stdarg(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | ATTRIBUTES | CONFORMING TO | BUGS | EXAMPLES | SEE ALSO | COLOPHON

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().

   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.

ATTRIBUTES         top

       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 │
       └──────────────────────┴───────────────┴─────────────────┘

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>

       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), vscanf(3), vsyslog(3)

COLOPHON         top

       This page is part of release 5.10 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
       https://www.kernel.org/doc/man-pages/.

                               2020-11-01                      STDARG(3)

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