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PROLOG | NAME | SYNOPSIS | DESCRIPTION | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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stdarg.h(0P) POSIX Programmer's Manual stdarg.h(0P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
stdarg.h — handle variable argument list
#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 functionality described on this reference page is aligned with
the ISO C standard. Any conflict between the requirements
described here and the ISO C standard is unintentional. This
volume of POSIX.1‐2017 defers to the ISO C standard.
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 <stdarg.h> header shall define the va_list type 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.
The following sections are informative.
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);
}
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 POSIX.1‐2017, exec(1p),
fprintf(3p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 stdarg.h(0P)
Pages that refer to this page: stdio.h(0p), wchar.h(0p), wctype.h(0p), va_arg(3p), vfprintf(3p), vfscanf(3p), vfwprintf(3p), vfwscanf(3p)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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