man:va_start
STDARG(3) Linux Programmer's Manual STDARG(3)
NAME
stdarg, va_start, va_arg, va_end, va_copy - variable argument lists
SYNOPSIS
#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);
DESCRIPTION
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 func-
tion 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 invo-
cation of va_end() in the same function. After the call va_end(ap) the
variable ap is undefined. Multiple traversals of the list, each brack-
eted 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 argu-
ment 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 invoca-
tion 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.
ATTRIBUTES
For an explanation of the terms used in this section, see
attributes(7).
allbox; lbw21 lb lb l l l. Interface Attribute Value T{ va_start(),
va_end(), va_copy() T} Thread safety MT-Safe T{ va_arg() T} Thread
safety MT-Safe race:ap
CONFORMING TO
The va_start(), va_arg(), and va_end() macros conform to C89. C99
defines the va_copy() macro.
NOTES
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.
BUGS
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).
EXAMPLE
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); }
COLOPHON
This page is part of release 4.16 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/.
2017-09-15 STDARG(3)
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