Fresh Fish 6/FreshFish_September1994.bin/bbs/gnu/gcc-2.6.0-src.lha/GNU/src/amiga/gcc-2.6.0/gcc.info-8

GNU Info File | 1994-07-15 | 50.0 KB | 1,221 lines

This is Info file gcc.info, produced by Makeinfo-1.54 from the input
file gcc.texi.

   This file documents the use and the internals of the GNU compiler.

   Published by the Free Software Foundation 675 Massachusetts Avenue
Cambridge, MA 02139 USA

   Copyright (C) 1988, 1989, 1992, 1993 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" 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" 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: 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.  Four attributes, `noreturn',
`const', `format', and `section' are currently defined for functions.
Other attributes, including `section' are supported for variables
declarations (*note Variable Attributes::.).

`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;

     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'.

`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 (".init")));

     puts the function `foobar' in the `.init' 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.

   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: Dollar Signs,  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.  */
     #if __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;
     }


File: gcc.info,  Node: Dollar Signs,  Next: Character Escapes,  Prev: Function Prototypes,  Up: C Extensions

Dollar Signs in Identifier Names
================================

   In GNU C, you may use dollar signs in identifier names.  This is
because many traditional C implementations allow such identifiers.

   On some machines, dollar signs are allowed in identifiers if you
specify `-traditional'.  On a few systems they are allowed by default,
even if you do not use `-traditional'.  But they are never allowed if
you specify `-ansi'.

   There are certain ANSI C programs (obscure, to be sure) that would
compile incorrectly if dollar signs were permitted in identifiers.  For
example:

     #define foo(a) #a
     #define lose(b) foo (b)
     #define test$
     lose (test)


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: Variable 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: Alignment,  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.  Four attributes are
currently defined for variables: `aligned', `mode', `packed', and
`section'.  Other attributes are defined for functions, and thus not
documented here; see *Note Function Attributes::.

`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.

     The `aligned' attribute can only increase the alignment; but you
     can decrease it by specifying `packed' as well.  See below.

     The linker of your operating system imposes a maximum alignment.
     If the linker aligns each object file on a four byte boundary,
     then it is beyond the compiler's power to cause anything to be
     aligned to a larger boundary than that.  For example, if  the
     linker happens to put this object file at address 136 (eight more
     than a multiple of 64), then the compiler cannot guarantee an
     alignment of more than 8 just by aligning variables in the object
     file.

`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.

`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_copy __attribute__ ((section ("INITDATACOPY"))) = 0;

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

            /* Initialize initialized data */
            memcpy (&init_data_copy, &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".

     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.

   To specify multiple attributes, separate them by commas within the
double parentheses: for example, `__attribute__ ((aligned (16),
packed))'.


File: gcc.info,  Node: Inline,  Next: Extended Asm,  Prev: Alignment,  Up: C Extensions

An Inline Function is As Fast As a Macro
========================================

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

   To declare a function inline, use the `inline' keyword in its
declaration, like this:

     inline int
     inc (int *a)
     {
       (*a)++;
     }

   (If you are writing a header file to be included in ANSI C programs,
write `__inline__' instead of `inline'.  *Note Alternate Keywords::.)

   You can also make all "simple enough" functions inline with the
option `-finline-functions'.  Note that certain usages in a function
definition can make it unsuitable for inline substitution.

   For C++ programs, GNU CC automatically inlines member functions even
if they are not explicitly declared `inline'.  (You can override this
with `-fno-default-inline'; *note Options Controlling C++ Dialect: C++
Dialect Options..)

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

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

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

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

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


File: gcc.info,  Node: Extended Asm,  Next: Asm Labels,  Prev: Inline,  Up: C Extensions

Assembler Instructions with C Expression Operands
=================================================

   In an assembler instruction using `asm', you can now specify the
operands of the instruction using C expressions.  This means no more
guessing which registers or memory locations will contain the data you
want to use.

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

   For example, here is how to use the 68881's `fsinx' instruction:

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

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

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

   If there are no output operands, and there are input operands, then
there must be two consecutive colons surrounding the place where the
output operands would go.

   Output operand expressions must be lvalues; the compiler can check
this.  The input operands need not be lvalues.  The compiler cannot
check whether the operands have data types that are reasonable for the
instruction being executed.  It does not parse the assembler
instruction template and does not know what it means, or whether it is
valid assembler input.  The extended `asm' feature is most often used
for machine instructions that the compiler itself does not know exist.

   The output operands must be write-only; GNU CC will assume that the
values in these operands before the instruction are dead and need not be
generated.  Extended asm does not support input-output or read-write
operands.  For this reason, the constraint character `+', which
indicates such an operand, may not be used.

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

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

The constraint `"0"' for operand 1 says that it must occupy the same
location as operand 0.  A digit in constraint is allowed only in an
input operand, and it must refer to an output operand.

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

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

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

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

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

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

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

   If your assembler instruction modifies memory in an unpredictable
fashion, add `memory' to the list of clobbered registers.  This will
cause GNU CC to not keep memory values cached in registers across the
assembler instruction.

   You can put multiple assembler instructions together in a single
`asm' template, separated either with newlines (written as `\n') or with
semicolons if the assembler allows such semicolons.  The GNU assembler
allows semicolons and all Unix assemblers seem to do so.  The input
operands are guaranteed not to use any of the clobbered registers, and
neither will the output operands' addresses, so you can read and write
the clobbered registers as many times as you like.  Here is an example
of multiple instructions in a template; it assumes that the subroutine
`_foo' accepts arguments in registers 9 and 10:

     asm ("movl %0,r9;movl %1,r10;call _foo"
          : /* no outputs */
          : "g" (from), "g" (to)
          : "r9", "r10");

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

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

     asm ("clr %0;frob %1;beq 0f;mov #1,%0;0:"
          : "g" (result)
          : "g" (input));

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

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

   Usually the most convenient way to use these `asm' instructions is to
encapsulate them in macros that look like functions.  For example,

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

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

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

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

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

     #define set_priority(x)  \
     asm volatile ("set_priority %0": /* no outputs */ : "g" (x))

An instruction without output operands will not be deleted or moved
significantly, regardless, unless it is unreachable.

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

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

   If you are writing a header file that should be includable in ANSI C
programs, write `__asm__' instead of `asm'.  *Note Alternate Keywords::.


File: gcc.info,  Node: Asm Labels,  Next: Explicit Reg Vars,  Prev: Extended Asm,  Up: C Extensions

Controlling Names Used in Assembler Code
========================================

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

     int foo asm ("myfoo") = 2;

This specifies that the name to be used for the variable `foo' in the
assembler code should be `myfoo' rather than the usual `_foo'.

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

   You cannot use `asm' in this way in a function *definition*; but you
can get the same effect by writing a declaration for the function
before its definition and putting `asm' there, like this:

     extern func () asm ("FUNC");

     func (x, y)
          int x, y;
     ...

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


File: gcc.info,  Node: Explicit Reg Vars,  Next: Alternate Keywords,  Prev: Asm Labels,  Up: C Extensions

Variables in Specified Registers
================================

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

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

   * Local register variables in specific registers do not reserve the
     registers.  The compiler's data flow analysis is capable of
     determining where the specified registers contain live values, and
     where they are available for other uses.

     These local variables are sometimes convenient for use with the
     extended `asm' feature (*note Extended Asm::.), if you want to
     write one output of the assembler instruction directly into a
     particular register.  (This will work provided the register you
     specify fits the constraints specified for that operand in the
     `asm'.)

* Menu:

* Global Reg Vars::
* Local Reg Vars::


File: gcc.info,  Node: Global Reg Vars,  Next: Local Reg Vars,  Up: Explicit Reg Vars

Defining Global Register Variables
----------------------------------

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

     register int *foo asm ("a5");

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

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

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

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

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

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

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

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

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

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

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

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

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

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


File: gcc.info,  Node: Local Reg Vars,  Prev: Global Reg Vars,  Up: Explicit Reg Vars

Specifying Registers for Local Variables
----------------------------------------

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

     register int *foo asm ("a5");

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

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

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

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

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


File: gcc.info,  Node: Alternate Keywords,  Next: Incomplete Enums,  Prev: Explicit Reg Vars,  Up: C Extensions

Alternate Keywords
==================

   The option `-traditional' disables certain keywords; `-ansi'
disables certain others.  This causes trouble when you want to use GNU C
extensions, or ANSI C features, in a general-purpose header file that
should be usable by all programs, including ANSI C programs and
traditional ones.  The keywords `asm', `typeof' and `inline' cannot be
used since they won't work in a program compiled with `-ansi', while
the keywords `const', `volatile', `signed', `typeof' and `inline' won't
work in a program compiled with `-traditional'.

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

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

     #ifndef __GNUC__
     #define __asm__ asm
     #endif

   `-pedantic' causes warnings for many GNU C extensions.  You can
prevent such warnings within one expression by writing `__extension__'
before the expression.  `__extension__' has no effect aside from this.


File: gcc.info,  Node: Incomplete Enums,  Next: Function Names,  Prev: Alternate Keywords,  Up: C Extensions

Incomplete `enum' Types
=======================

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

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

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


File: gcc.info,  Node: Function Names,  Prev: Incomplete Enums,  Up: C Extensions

Function Names as Strings
=========================

   GNU CC predefines two string variables to be the name of the current
function.  The variable `__FUNCTION__' is the name of the function as
it appears in the source.  The variable `__PRETTY_FUNCTION__' is the
name of the function pretty printed in a language specific fashion.

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

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

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

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

gives this output:

     __FUNCTION__ = sub
     __PRETTY_FUNCTION__ = int  a::sub (int)


File: gcc.info,  Node: C++ Extensions,  Next: Trouble,  Prev: C Extensions,  Up: Top

Extensions to the C++ Language
******************************

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

* Menu:

* Naming Results::      Giving a name to C++ function return values.
* Min and Max::        C++ Minimum and maximum operators.
* Destructors and Goto:: Goto is safe to use in C++ even when destructors
                           are needed.
* C++ Interface::       You can use a single C++ header file for both
                         declarations and definitions.
* C++ Signatures::    You can specify abstract types to get subtype
             polymorphism independent from inheritance.


File: gcc.info,  Node: Naming Results,  Next: Min and Max,  Up: C++ Extensions

Named Return Values in C++
==========================

   GNU C++ extends the function-definition syntax to allow you to
specify a name for the result of a function outside the body of the
definition, in C++ programs:

     TYPE
     FUNCTIONNAME (ARGS) return RESULTNAME;
     {
       ...
       BODY
       ...
     }

   You can use this feature to avoid an extra constructor call when a
function result has a class type.  For example, consider a function
`m', declared as `X v = m ();', whose result is of class `X':

     X
     m ()
     {
       X b;
       b.a = 23;
       return b;
     }

   Although `m' appears to have no arguments, in fact it has one
implicit argument: the address of the return value.  At invocation, the
address of enough space to hold `v' is sent in as the implicit argument.
Then `b' is constructed and its `a' field is set to the value 23.
Finally, a copy constructor (a constructor of the form `X(X&)') is
applied to `b', with the (implicit) return value location as the
target, so that `v' is now bound to the return value.

   But this is wasteful.  The local `b' is declared just to hold
something that will be copied right out.  While a compiler that
combined an "elision" algorithm with interprocedural data flow analysis
could conceivably eliminate all of this, it is much more practical to
allow you to assist the compiler in generating efficient code by
manipulating the return value explicitly, thus avoiding the local
variable and copy constructor altogether.

   Using the extended GNU C++ function-definition syntax, you can avoid
the temporary allocation and copying by naming `r' as your return value
as the outset, and assigning to its `a' field directly:

     X
     m () return r;
     {
       r.a = 23;
     }

The declaration of `r' is a standard, proper declaration, whose effects
are executed *before* any of the body of `m'.

   Functions of this type impose no additional restrictions; in
particular, you can execute `return' statements, or return implicitly by
reaching the end of the function body ("falling off the edge").  Cases
like

     X
     m () return r (23);
     {
       return;
     }

(or even `X m () return r (23); { }') are unambiguous, since the return
value `r' has been initialized in either case.  The following code may
be hard to read, but also works predictably:

     X
     m () return r;
     {
       X b;
       return b;
     }

   The return value slot denoted by `r' is initialized at the outset,
but the statement `return b;' overrides this value.  The compiler deals
with this by destroying `r' (calling the destructor if there is one, or
doing nothing if there is not), and then reinitializing `r' with `b'.

   This extension is provided primarily to help people who use
overloaded operators, where there is a great need to control not just
the arguments, but the return values of functions.  For classes where
the copy constructor incurs a heavy performance penalty (especially in
the common case where there is a quick default constructor), this is a
major savings.  The disadvantage of this extension is that you do not
control when the default constructor for the return value is called: it
is always called at the beginning.


File: gcc.info,  Node: Min and Max,  Next: Destructors and Goto,  Prev: Naming Results,  Up: C++ Extensions

Minimum and Maximum Operators in C++
====================================

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

`A <? B'
     is the "minimum", returning the smaller of the numeric values A
     and B;

`A >? B'
     is the "maximum", returning the larger of the numeric values A and
     B.

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

     #define MIN(X,Y) ((X) < (Y) ? : (X) : (Y))

You might then use `int min = MIN (i, j);' to set MIN to the minimum
value of variables I and J.

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

   Since `<?' and `>?' are built into the compiler, they properly
handle expressions with side-effects;  `int min = i++ <? j++;' works
correctly.


File: gcc.info,  Node: Destructors and Goto,  Next: C++ Interface,  Prev: Min and Max,  Up: C++ Extensions

`goto' and Destructors in GNU C++
=================================

   In C++ programs, you can safely use the `goto' statement.  When you
use it to exit a block which contains aggregates requiring destructors,
the destructors will run before the `goto' transfers control.  (In ANSI
C++, `goto' is restricted to targets within the current block.)

   The compiler still forbids using `goto' to *enter* a scope that
requires constructors.