stdarg(3) - Linux manual page (original) (raw)
stdarg(3) Library Functions Manual stdarg(3)
NAME top
stdarg, va_start, va_arg, va_end, va_copy - variable argument
listsLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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 top
A function may be called with a varying number of arguments of
varying types. The include file _<stdarg.h>_ declares a type
_valist_ 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 _valist_ 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 valist 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 _valist_ 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.ATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌──────────────────────────────┬───────────────┬─────────────────┐
│ **Interface** │ **Attribute** │ **Value** │
├──────────────────────────────┼───────────────┼─────────────────┤
│ **va_start**(), **va_end**(), │ Thread safety │ MT-Safe │
│ **va_copy**() │ │ │
├──────────────────────────────┼───────────────┼─────────────────┤
│ **va_arg**() │ Thread safety │ MT-Safe race:ap │
└──────────────────────────────┴───────────────┴─────────────────┘STANDARDS top
C11, POSIX.1-2008.HISTORY top
**va_start**()
**va_arg**()
**va_end**()
C89, POSIX.1-2001.
**va_copy**()
C99, POSIX.1-2001.CAVEATS top
Unlike the historical **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 _valist_ argument, such as [vfprintf(3)](../man3/vfprintf.3.html).EXAMPLES top
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, ...) /* '...' is C syntax for a variadic function */
{
va_list ap;
int d;
char c;
char *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);
}SEE ALSO top
[vprintf(3)](../man3/vprintf.3.html), [vscanf(3)](../man3/vscanf.3.html), [vsyslog(3)](../man3/vsyslog.3.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 stdarg(3)
Pages that refer to this page:pam_error(3), pam_info(3), pam_syslog(3), printf(3), scanf(3), sd_bus_error(3), sd_journal_print(3), syslog(3), va_list(3type)