extend.texi

早期freebsd实现

@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{##} adjacent
to a rest argument discards the token on the other side of the
@samp{##}, if the rest argument value is empty.

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

struct foo f();

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

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.

To specify an array index, write @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.

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

You can also use an element label 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 like any other cast, 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.

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 @code{volatile} applied to function
@cindex @code{const} applied to function

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

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{volatile} to tell the compiler this fact.  For example,

@example
extern void volatile fatal ();

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

The @code{volatile} keyword tells the compiler to assume that
@code{fatal} cannot return.  This makes slightly better code, but more
importantly it helps avoid spurious warnings of uninitialized variables.

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

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
@code{const}.  For example,

@example
extern int const square ();
@end example

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

@cindex pointer arguments
Note that a function that has pointer arguments and examines the data