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NAME |LIBRARY |SYNOPSIS |DESCRIPTION |ATTRIBUTES |STANDARDS |HISTORY |CAVEATS |EXAMPLES |SEE ALSO |COLOPHON | |
stdarg(3) Library Functions Manualstdarg(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_listap,last);typeva_arg(va_listap,type);void va_end(va_listap);void va_copy(va_listdest, va_listsrc);
A function may be called with a varying number of arguments of varying types. The include file<stdarg.h> declares a typeva_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 typeva_list which is used by the macrosva_start(),va_arg(), andva_end().va_start() Theva_start() macro initializesap for subsequent use byva_arg() andva_end(), and must be called first. The argumentlast 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 theva_start() macro, it should not be declared as a register variable, or as a function or an array type.va_arg() Theva_arg() macro expands to an expression that has the type and value of the next argument in the call. The argumentap is theva_list ap initialized byva_start(). Each call tova_arg() modifiesap so that the next call returns the next argument. The argumenttype 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 * totype. The first use of theva_arg() macro after that of theva_start() macro returns the argument afterlast. Successive invocations return the values of the remaining arguments. If there is no next argument, or iftype is not compatible with the type of the actual next argument (as promoted according to the default argument promotions), random errors will occur. Ifap is passed to a function that usesva_arg(ap,type),then the value ofap is undefined after the return of that function.va_end() Each invocation ofva_start() must be matched by a corresponding invocation ofva_end() in the same function. After the callva_end(ap)the variableap is undefined. Multiple traversals of the list, each bracketed byva_start() andva_end() are possible.va_end() may be a macro or a function.va_copy() Theva_copy() macro copies the (previously initialized) variable argument listsrc todest. The behavior is as ifva_start() were applied todest with the samelast argument, followed by the same number ofva_arg() invocations that was used to reach the current state ofsrc. An obvious implementation would have ava_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 forva_start() to allocate memory, store the arguments there, and also an indication of which argument is next, so thatva_arg() can step through the list. Nowva_end() can free the allocated memory again. To accommodate this situation, C99 adds a macrova_copy(), so that the above assignment can be replaced by va_list aq; va_copy(aq, ap); ... va_end(aq); Each invocation ofva_copy() must be matched by a corresponding invocation ofva_end() in the same function. Some systems that do not supplyva_copy() have__va_copyinstead, since that was the name used in the draft proposal.
For an explanation of the terms used in this section, seeattributes(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 historicalvarargsmacros, thestdargmacros do not permit programmers to code a function with no fixed arguments. This problem generates work mainly when convertingvarargscode tostdargcode, but it also creates difficulties for variadic functions that wish to pass all of their arguments on to a function that takes ava_list argument, such asvfprintf(3).
The functionfoo 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)
This page is part of theman-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which isnot part of the original manual page), send a mail to man-pages@man7.orgLinux man-pages 6.15 2025-05-17stdarg(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)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |