VARARGS(5)           INTERACTIVE UNIX System           VARARGS(5)



          NAME
               varargs - handle variable argument list

          SYNOPSIS
               #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,
               are possible.








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          VARARGS(5)           INTERACTIVE UNIX System           VARARGS(5)



          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), vprintf(3S).

          NOTES
               It is up to the calling routine to specify how many argu-
               ments there are, since it is not always possible to deter-
               mine 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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