


      VARARGS(5)                  (UNIX 5.2)                 VARARGS(5)



      NAME
           varargs - handle variable argument list

      SYNTAX
           #include <varargs.h>

           va_alist

           va_dcl

           void va_start(pvar)
           va_list pvar;

           type va_arg(pvar, type)
           va_list pvar;

           void va_end(pvar)
           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 nonportable, 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 ...  va_end,









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      VARARGS(5)                  (UNIX 5.2)                 VARARGS(5)



           are possible.

      EXAMPLE
           This example is a possible implementation of execl(2).

                #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];
                     int argno = 0;

                     va_start(ap);
                     file = va_arg(ap, char *);
                     while ((args[argno++] = va_arg(ap, char *)) != (char *)0)
                          ;
                     va_end(ap);
                     return execv(file, args);
                }

      SEE ALSO
           exec(2), printf(3S).

      BUGS
           It is up to the calling routine to specify how many
           arguments there are, since it is not always possible to
           determine this 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.














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