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stdarg(3)                BSD Library Functions Manual                stdarg(3)


NAME

     stdarg -- variable argument lists


SYNOPSIS

     #include <stdarg.h>

     void
     va_start(va_list ap, last);

     type
     va_arg(va_list ap, type);

     void
     va_copy(va_list dest, va_list src);

     void
     va_end(va_list ap);


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(), va_copy(), and va_end().

     The va_start() macro must be called first, and it initializes ap, which
     can be passed to va_arg() for each argument to be processed.  Calling
     va_end() signals that there are no further arguments, and causes ap to be
     invalidated.  Note that each call to va_start() must be matched by a call
     to va_end(), from within the same function.

     The parameter last is the name of the last parameter before the variable
     argument list, i.e., the last parameter of which the calling function
     knows the type.

     Because the address of this parameter is 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 parameter 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 parameter type is a type name
     specified so that the type of a pointer to an object that has the speci-
     fied type can be obtained simply by adding a * to type.

     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 argu-
     ment promotions), random errors will occur.

     The first use of the va_arg() macro after that of the va_start() macro
     returns the argument after last.  Successive invocations return the val-
     ues of the remaining arguments.

     The va_copy() macro copies the state of the variable argument list, src,
     previously initialized by va_start(), to the variable argument list,
     dest, which must not have been previously initialized by va_start(),
     without an intervening call to va_end().  The state preserved in dest is
     equivalent to calling va_start() and va_arg() on dest in the same way as
     was used on src.  The copied variable argument list can subsequently be
     passed to va_arg(), and must finally be passed to va_end() when through
     with it.

     After a variable argument list is invalidated by va_end(), it can be
     reinitialized with va_start() or made a copy of another variable argument
     list with va_copy().


EXAMPLES

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

           void foo(char *fmt, ...)
           {
                   va_list ap, ap2;
                   int d;
                   char c, *s;

                   va_start(ap, fmt);
                   va_copy(ap2, ap);
                   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 */
                                   /* Note: char is promoted to int. */
                                   c = va_arg(ap, int);
                                   printf("char %c\n", c);
                                   break;
                           }
                   va_end(ap);
                   ...
                   /* use ap2 to iterate over the arguments again */
                   ...
                   va_end(ap2);
           }


COMPATIBILITY

     These macros are not compatible with the historic macros they replace.  A
     backward compatible version can be found in the include file <varargs.h>.


STANDARDS

     The va_start(), va_arg(), va_copy(), and va_end() macros conform to
     ISO/IEC 9899:1999 (``ISO C99'').


BUGS

     Unlike the 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 argu-
     ments on to a function that takes a va_list argument, such as
     vfprintf(3).

BSD                            October 25, 2002                            BSD

Mac OS X 10.8 - Generated Fri Aug 31 05:33:11 CDT 2012