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 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 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
       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.

       For an explanation of the terms used in this section, see attributes(7).

       │Interface                             Attribute     Value           │
       │va_start(), va_end(), va_copy()       │ Thread safety │ MT-Safe         │
       │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.

       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

       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);

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

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                                   2021-03-22                          STDARG(3)