varargs(5)                       UNIX System V                       varargs(5)


NAME
      varargs - 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)


10/89                                                                    Page 1

varargs(5)                       UNIX System V                       varargs(5)


            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.























Page 2                                                                    10/89