stdarg.h(3)                POSIX Programmer's Manual               stdarg.h(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       stdarg.h -- handle variable argument list

SYNOPSIS
       #include <stdarg.h>

       void va_start(va_list ap, argN);
       void va_copy(va_list dest, va_list src);
       type va_arg(va_list ap, type);
       void va_end(va_list ap);

DESCRIPTION
       The functionality described on this reference page is aligned with the
       ISO C standard. Any conflict between the requirements described here
       and the ISO C standard is unintentional. This volume of POSIX.1-2017
       defers to the ISO C standard.

       The <stdarg.h> header shall contain a set of macros which allows
       portable functions that accept variable argument lists to be written.
       Functions that have variable argument lists (such as printf()) but do
       not use these macros are inherently non-portable, as different systems
       use different argument-passing conventions.

       The <stdarg.h> header shall define the va_list type for variables used
       to traverse the list.

       The va__start() macro is invoked to initialize ap to the beginning of
       the list before any calls to va__arg().

       The va__copy() macro initializes dest as a copy of src, as if the
       va__start() macro had been applied to dest followed by the same sequence
       of uses of the va__arg() macro as had previously been used to reach the
       present state of src.  Neither the va__copy() nor va__start() macro shall
       be invoked to reinitialize dest without an intervening invocation of
       the va__end() macro for the same dest.

       The object ap may be passed as an argument to another function; if that
       function invokes the va__arg() macro with parameter ap, the value of ap
       in the calling function is unspecified and shall be passed to the
       va__end() macro prior to any further reference to ap.  The parameter
       argN is the identifier of the rightmost parameter in the variable
       parameter list in the function definition (the one just before the
       ...). If the parameter argN is declared with the register storage
       class, with a function type or array type, or with a type that is not
       compatible with the type that results after application of the default
       argument promotions, the behavior is undefined.

       The va__arg() macro shall return the next argument in the list pointed
       to by ap.  Each invocation of va__arg() modifies ap so that the values
       of successive arguments are returned in turn. The type parameter shall
       be a type name specified such that the type of a pointer to an object
       that has the specified type can be obtained simply by postfixing a ''**''
       to type. If there is no actual next argument, or if type is not
       compatible with the type of the actual next argument (as promoted
       according to the default argument promotions), the behavior is
       undefined, except for the following cases:

        *  One type is a signed integer type, the other type is the
           corresponding unsigned integer type, and the value is representable
           in both types.

        *  One type is a pointer to void and the other is a pointer to a
           character type.

        *  Both types are pointers.

       Different types can be mixed, but it is up to the routine to know what
       type of argument is expected.

       The va__end() macro is used to clean up; it invalidates ap for use
       (unless va__start() or va__copy() is invoked again).

       Each invocation of the va__start() and va__copy() macros shall be matched
       by a corresponding invocation of the va__end() macro in the same
       function.

       Multiple traversals, each bracketed by va__start() ...  va__end(), are
       possible.

       The following sections are informative.

EXAMPLES
       This example is a possible implementation of execl():


           #include <stdarg.h>

           #define  MAXARGS     31

           /*
            * execl is called by
            * execl(file, arg1, arg2, ..., (char *)(0));
            */
           int execl(const char *file, const char *args, ...)
           {
               va_list ap;
               char *array[MAXARGS +1];
               int argno = 0;

               va_start(ap, args);
               while (args != 0 && argno < MAXARGS)
               {
                   array[argno++] = args;
                   args = va_arg(ap, const char *);
               }
               array[argno] = (char *) 0;
               va_end(ap);
               return execv(file, array);
           }

APPLICATION USAGE
       It is up to the calling routine to communicate to the called routine
       how many arguments there are, since it is not always possible for the
       called routine to determine this in any other way. For example, execl()
       is passed a null pointer to signal the end of the list. The printf()
       function can tell how many arguments are there by the format argument.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       The System Interfaces volume of POSIX.1-2017, exec, fprintf()

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                          stdarg.h(3)
