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

NAME
     ssttddaarrgg - variable argument lists

SYNOPSIS
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     void
     vvaassttaarrtt(valist ap, last);

     type
     vvaaaarrgg(valist ap, type);

     void
     vvaaeenndd(valist ap);

DESCRIPTION
     A function may be called with a varying number of arguments of varying
     types.  The include file  declares a type (valist) 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 valist which is used
     by the macros vvaassttaarrtt(), vvaaaarrgg(), and vvaaeenndd().

     The vvaassttaarrtt() macro initializes ap for subsequent use by vvaaaarrgg() and
     vvaaeenndd(), 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 vvaassttaarrtt() macro, it
     should not be declared as a register variable, or as a function or an
     array type.

     The vvaassttaarrtt() macro returns no value.

     The vvaaaarrgg() macro expands to an expression that has the type and value
     of the next argument in the call.  The parameter ap is the valist ap
     initialized by vvaassttaarrtt().  Each call to vvaaaarrgg() 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 vvaaaarrgg() macro after that of the vvaassttaarrtt() macro
     returns the argument after last.  Successive invocations return the val-
     ues of the remaining arguments.

     The vvaaeenndd() macro handles a normal return from the function whose vari-
     able argument list was initialized by vvaassttaarrtt().

     The vvaaeenndd() macro returns no value.

EEXXAAMMPPLLEESS
     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, ...)
           {
                   valist ap;
                   int d;
                   char c, *p, *s;

                   vastart(ap, fmt);
                   while (*fmt)
                           switch(*fmt++) {
                           case 's':                       /* string */
                                   s = vaarg(ap, char *);
                                   printf("string %s\n", s);
                                   break;
                           case 'd':                       /* int */
                                   d = vaarg(ap, int);
                                   printf("int %d\n", d);
                                   break;
                           case 'c':                       /* char */
                                   c = vaarg(ap, char);
                                   printf("char %c\n", c);
                                   break;
                           }
                   vaend(ap);
           }

STANDARDS
     The vvaassttaarrtt(), vvaaaarrgg(), and vvaaeenndd() macros conform to ANSI X3.159-1989
     (``ANSI C89'').

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

BUGS
     Unlike the varargs macros, the ssttddaarrgg macros do not permit programmers to
     code a function with no fixed arguments.  This problem generates work
     mainly when converting varargs code to ssttddaarrgg 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 valist argument, such as
     vfprintf(3).

BSD                              June 5, 1993                              BSD