varargs(5)                        SDK R4.11                       varargs(5)


NAME
       varargs: vastart, vaarg, vaend - handle variable argument list

SYNOPSIS
       #include <varargs.h>

       vaalist

       vadcl

       valist pvar;

       void vastart(valist pvar);

       type vaarg(valist pvar, type);

       void vaend(valist pvar);

DESCRIPTION
       This set of macros allows portable procedures that accept variable
       argument lists to be written.  Routines that have variable argument
       lists [such as printf(3S)] but do not use varargs are inherently non-
       portable, as different machines use different argument-passing
       conventions.

       vaalist is used as the parameter list in a function header.

       vadcl is a declaration for vaalist.  No semicolon should follow
       vadcl.

       valist is a type defined for the variable used to traverse the list.

       vastart is called to initialize pvar to the beginning of the list.

       vaarg will return the next argument in the list pointed to by pvar.
       type is the type the argument is expected to be.  Different types can
       be mixed, but it is up to the routine to know what type of argument
       is expected, as it cannot be determined at runtime.

       vaend is used to clean up.

       Multiple traversals, each bracketed by vastart and vaend, are
       possible.

EXAMPLE
       This example is a possible implementation of execl [see exec(2)].

              #include <unistd.h>
              #include <varargs.h>
              #define MAXARGS     100

              /*   execl is called by
                        execl(file, arg1, arg2, ..., (char *)0);
              */
              execl(vaalist)
              vadcl
              {
                   valist ap;
                   char *file;
                   char *args[MAXARGS];          /* assumed big enough*/
                   int argno = 0;

                   vastart(ap);
                   file = vaarg(ap, char *);
                   while ((args[argno++] = vaarg(ap, char *)) != 0)
                        ;
                   vaend(ap);
                   return execv(file, args);
              }

SEE ALSO
       exec(2), printf(3S), vprintf(3S), stdarg(5).

NOTES
       It is up to the calling routine to specify in some manner how many
       arguments there are, since it is not always possible to determine the
       number of arguments from the stack frame.  For example, execl is
       passed a zero pointer to signal the end of the list.  printf can tell
       how many arguments are there by the format.

       It is non-portable to specify a second argument of char, short, or
       float to vaarg, since arguments seen by the called function are not
       char, short, or float.  C converts char and short arguments to int
       and converts float arguments to double before passing them to a
       function.

       stdarg is the preferred interface.


Licensed material--property of copyright holder(s)