VARARGS(5-SVR4)     RISC/os Reference Manual      VARARGS(5-SVR4)



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.

     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)].





                        Printed 11/19/92                   Page 1

VARARGS(5-SVR4)     RISC/os Reference Manual      VARARGS(5-SVR4)



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

          /*   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 possi-
     ble to determine the number of arguments from the stack
     frame.  For example, execl is passed a zero pointer to sig-
     nal 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                 Printed 11/19/92