extend.texi

早期freebsd实现

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

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

@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.
* 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}.
* 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.
* Dollar Signs::        Dollar sign is allowed in identifiers.
* Character Escapes::   @samp{\e} stands for the character @key{ESC}.
* Variable Attributes::	Specifying attributes of variables.
* 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.
@end menu

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

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

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

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

  return square (a) + square (b);
@}
@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

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.

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 the file
@file{pub/tmb/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
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 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 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