STDARG(3)                 (2020-11-01)                  STDARG(3)

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

     SYNOPSIS
          #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
          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 argu-
          ment 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 speci-
          fied 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

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          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 corre-
          sponding 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 func-
          tion.

        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 argu-
          ment is next, so that va_arg() can step through the list.
          Now va_end() can free the allocated memory again.  To accom-
          modate 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 corre-
          sponding 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.

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     ATTRIBUTES
          For an explanation of the terms used in this section, see
          attributes(7).  allbox; lbw21 lb lb l l l.
          Interface Attribute Value T{ va_start(), va_end(), va_copy()
          T}   Thread safety  MT-Safe T{ va_arg() T}   Thread
          safety  MT-Safe race:ap

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

     BUGS
          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 convert-
          ing varargs code to stdarg code, but it also creates diffi-
          culties for variadic functions that wish to pass all of
          their arguments on to a function that takes a va_list argu-
          ment, such as vfprintf(3).

     EXAMPLES
          The function foo takes a string of format characters and
          prints out the argument associated with each format charac-
          ter based on the type.

          #include <stdio.h>
          #include <stdarg.h>

          void
          foo(char *fmt, ...)   /* aq...aq 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 aqsaq:              /* string */
                      s = va_arg(ap, char *);
                      printf("string %s\n", s);
                      break;
                  case aqdaq:              /* int */
                      d = va_arg(ap, int);
                      printf("int %d\n", d);
                      break;
                  case aqcaq:              /* char */
                      /* need a cast here since va_arg only
                         takes fully promoted types */
                      c = (char) va_arg(ap, int);

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                      printf("char %c\n", c);
                      break;
                  }
              va_end(ap);
          }

     SEE ALSO
          vprintf(3), vscanf(3), vsyslog(3)

     COLOPHON
          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/.

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