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NAME | SYNOPSIS | DESCRIPTION | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | EXAMPLE | COLOPHON |
STDARG(3) Linux Programmer's Manual STDARG(3)
stdarg, va_start, va_arg, va_end, va_copy - variable argument lists
#include <stdarg.h>
void va_start(va_list ap, last);
type va_arg(va_list ap, type);
void va_end(va_list ap);
void va_copy(va_list dest, va_list src);
A function may be called with a varying number of arguments of
varying types. The include file <stdarg.h> declares a type va_list
and defines three macros for stepping through a list of arguments
whose number and types are not known to the called function.
The called function must declare an object of type va_list which is
used by the macros va_start(), va_arg(), and va_end().
va_start()
The va_start() macro initializes ap for subsequent use by va_arg()
and va_end(), and must be called first.
The argument last is the name of the last argument before the
variable argument list, that is, the last argument of which the
calling function knows the type.
Because the address of this argument may be used in the va_start()
macro, it should not be declared as a register variable, or as a
function or an array type.
va_arg()
The va_arg() macro expands to an expression that has the type and
value of the next argument in the call. The argument ap is the
va_list ap initialized by va_start(). Each call to va_arg() modifies
ap so that the next call returns the next argument. The argument
type is a type name specified so that the type of a pointer to an
object that has the specified type can be obtained simply by adding a
* to type.
The first use of the va_arg() macro after that of the va_start()
macro returns the argument after last. Successive invocations return
the values of the remaining arguments.
If there is no next argument, or if type is not compatible with the
type of the actual next argument (as promoted according to the
default argument promotions), random errors will occur.
If ap is passed to a function that uses va_arg(ap,type), then the
value of ap is undefined after the return of that function.
va_end()
Each invocation of va_start() must be matched by a corresponding
invocation of va_end() in the same function. After the call
va_end(ap) the variable ap is undefined. Multiple traversals of the
list, each bracketed by va_start() and va_end() are possible.
va_end() may be a macro or a function.
va_copy()
The va_copy() macro copies the (previously initialized) variable
argument list src to dest. The behavior is as if va_start() were
applied to dest with the same last argument, followed by the same
number of va_arg() invocations that was used to reach the current
state of src.
An obvious implementation would have a va_list be a pointer to the
stack frame of the variadic function. In such a setup (by far the
most common) there seems nothing against an assignment
va_list aq = ap;
Unfortunately, there are also systems that make it an array of
pointers (of length 1), and there one needs
va_list aq;
*aq = *ap;
Finally, on systems where arguments are passed in registers, it may
be necessary for va_start() to allocate memory, store the arguments
there, and also an indication of which argument is next, so that
va_arg() can step through the list. Now va_end() can free the
allocated memory again. To accommodate this situation, C99 adds a
macro va_copy(), so that the above assignment can be replaced by
va_list aq;
va_copy(aq, ap);
...
va_end(aq);
Each invocation of va_copy() must be matched by a corresponding
invocation of va_end() in the same function. Some systems that do
not supply va_copy() have __va_copy instead, since that was the name
used in the draft proposal.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────┬───────────────┬─────────────────┐
│Interface │ Attribute │ Value │
├──────────────────────┼───────────────┼─────────────────┤
│va_start(), va_end(), │ Thread safety │ MT-Safe │
│va_copy() │ │ │
├──────────────────────┼───────────────┼─────────────────┤
│va_arg() │ Thread safety │ MT-Safe race:ap │
└──────────────────────┴───────────────┴─────────────────┘
The va_start(), va_arg(), and va_end() macros conform to C89. C99
defines the va_copy() macro.
These macros are not compatible with the historic macros they
replace. A backward-compatible version can be found in the include
file <varargs.h>.
The historic setup is:
#include <varargs.h>
void
foo(va_alist)
va_dcl
{
va_list ap;
va_start(ap);
while (...) {
...
x = va_arg(ap, type);
...
}
va_end(ap);
}
On some systems, va_end contains a closing '}' matching a '{' in
va_start, so that both macros must occur in the same function, and in
a way that allows this.
Unlike the varargs macros, the stdarg macros do not permit
programmers to code a function with no fixed arguments. This problem
generates work mainly when converting varargs code to stdarg code,
but it also creates difficulties for variadic functions that wish to
pass all of their arguments on to a function that takes a va_list
argument, such as vfprintf(3).
The function foo takes a string of format characters and prints out
the argument associated with each format character based on the type.
#include <stdio.h>
#include <stdarg.h>
void
foo(char *fmt, ...)
{
va_list ap;
int d;
char c, *s;
va_start(ap, fmt);
while (*fmt)
switch (*fmt++) {
case 's': /* string */
s = va_arg(ap, char *);
printf("string %s\n", s);
break;
case 'd': /* int */
d = va_arg(ap, int);
printf("int %d\n", d);
break;
case 'c': /* char */
/* need a cast here since va_arg only
takes fully promoted types */
c = (char) va_arg(ap, int);
printf("char %c\n", c);
break;
}
va_end(ap);
}
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
2015-03-02 STDARG(3)
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