extend.texi

gcc库的原代码,对编程有很大帮助.

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.