varargs(5)                                                       varargs(5)


   NAME
         varargs - handle variable argument list

   SYNOPSIS
         #include <varargs.h>

         va_alist

         va_dcl

         va_list pvar;

         void va_start(va_list pvar);

         type va_arg(va_list pvar, type);

         void va_end(va_list 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.

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

         va_dcl is a declaration for va_alist.  No semicolon should follow
         va_dcl.

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

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

         va_arg 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.

         va_end is used to clean up.

         Multiple traversals, each bracketed by va_start and va_end, are
         possible.

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

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



   7/91                                                                 Page 1

   varargs(5)                                                       varargs(5)


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

                     va_start(ap);
                     file = va_arg(ap, char *);
                     while ((args[argno++] = va_arg(ap, char *)) != 0)
                           ;
                     va_end(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 va_arg, 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                                                                 7/91