source: trunk/third/gcc/extend.texi @ 8834

@c Copyright (C) 1988, 1989, 1992, 1993, 1994 Free Software Foundation, Inc.
@c This is part of the GCC manual.
@c For copying conditions, see the file gcc.texi.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

A local label declaration looks like this:

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

@noindent
or

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

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

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

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

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

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

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

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

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

@example
goto *ptr;
@end example

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

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

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

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

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

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

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

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

You can use this mechanism to jump to code in a different function.  If
you do that, totally unpredictable things will happen.  The best way to
avoid this is to store the label address only in automatic variables and
never pass it as an argument.

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

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

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

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

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

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

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

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

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

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

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

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

GNU CC implements taking the address of a nested function using a
technique called @dfn{trampolines}.  A paper describing them is
available from @samp{maya.idiap.ch} in directory @file{pub/tmb},
file @file{usenix88-lexic.ps.Z}.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Here is an example with a typename as the argument:

@example
typeof (int *)
@end example

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

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

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

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

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

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

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

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

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

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

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

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

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

@noindent
Now the declaration can be rewritten this way:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Therefore, the expression

@example
x ? : y
@end example

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

This example is perfectly equivalent to

@example
x ? x : y
@end example

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

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

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

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

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

@node Complex
@section Complex Numbers
@cindex complex numbers

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@node Macro Varargs
@section Macros with Variable Numbers of Arguments
@cindex variable number of arguments
@cindex macro with variable arguments
@cindex rest argument (in macro)

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:

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

Here @code{args} is a @dfn{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 @code{args}, which is substituted into
the macro body where @code{args} is used.  Thus, we have this expansion:

@example
eprintf ("%s:%d: ", input_file_name, line_number)
@expansion{}
fprintf (stderr, "%s:%d: " , input_file_name, line_number)
@end example

@noindent
Note that the comma after the string constant comes from the definition
of @code{eprintf}, whereas the last comma comes from the value of
@code{args}.

The reason for using @samp{##} is to handle the case when @code{args}
matches no arguments at all.  In this case, @code{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:

@example
fprintf (stderr, "success!\n" , )
@end example

@noindent
which is invalid C syntax.  @samp{##} gets rid of the comma, so we get
the following instead:

@example
fprintf (stderr, "success!\n")
@end example

This is a special feature of the GNU C preprocessor: @samp{##} 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.

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

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

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

struct foo f();

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@noindent
This is equivalent to writing the following:

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

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

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

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

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

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

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

Standard 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 @samp{[@var{index}]} or
@samp{[@var{index}] =} before the element value.  For example,

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

@noindent
is equivalent to

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

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

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

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

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

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

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

@noindent
the following initialization

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

@noindent
is equivalent to

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

Another syntax which has the same meaning is @samp{.@var{fieldname} =}.,
as shown here:

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

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

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

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

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

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

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

@noindent
is equivalent to

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

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

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

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

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

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

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

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

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

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

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

@noindent 
rather than this:

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

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

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

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

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

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

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

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

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

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

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

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

The keyword @code{__attribute__} allows you to specify special
attributes when making a declaration.  This keyword is followed by an
attribute specification inside double parentheses.  Eight attributes,
@code{noreturn}, @code{const}, @code{format}, @code{section},
@code{constructor}, @code{destructor}, @code{unused} and @code{weak} are
currently defined for functions.  Other attributes, including
@code{section} are supported for variables declarations (@pxref{Variable
Attributes}) and for types (@pxref{Type Attributes}).

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

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

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

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

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

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

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

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

@smallexample  
typedef void voidfn ();

volatile voidfn fatal;
@end smallexample

@cindex @code{const} function attribute
@item 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 @code{const}.  For example,

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

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

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

@smallexample
typedef int intfn ();

extern const intfn square;
@end smallexample

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

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

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

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

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

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

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

The @code{format} attribute allows you to identify your own functions
which take format strings as arguments, so that GNU CC can check the
calls to these functions for errors.  The compiler always checks formats
for the ANSI library functions @code{printf}, @code{fprintf},
@code{sprintf}, @code{scanf}, @code{fscanf}, @code{sscanf},
@code{vprintf}, @code{vfprintf} and @code{vsprintf} whenever such
warnings are requested (using @samp{-Wformat}), so there is no need to
modify the header file @file{stdio.h}.

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

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

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

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

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

These attributes are not currently implemented for Objective C.

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

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

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

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

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

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

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

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

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

@cindex @code{#pragma}, reason for not using
@cindex pragma, reason for not using
Some people object to the @code{__attribute__} feature, suggesting that ANSI C's
@code{#pragma} should be used instead.  There are two reasons for not
doing this.

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

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

These two reasons apply to almost any application that might be proposed
for @code{#pragma}.  It is basically a mistake to use @code{#pragma} for
@emph{anything}.

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

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

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

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

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

Suppose the type @code{uid_t} happens to be @code{short}.  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 @code{int}, which does not
match the prototype argument type of @code{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 @code{uid_t} type is @code{short}, @code{int}, or
@code{long}.  Therefore, in cases like these GNU C allows a prototype
to override a later old-style definition.  More precisely, in GNU C, a
function prototype argument type overrides the argument type specified
by a later old-style definition if the former type is the same as the
latter type before promotion.  Thus in GNU C the above example is
equivalent to the following:

@example
int isroot (uid_t);

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

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

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

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

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

In GNU C, you may 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
@w{@samp{-traditional}}.  On a few systems they are allowed by default,
even if you do not use @w{@samp{-traditional}}.  But they are never
allowed if you specify @w{@samp{-ansi}}.

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

@example
#define foo(a) #a
#define lose(b) foo (b)
#define test$
lose (test)
@end example

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

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

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

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

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

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

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

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

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

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

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

The keyword @code{__attribute__} allows you to specify special
attributes of variables or structure fields.  This keyword is followed
by an attribute specification inside double parentheses.  Eight
attributes are currently defined for variables: @code{aligned},
@code{mode}, @code{nocommon}, @code{packed}, @code{section},
@code{transparent_union}, @code{unused}, and @code{weak}.  Other
attributes are available for functions (@pxref{Function Attributes}) and
for types (@pxref{Type Attributes}).

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@item transparent_union
This attribute, attached to a function argument variable which is a
union, means to pass the argument in the same way that the first union
member would be passed.  You can also use this attribute on a
@code{typedef} for a union data type; then it applies to all function
arguments with that type.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@item transparent_union
This attribute, attached to a @code{union} type definition, indicates
that any variable having that union type should, if passed to a
function, be passed in the same way that the first union member would be
passed.  For example:

@example
union foo
@{
  char a;
  int x[2];
@} __attribute__ ((transparent_union));
@end example
@end table

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

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

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

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

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

(If you are writing a header file to be included in ANSI C programs, write
@code{__inline__} instead of @code{inline}.  @xref{Alternate Keywords}.)

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

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

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

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

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

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

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

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

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

In an assembler instruction using @code{asm}, you can 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 @code{fsinx} instruction:

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

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

Each operand is described by an operand-constraint string followed by the C
expression in parentheses.  A colon separates the assembler template from
the first output operand, and another separates the last output operand
from the first input, if any.  Commas separate 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 @code{asm} feature is most often used for machine instructions
that the compiler itself does not know exist.  If the output expression
cannot be directly addressed (for example, it is a bit field), your
constraint must allow a register.  In that case, GNU CC will use
the register as the output of the @code{asm}, and then store that
register into the output.

The 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 @samp{+}, 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) @samp{combine} instruction with @code{bar} as its read-only
source operand and @code{foo} as its read-write destination:

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

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

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

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

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

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

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

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 @samp{%}; to produce one
@samp{%} in the assembler code, you must write @samp{%%} in the input.

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

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

You can put multiple assembler instructions together in a single @code{asm}
template, separated either with newlines (written as @samp{\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
@code{_foo} accepts arguments in registers 9 and 10:

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

Unless an output operand has the @samp{&} constraint modifier, GNU CC may
allocate it in the same register as an unrelated input operand, on the
assumption 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 @samp{&} for each output
operand that may not overlap an input.
@xref{Modifiers}.

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

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

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

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

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

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

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

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

If an @code{asm} has output operands, GNU CC assumes for optimization
purposes 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 @code{asm} instruction from being deleted, moved
significantly, or combined, by writing the keyword @code{volatile} after
the @code{asm}.  For example:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

The option @samp{-traditional} disables certain keywords; @samp{-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 @code{asm}, @code{typeof} and @code{inline} cannot be
used since they won't work in a program compiled with @samp{-ansi}, while
the keywords @code{const}, @code{volatile}, @code{signed}, @code{typeof}
and @code{inline} won't work in a program compiled with
@samp{-traditional}.@refill

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

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

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

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

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

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

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

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

This extension is not supported by GNU C++.

@node Function Names
@section Function Names as Strings

GNU CC predefines two string variables to be the name of the current function.
The variable @code{__FUNCTION__} is the name of the function as it appears
in the source.  The variable @code{__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:

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

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

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

@noindent
gives this output:

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

These names are not macros: they are predefined string variables.
For example, @samp{#ifdef __FUNCTION__} does not have any special
meaning inside a function, since the preprocessor does not do anything
special with the identifier @code{__FUNCTION__}.

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

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

@menu
* 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.
* Template Instantiation:: Methods for ensuring that exactly one copy of
                         each needed template instantiation is emitted.
* C++ Signatures::      You can specify abstract types to get subtype
                         polymorphism independent from inheritance.
@end menu

@node Naming Results
@section Named Return Values in C++

@cindex @code{return}, in C++ function header
@cindex return value, named, in C++
@cindex named return value in C++
@cindex C++ named return value
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:

@example
@group
@var{type}
@var{functionname} (@var{args}) return @var{resultname};
@{ 
  @dots{}
  @var{body}
  @dots{}
@}
@end group
@end example

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

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

@cindex implicit argument: return value
Although @code{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 @code{v} is sent in as the implicit argument.
Then @code{b} is constructed and its @code{a} field is set to the value
23.  Finally, a copy constructor (a constructor of the form @samp{X(X&)})
is applied to @code{b}, with the (implicit) return value location as the
target, so that @code{v} is now bound to the return value.

But this is wasteful.  The local @code{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 @code{r} as your return value
at the outset, and assigning to its @code{a} field directly:

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

@noindent
The declaration of @code{r} is a standard, proper declaration, whose effects
are executed @strong{before} any of the body of @code{m}.

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

@example
X
m () return r (23);
@{
  return; 
@}
@end example

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

@example
X
m () return r;
@{
  X b;
  return b; 
@}
@end example

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

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

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

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

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

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

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

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

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

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

@node Destructors and Goto
@section @code{goto} and Destructors in GNU C++

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

@cindex constructors vs @code{goto}
The compiler still forbids using @code{goto} to @emph{enter} a scope
that requires constructors.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

@cindex template instantiation

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

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

@item Cfront model
The AT&T C++ translator, Cfront, solved the template instantiation
problem by creating the notion of a template repository, an
automatically maintained place where template instances are stored.  As
individual object files are built, notes are placed in the repository to
record where templates and potential type arguments were seen so that
the subsequent instantiation step knows where to find them.  At link
time, any needed instances are generated and linked in.  The advantages
of this model are more optimal compilation speed and the ability to use
the system linker; to implement the Borland model a compiler vendor also
needs to replace the linker.  The disadvantages are vastly increased
complexity, and thus potential for error; theoretically, this should be
just as transparent, but in practice it has been very difficult to build
multiple programs in one directory and one program in multiple
directories using Cfront.  Code written for this model tends to separate
definitions of non-inline member templates into a separate file, which
is magically found by the link preprocessor when a template needs to be
instantiated.
@end table

Currently, g++ implements neither automatic model.  In the mean time,
you have three options for dealing with template instantiations:

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

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

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

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

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

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

@item
Explicitly instantiate all the template instances you use, and compile
with -fno-implicit-templates.  This is probably your best bet; it may
require more knowledge of exactly which templates you are using, but
it's less mysterious than the previous approach, and it doesn't require
any @samp{#pragma}s or other g++-specific code.  You can scatter the
instantiations throughout your program, you can create one big file to
do all the instantiations, or you can create tiny files like

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

template class Foo<int>;
@end example

for each instance you need, and create a template instantiation library
from those.  I'm partial to the last, but your mileage may vary.  If you
are using Cfront-model code, you can probably get away with not using
-fno-implicit-templates when compiling files that don't @samp{#include}
the member template definitions.
@end enumerate

@node C++ Signatures
@section Type Abstraction using Signatures

@findex signature
@cindex type abstraction, C++
@cindex C++ type abstraction
@cindex subtype polymorphism, C++
@cindex C++ subtype polymorphism
@cindex signatures, C++
@cindex C++ signatures

In GNU C++, you can use the keyword @code{signature} to define a
completely abstract class interface as a datatype.  You can connect this
abstraction with actual classes using signature pointers.  If you want
to use signatures, run the GNU compiler with the
@samp{-fhandle-signatures} command-line option.  (With this option, the
compiler reserves a second keyword @code{sigof} as well, for a future
extension.)

Roughly, signatures are type abstractions or interfaces of classes.
Some other languages have similar facilities.  C++ signatures are
related to ML's signatures, Haskell's type classes, definition modules
in Modula-2, interface modules in Modula-3, abstract types in Emerald,
type modules in Trellis/Owl, categories in Scratchpad II, and types in
POOL-I.  For a more detailed discussion of signatures, see
@cite{Signatures: A Language Extension for Improving Type Abstraction and
Subtype Polymorphism in C++}
by @w{Gerald} Baumgartner and Vincent F. Russo (Tech report
CSD--TR--95--051, Dept. of Computer Sciences, Purdue University,
August 1995, a slightly improved version appeared in
@emph{Software---Practice & Experience}, @b{25}(8), pp. 863--889,
August 1995).  You can get the tech report by anonymous FTP from
@code{ftp.cs.purdue.edu} in @file{pub/gb/Signature-design.ps.gz}.

Syntactically, a signature declaration is a collection of
member function declarations and nested type declarations.
For example, this signature declaration defines a new abstract type
@code{S} with member functions @samp{int foo ()} and @samp{int bar (int)}:

@example
signature S
@{
  int foo ();
  int bar (int);
@};
@end example

Since signature types do not include implementation definitions, you
cannot write an instance of a signature directly.  Instead, you can
define a pointer to any class that contains the required interfaces as a
@dfn{signature pointer}.  Such a class @dfn{implements} the signature
type.
@c Eventually signature references should work too.

To use a class as an implementation of @code{S}, you must ensure that
the class has public member functions @samp{int foo ()} and @samp{int
bar (int)}.  The class can have other member functions as well, public
or not; as long as it offers what's declared in the signature, it is
suitable as an implementation of that signature type.

For example, suppose that @code{C} is a class that meets the
requirements of signature @code{S} (@code{C} @dfn{conforms to}
@code{S}).  Then

@example
C obj;
S * p = &obj;
@end example

@noindent
defines a signature pointer @code{p} and initializes it to point to an
object of type @code{C}.  
The member function call @w{@samp{int i = p->foo ();}}
executes @samp{obj.foo ()}.

@cindex @code{signature} in C++, advantages
Abstract virtual classes provide somewhat similar facilities in standard
C++.  There are two main advantages to using signatures instead:

@enumerate
@item
Subtyping becomes independent from inheritance.  A class or signature
type @code{T} is a subtype of a signature type @code{S} independent of
any inheritance hierarchy as long as all the member functions declared
in @code{S} are also found in @code{T}.  So you can define a subtype
hierarchy that is completely independent from any inheritance
(implementation) hierarchy, instead of being forced to use types that
mirror the class inheritance hierarchy.

@item
Signatures allow you to work with existing class hierarchies as
implementations of a signature type.  If those class hierarchies are
only available in compiled form, you're out of luck with abstract virtual
classes, since an abstract virtual class cannot be retrofitted on top of
existing class hierarchies.  So you would be required to write interface
classes as subtypes of the abstract virtual class.
@end enumerate

@cindex default implementation, signature member function
@cindex signature member function default implementation
There is one more detail about signatures.  A signature declaration can
contain member function @emph{definitions} as well as member function
declarations.  A signature member function with a full definition is
called a @emph{default implementation}; classes need not contain that
particular interface in order to conform.  For example, a
class @code{C} can conform to the signature

@example
signature T
@{
  int f (int);
  int f0 () @{ return f (0); @};
@};
@end example

@noindent
whether or not @code{C} implements the member function @samp{int f0 ()}.
If you define @code{C::f0}, that definition takes precedence;
otherwise, the default implementation @code{S::f0} applies.

@ignore
There will be more support for signatures in the future.
Add to this doc as the implementation grows.
In particular, the following features are planned but not yet
implemented:
@itemize @bullet
@item signature references,
@item signature inheritance,
@item the @code{sigof} construct for extracting the signature information
      of a class,
@item views for renaming member functions when matching a class type
      with a signature type,
@item specifying exceptions with signature member functions, and
@item signature templates.
@end itemize
This list is roughly in the order in which we intend to implement
them.  Watch this space for updates.
@end ignore