varargs

STDARG(3)                 BSD Library Functions Manual                 STDARG(3)

NAME
     stdarg β€” 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
     four 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 initializes ap for subsequent use by va_arg(),
     va_copy(), and va_end(), and must be called first.

     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() or va_copy().  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
     specified 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 argument
     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 values
     of the remaining arguments.

     The va_copy() macro copies a variable argument list, previously initialized
     by va_start(), from src to dest.  The state is preserved such that it is
     equivalent to calling va_start() with the same second argument used with
     src, and calling va_arg() the same number of times as called with src.

     The va_end() macro cleans up any state associated with the variable
     argument list ap.

     Each invocation of va_start() or va_copy() must be paired with a
     corresponding invocation of va_end() in the same function.

RETURN VALUES
     The va_arg() macro returns the value of the next argument.

     The va_start(), va_copy(), and va_end() macros return no value.

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;
                   int d;
                   char c, *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 */
                                   /* Note: char is promoted to int. */
                                   c = va_arg(ap, int);
                                   printf("char %c\n", c);
                                   break;
                           }
                   va_end(ap);
           }

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 (β€œISO 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
     arguments on to a function that takes a va_list argument, such as
     vfprintf(3).

BSD                             October 25, 2002                             BSD