pointer Arith, Prev: Macro Varargs, Up: C Extensions
Non-Lvalue Arrays May Have Subscripts
=====================================
Subscripting is allowed on arrays that are not lvalues, even though
the unary `&' operator is not. For example, this is valid in GNU C
though not valid in other C dialects:
struct foo {int a[4];};
struct foo f();
bar (int index)
{
return f().a[index];
}

File: gcc.info, Node: Pointer Arith, Next: Initializers, Prev: Subscripting, Up: C Extensions
Arithmetic on `void'- and Function-Pointers
===========================================
In GNU C, addition and subtraction operations are supported on
pointers to `void' and on pointers to functions. This is done by
treating the size of a `void' or of a function as 1.
A consequence of this is that `sizeof' is also allowed on `void' and
on function types, and returns 1.
The option `-Wpointer-arith' requests a warning if these extensions
are used.

File: gcc.info, Node: Initializers, Next: Constructors, Prev: Pointer Arith, Up: C Extensions
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:
foo (float f, float g)
{
float beat_freqs[2] = { f-g, f+g };
...
}

File: gcc.info, Node: Constructors, Next: Labeled Elements, Prev: Initializers, Up: C Extensions
Constructor Expressions
=======================
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 `struct
foo' and `structure' are declared as shown:
struct foo {int a; char b[2];} structure;
Here is an example of constructing a `struct foo' with a constructor:
structure = ((struct foo) {x + y, 'a', 0});
This is equivalent to writing the following:
{
struct foo temp = {x + y, 'a', 0};
structure = temp;
}
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:
char **foo = (char *[]) { "x", "y", "z" };
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 `switch'
statement, while the latter does the same thing an ordinary C
initializer would do. Here is an example of subscripting an array
constructor:
output = ((int[]) { 2, x, 28 }) [input];
Constructor expressions for scalar types and union types are is also
allowed, but then the constructor expression is equivalent to a cast.

File: gcc.info, Node: Labeled Elements, Next: Cast to Union, Prev: Constructors, Up: C Extensions
Labeled Elements in Initializers
================================
Standard C 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 GNU C you can give the elements in any order, specifying the array
indices or structure field names they apply to. This extension is not
implemented in GNU C++.
To specify an array index, write `[INDEX]' or `[INDEX] =' before the
element value. For example,
int a[6] = { [4] 29, [2] = 15 };
is equivalent to
int a[6] = { 0, 0, 15, 0, 29, 0 };
The index values must be constant expressions, even if the array being
initialized is automatic.
To initialize a range of elements to the same value, write `[FIRST
... LAST] = VALUE'. For example,
int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };
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 `FIELDNAME:' before the element value. For example, given the
following structure,
struct point { int x, y; };
the following initialization
struct point p = { y: yvalue, x: xvalue };
is equivalent to
struct point p = { xvalue, yvalue };
Another syntax which has the same meaning is `.FIELDNAME ='., as
shown here:
struct point p = { .y = yvalue, .x = xvalue };
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,
union foo { int i; double d; };
union foo f = { d: 4 };
will convert 4 to a `double' to store it in the union using the second
element. By contrast, casting 4 to type `union foo' would store it
into the union as the integer `i', since it is an integer. (*Note 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,
int a[6] = { [1] = v1, v2, [4] = v4 };
is equivalent to
int a[6] = { 0, v1, v2, 0, v4, 0 };
Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an `enum' type. For
example:
int whitespace[256]
= { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };

File: gcc.info, Node: Case Ranges, Next: Function Attributes, Prev: Cast to Union, Up: C Extensions
Case Ranges
===========
You can specify a range of consecutive values in a single `case'
label, like this:
case LOW ... HIGH:
This has the same effect as the proper number of individual `case'
labels, one for each integer value from LOW to HIGH, inclusive.
This feature is especially useful for ranges of ASCII character
codes:
case 'A' ... 'Z':
*Be careful:* Write spaces around the `...', for otherwise it may be
parsed wrong when you use it with integer values. For example, write
this:
case 1 ... 5:
rather than this:
case 1...5:

File: gcc.info, Node: Cast to Union, Next: Case Ranges, Prev: Labeled Elements, Up: C Extensions
Cast to a Union Type
====================
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 `union
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. (*Note 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:
union foo { int i; double d; };
int x;
double y;
both `x' and `y' can be cast to type `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:
union foo u;
...
u = (union foo) x == u.i = x
u = (union foo) y == u.d = y
You can also use the union cast as a function argument:
void hack (union foo);
...
hack ((union foo) x);

File: gcc.info, Node: Function Attributes, Next: Function Prototypes, Prev: Case Ranges, Up: C Extensions
Declaring Attributes of Functions
=================================
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 `__attribute__' allows you to specify special attributes
when making a declaration. This keyword is followed by an attribute
specification inside double parentheses. Eight attributes, `noreturn',
`const', `format', `section', `constructor', `destructor', `unused' and
`weak' are currently defined for functions. Other attributes, including
`section' are supported for variables declarations (*note Variable
Attributes::.) and for types (*note Type Attributes::.).
You may also specify attributes with `__' 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 `__noreturn__' instead of `noreturn'.
`noreturn'
A few standard library functions, such as `abort' and `exit',
cannot return. GNU CC knows this automatically. Some programs
define their own functions that never return. You can declare them
`noreturn' to tell the compiler this fact. For example,
void fatal () __attribute__ ((noreturn));
void
fatal (...)
{
... /* Print error message. */ ...
exit (1);
}
The `noreturn' keyword tells the compiler to assume that `fatal'
cannot return. It can then optimize without regard to what would
happen if `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 `noreturn' function.
It does not make sense for a `noreturn' function to have a return
type other than `void'.
The attribute `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:
typedef void voidfn ();
volatile voidfn fatal;
`const'
Many functions do not examine any values except their arguments,
and have no effects except the return value. 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 `const'. For
example,
int square (int) __attribute__ ((const));
says that the hypothetical function `square' is safe to call fewer
times than the program says.
The attribute `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:
typedef int intfn ();
extern const intfn square;
This approach does not work in GNU C++ from 2.6.0 on, since the
language specifies that the `const' must be attached to the return
value.
Note that a function that has pointer arguments and examines the
data pointed to must *not* be declared `const'. Likewise, a
function that calls a non-`const' function usually must not be
`const'. It does not make sense for a `const' function to return
`void'.
`format (ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK)'
The `format' attribute specifies that a function takes `printf' or
`scanf' style arguments which should be type-checked against a
format string. For example, the declaration:
extern int
my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format (printf, 2, 3)));
causes the compiler to check the arguments in calls to `my_printf'
for consistency with the `printf' style format string argument
`my_format'.
The parameter ARCHETYPE determines how the format string is
interpreted, and should be either `printf' or `scanf'. The
parameter STRING-INDEX specifies which argument is the format
string argument (starting from 1), while 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 `vprintf'), specify the third parameter as zero. In this
case the compiler only checks the format string for consistency.
In the example above, the format string (`my_format') is the second
argument of the function `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 `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
checks formats for the ANSI library functions `printf', `fprintf',
`sprintf', `scanf', `fscanf', `sscanf', `vprintf', `vfprintf' and
`vsprintf' whenever such warnings are requested (using
`-Wformat'), so there is no need to modify the header file
`stdio.h'.
`format_arg (STRING-INDEX)'
The `format_arg' attribute specifies that a function takes
`printf' or `scanf' style arguments, modifies it (for example, to
translate it into another language), and passes it to a `printf'
or `scanf' style function. For example, the declaration:
extern char *
my_dgettext (char *my_domain, const char *my_format)
__attribute__ ((format_arg (2)));
causes the compiler to check the arguments in calls to
`my_dgettext' whose result is passed to a `printf' or `scanf' type
function for consistency with the `printf' style format string
argument `my_format'.
The parameter STRING-INDEX specifies which argument is the format
string argument (starting from 1).
The `format-arg' attribute allows you to identify your own
functions which modify format strings, so that GNU CC can check the
calls to `printf' and `scanf' function whose operands are a call
to one of your own function. The compiler always treats
`gettext', `dgettext', and `dcgettext' in this manner.
`section ("section-name")'
Normally, the compiler places the code it generates in the `text'
section. Sometimes, however, you need additional sections, or you
need certain particular functions to appear in special sections.
The `section' attribute specifies that a function lives in a
particular section. For example, the declaration:
extern void foobar (void) __attribute__ ((section ("bar")));
puts the function `foobar' in the `bar' section.
Some file formats do not support arbitrary sections so the
`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.
`constructor'
`destructor'
The `constructor' attribute causes the function to be called
automatically before execution enters `main ()'. Similarly, the
`destructor' attribute causes the function to be called
automatically after `main ()' has completed or `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.
`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++.
`weak'
The `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.
`alias ("target")'
The `alias' attribute causes the declaration to be emitted as an
alias for another symbol, which must be specified. For instance,
void __f () { /* do something */; }
void f () __attribute__ ((weak, alias ("__f")));
declares `f' to be a weak alias for `__f'. In C++, the mangled
name for the target must be used.
Not all target machines support this attribute.
`regparm (NUMBER)'
On the Intel 386, the `regparm' attribute causes the compiler to
pass up to NUMBER integer arguments in registers EAX, EDX, and 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.
`stdcall'
On the Intel 386, the `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 `stdcall'
attribute.
`cdecl'
On the Intel 386, the `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
`-mrtd' switch.
The PowerPC compiler for Windows NT currently ignores the `cdecl'
attribute.
`longcall'
On the RS/6000 and PowerPC, the `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.
`dllimport'
On the PowerPC running Windows NT, the `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 `__imp_' and the function
name.
`dllexport'
On the PowerPC running Windows NT, the `dllexport' attribute causes
the compiler to provide a global pointer to the function pointer,
so that it can be called with the `dllimport' attribute. The
pointer name is formed by combining `__imp_' and the function name.
`exception (EXCEPT-FUNC [, EXCEPT-ARG])'
On the PowerPC running Windows NT, the `exception' attribute causes
the compiler to modify the structured exception table entry it
emits for the declared function. The string or identifier
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 EXCEPT-ARG is placed in the
fourth entry of the structured exception table.
`function_vector'
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.
`interrupt_handler'
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.
`eightbit_data'
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.
`tiny_data'
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.
`interrupt'
Use this option on the M32R/D 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.
`model (MODEL-NAME)'
Use this attribute on the M32R/D to set the addressability of an
object, and the code generated for a function. The identifier
MODEL-NAME is one of `small', `medium', or `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 `ld24' instruction), and are
callable with the `bl' instruction.
Medium model objects may live anywhere in the 32 bit address space
(the compiler will generate `seth/add3' instructions to load their
addresses), and are callable with the `bl' instruction.
Large model objects may live anywhere in the 32 bit address space
(the compiler will generate `seth/add3' instructions to load their
addresses), and may not be reachable with the `bl' instruction
(the compiler will generate the much slower `seth/add3/jl'
instruction sequence).
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.
Some people object to the `__attribute__' feature, suggesting that
ANSI C's `#pragma' should be used instead. There are two reasons for
not doing this.
1. It is impossible to generate `#pragma' commands from a macro.
2. There is no telling what the same `#pragma' might mean in another
compiler.
These two reasons apply to almost any application that might be
proposed for `#pragma'. It is basically a mistake to use `#pragma' for
*anything*.

File: gcc.info, Node: Function Prototypes, Next: C++ Comments, Prev: Function Attributes, Up: C Extensions
Prototypes and Old-Style Function Definitions
=============================================
GNU C extends ANSI C to allow a function prototype to override a
later old-style non-prototype definition. Consider the following
example:
/* Use prototypes unless the compiler is old-fashioned. */
#ifdef __STDC__
#define P(x) x
#else
#define P(x) ()
#endif
/* Prototype function declaration. */
int isroot P((uid_t));
/* Old-style function definition. */
int
isroot (x) /* ??? lossage here ??? */
uid_t x;
{
return x == 0;
}
Suppose the type `uid_t' happens to be `short'. ANSI 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 `int', which does not match
the prototype argument type of `short'.
This restriction of ANSI C makes it hard to write code that is
portable to traditional C compilers, because the programmer does not
know whether the `uid_t' type is `short', `int', or `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:
int isroot (uid_t);
int
isroot (uid_t x)
{
return x == 0;
}
GNU C++ does not support old-style function definitions, so this
extension is irrelevant.

File: gcc.info, Node: C++ Comments, Next: Dollar Signs, Prev: Function Prototypes, Up: C Extensions
C++ Style Comments
==================
In GNU C, you may use C++ style comments, which start with `//' 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 `-ansi'
or `-traditional', since they are incompatible with traditional
constructs like `dividend//*comment*/divisor'.

File: gcc.info, Node: Dollar Signs, Next: Character Escapes, Prev: C++ Comments, Up: C Extensions
Dollar Signs in Identifier Names
================================
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.

File: gcc.info, Node: Character Escapes, Next: Variable Attributes, Prev: Dollar Signs, Up: C Extensions
The Character <ESC> in Constants
================================
You can use the sequence `\e' in a string or character constant to
stand for the ASCII character <ESC>.

File: gcc.info, Node: Alignment, Next: Inline, Prev: Type Attributes, Up: C Extensions
Inquiring on Alignment of Types or Variables
============================================
The keyword `__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 `sizeof'.
For example, if the target machine requires a `double' value to be
aligned on an 8-byte boundary, then `__alignof__ (double)' is 8. This
is true on many RISC machines. On more traditional machine designs,
`__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,
`__alignof__' reports the *recommended* alignment of a type.
When the operand of `__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:
struct foo { int x; char y; } foo1;
the value of `__alignof__ (foo1.y)' is probably 2 or 4, the same as
`__alignof__ (int)', even though the data type of `foo1.y' does not
itself demand any alignment.
A related feature which lets you specify the alignment of an object
is `__attribute__ ((aligned (ALIGNMENT)))'; see the following section.

File: gcc.info, Node: Variable Attributes, Next: Type Attributes, Prev: Character Escapes, Up: C Extensions
Specifying Attributes of Variables
==================================
The keyword `__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: `aligned', `mode', `nocommon',
`packed', `section', `transparent_union', `unused', and `weak'. Other
attributes are available for functions (*note Function Attributes::.)
and for types (*note Type Attributes::.).
You may also specify attributes with `__' 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 `__aligned__' instead of `aligned'.
`aligned (ALIGNMENT)'
This attribute specifies a minimum alignment for the variable or
structure field, measured in bytes. For example, the declaration:
int x __attribute__ ((aligned (16))) = 0;
causes the compiler to allocate the global variable `x' on a
16-byte boundary. On a 68040, this could be used in conjunction
with an `asm' expression to access the `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 `int' pair, you could
write:
struct foo { int x[2] __attribute__ ((aligned (8))); };
This is an alternative to creating a union with a `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:
short array[3] __attribute__ ((aligned));
Whenever you leave out the alignment factor in an `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 `aligned' attribute can only increase the alignment; but you
can decrease it by specifying `packed' as well. See below.
Note that the effectiveness of `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 `aligned(16)' in an `__attribute__' will still
only provide you with 8 byte alignment. See your linker
documentation for further information.
`mode (MODE)'
This attribute specifies the data type for the
declaration--whichever type corresponds to the mode MODE. This in
effect lets you request an integer or floating point type
according to its width.
You may also specify a mode of `byte' or `__byte__' to indicate
the mode corresponding to a one-byte integer, `word' or `__word__'
for the mode of a one-word integer, and `pointer' or `__pointer__'
for the mode used to represent pointers.
`nocommon'
This attribute specifies requests GNU CC not to place a variable
"common" but instead to allocate space for it directly. If you
specify the `-fno-common' flag, GNU CC will do this for all
variables.
Specifying the `nocommon' attribute for a variable provides an
initialization of zeros. A variable may only be initialized in one
source file.
`packed'
The `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 `aligned' attribute.
Here is a structure in which the field `x' is packed, so that it
immediately follows `a':
struct foo
{
char a;
int x[2] __attribute__ ((packed));
};
`section ("section-name")'
Normally, the compiler places the objects it generates in sections
like `data' and `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
`section' attribute specifies that a variable (or function) lives
in a particular section. For example, this small program uses
several specific section names:
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);
}
Use the `section' attribute with an *initialized* definition of a
*global* variable, as shown in the example. GNU CC issues a
warning and otherwise ignores the `section' attribute in
uninitialized variable declarations.
You may only use the `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 `common' (or `bss')
section and can be multiply "defined". You can force a variable
to be initialized with the `-fno-common' flag or the `nocommon'
attribute.
Some file formats do not support arbitrary sections so the
`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.
`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 *Note Type
Attributes::. You can also use this attribute on a `typedef' for
a union data type; then it applies to all function parameters with
that type.
`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.
`weak'
The `weak' attribute is described in *Note Function Attributes::.
`model (MODEL-NAME)'
Use this attribute on the M32R/D to set the addressability of an
object. The identifier MODEL-NAME is one of `small', `medium', or
`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 `ld24' instruction).
Medium and large model objects may live anywhere in the 32 bit
address space (the compiler will generate `seth/add3' instructions
to load their addresses).
To specify multiple attributes, separate them by commas within the
double parentheses: for example, `__attribute__ ((aligned (16),
packed))'.

File: gcc.info, Node: Type Attributes, Next: Alignment, Prev: Variable Attributes, Up: C Extensions
Specifying Attributes of Types
==============================
The keyword `__attribute__' allows you to specify special attributes
of `struct' and `union' types when you define such types. This keyword
is followed by an attribute specification inside double parentheses.
Three attributes are currently defined for types: `aligned', `packed',
and `transparent_union'. Other attributes are defined for functions
(*note Function Attributes::.) and for variables (*note Variable
Attributes::.).
You may also specify any one of these attributes with `__' 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 `__aligned__' instead of `aligned'.
You may specify the `aligned' and `transparent_union' attributes
either in a `typedef' declaration or just past the closing curly brace
of a complete enum, struct or union type *definition* and the `packed'
attribute only past the closing brace of a definition.
`aligned (ALIGNMENT)'
This attribute specifies a minimum alignment (in bytes) for
variables of the specified type. For example, the declarations:
struct S { short f[3]; } __attribute__ ((aligned (8)));
typedef int more_aligned_int __attribute__ ((aligned (8)));
force the compiler to insure (as far as it can) that each variable
whose type is `struct S' or `more_aligned_int' will be allocated
and aligned *at least* on a 8-byte boundary. On a Sparc, having
all variables of type `struct S' aligned to 8-byte boundaries
allows the compiler to use the `ldd' and `std' (doubleword load and
store) instructions when copying one variable of type `struct S' to
another, thus improving run-time efficiency.
Note that the alignment of any given `struct' or `union' type is
required by the ANSI C standard to be at least a perfect multiple
of the lowest common multiple of the alignments of all of the
members of the `struct' or `union' in question. This means that
you *can* effectively adjust the alignment of a `struct' or `union'
type by attaching an `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 `struct' or `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
`struct' or `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:
struct S { short f[3]; } __attribute__ ((aligned));
Whenever you leave out the alignment factor in an `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 `short' is 2 bytes, then
the size of the entire `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 `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 `aligned' attribute can only increase the alignment; but you
can decrease it by specifying `packed' as well. See below.
Note that the effectiveness of `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 `aligned(16)' in an `__attribute__' will still
only provide you with 8 byte alignment. See your linker
documentation for further information.
`packed'
This attribute, attached to an `enum', `struct', or `union' type
definition, specified that the minimum required memory be used to
represent the type.
Specifying this attribute for `struct' and `union' types is
equivalent to specifying the `packed' attribute on each of the
structure or union members. Specifying the `-fshort-enums' flag
on the line is equivalent to specifying the `packed' attribute on
all `enum' definitions.
You may only specify this attribute after a closing curly brace on
an `enum' definition, not in a `typedef' declaration, unless that
declaration also contains the definition of the `enum'.
`transparent_union'
This attribute, attached to a `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 `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 `wait' function must accept either a value of type
`int *' to comply with Posix, or a value of type `union wait *' to
comply with the 4.1BSD interface. If `wait''s parameter were
`void *', `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, `<sys/wait.h>' might
define the interface as follows:
typedef union
{
int *__ip;
union wait *__up;
} wait_status_ptr_t __attribute__ ((__transparent_union__));
pid_t wait (wait_status_ptr_t);
This interface allows either `int *' or `union wait *' arguments
to be passed, using the `int *' calling convention. The program
can call `wait' with arguments of either type:
int w1 () { int w; return wait (&w); }
int w2 () { union wait w; return wait (&w); }
With this interface, `wait''s implementation might look like this:
pid_t wait (wait_status_ptr_t p)
{
return waitpid (-1, p.__ip, 0);
}
`unused'
When attached to a type (including a `union' or a `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.
To specify multiple attributes, separate them by commas within the
double parentheses: for example, `__attribute__ ((aligned (16),
packed))'.