extend.texi

gcc库的原代码,对编程有很大帮助.

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