Originální popis anglicky:
stdarg.h - handle variable argument list
Návod, kniha: POSIX Programmer's Manual
#include <stdarg.h>
void va_start(va_list
ap,
argN);
void va_copy(va_list
dest, va_list
src );
type va_arg(va_list
ap,
type);
void va_end(va_list
ap);
The
<stdarg.h> header shall contain a set of macros which allows
portable functions that accept variable argument lists to be written.
Functions that have variable argument lists (such as
printf()) but do
not use these macros are inherently non-portable, as different systems use
different argument-passing conventions.
The type
va_list shall be defined for variables used to traverse the
list.
The
va_start() macro is invoked to initialize
ap to the beginning
of the list before any calls to
va_arg().
The
va_copy() macro initializes
dest as a copy of
src, as
if the
va_start() macro had been applied to
dest followed by the
same sequence of uses of the
va_arg() macro as had previously been used
to reach the present state of
src. Neither the
va_copy() nor
va_start() macro shall be invoked to reinitialize
dest without
an intervening invocation of the
va_end() macro for the same
dest.
The object
ap may be passed as an argument to another function; if that
function invokes the
va_arg() macro with parameter
ap, the value
of
ap in the calling function is unspecified and shall be passed to the
va_end() macro prior to any further reference to
ap. The
parameter
argN is the identifier of the rightmost parameter in the
variable parameter list in the function definition (the one just before the
...). If the parameter
argN is declared with the
register
storage class, with a function type or array type, or with a type that is not
compatible with the type that results after application of the default
argument promotions, the behavior is undefined.
The
va_arg() macro shall return the next argument in the list pointed to
by
ap. Each invocation of
va_arg() modifies
ap so that
the values of successive arguments are returned in turn. The
type
parameter shall be a type name specified such that the type of a pointer to an
object that has the specified type can be obtained simply by postfixing a
'*' to type. If there is no actual next argument, or if
type is
not compatible with the type of the actual next argument (as promoted
according to the default argument promotions), the behavior is undefined,
except for the following cases:
- *
- One type is a signed integer type, the other type is the
corresponding unsigned integer type, and the value is representable in
both types.
- *
- One type is a pointer to void and the other is a
pointer to a character type.
- *
- Both types are pointers.
Different types can be mixed, but it is up to the routine to know what type of
argument is expected.
The
va_end() macro is used to clean up; it invalidates
ap for use
(unless
va_start() or
va_copy() is invoked again).
Each invocation of the
va_start() and
va_copy() macros shall be
matched by a corresponding invocation of the
va_end() macro in the same
function.
Multiple traversals, each bracketed by
va_start() ...
va_end(),
are possible.
This example is a possible implementation of
execl():
#include <stdarg.h>
#define MAXARGS 31
/*
* execl is called by
* execl(file, arg1, arg2, ..., (char *)(0));
*/
int execl(const char *file, const char *args, ...)
{
va_list ap;
char *array[MAXARGS +1];
int argno = 0;
va_start(ap, args);
while (args != 0 && argno < MAXARGS)
{
array[argno++] = args;
args = va_arg(ap, const char *);
}
array[argno] = (char *) 0;
va_end(ap);
return execv(file, array);
}
The following sections are informative.
It is up to the calling routine to communicate to the called routine how many
arguments there are, since it is not always possible for the called routine to
determine this in any other way. For example,
execl() is passed a null
pointer to signal the end of the list. The
printf() function can tell
how many arguments are there by the
format argument.
None.
None.
The System Interfaces volume of IEEE Std 1003.1-2001,
exec,
printf()
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
