stdarg manpage

Search topic Section

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().

       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.

       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.

       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.

       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);

       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.

       For  an	explanation  of	 the  terms  used   in	 this	section,   see

       |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

       The historic setup is:

	   #include <varargs.h>

	       va_list ap;

	       while (...) {
		   x = va_arg(ap, type);

       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

       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>

       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);
	       case 'd':	      /* int */
		   d = va_arg(ap, int);
		   printf("int %d\n", d);
	       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);

       This  page  is  part of release 4.10 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

				  2015-03-02			     STDARG(3)