va_copy(3) — Linux manual page


STDARG(3)                 Linux Programmer's Manual                STDARG(3)

NAME         top

       stdarg, va_start, va_arg, va_end, va_copy - variable argument lists

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 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
       function 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
       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.

       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 point‐
       ers (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 allo‐
       cated 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 invo‐
       cation 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

       │Interface             Attribute     Value           │
       │va_start(), va_end(), │ Thread safety │ MT-Safe         │
       │va_copy()             │               │                 │
       │va_arg()              │ Thread safety │ MT-Safe race:ap │

CONFORMING TO         top

       The va_start(), va_arg(), and va_end() macros conform to C89.  C99
       defines the va_copy() macro.

BUGS         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 va_list
       argument, such as vfprintf(3).

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>

       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);
               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);

SEE ALSO         top

       vprintf(3), vscanf(3), vsyslog(3)

COLOPHON         top

       This page is part of release 5.09 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

                                 2020-11-01                        STDARG(3)

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