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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | ATTRIBUTES | STANDARDS | HISTORY | CAVEATS | EXAMPLES | SEE ALSO | COLOPHON |
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stdarg(3) Library Functions Manual stdarg(3)
stdarg, va_start, va_arg, va_end, va_copy - variable argument
lists
Standard C library (libc, -lc)
#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 │
└─────────────────────────────┴───────────────┴─────────────────┘
C11, POSIX.1-2008.
va_start()
va_arg()
va_end()
C89, POSIX.1-2001.
va_copy()
C99, POSIX.1-2001.
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 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, ...) /* '...' 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);
}
vprintf(3), vscanf(3), vsyslog(3)
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⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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man-pages@man7.org
Linux man-pages 6.9.1 2024-06-15 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)
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HTML rendering created 2024-06-26 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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