varargs(5)                                                       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.h are inherently
     non-portable, as different machines use different argument-passing
     conventions.

     The following data types are defined in varargs.h:

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

     vadcl      is a declaration for vaalist. No semicolon may follow
                 vadcl.

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

     The following macros are contained in varargs.h:

     vastart()  is called to initialize pvar to the beginning of the list.

     vaarg()    returns 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 deter-
                 mined at runtime.

     vaend()    is used to clean up.

     Multiple traversals, each bracketed by vastart() and vaend(), are
     possible.











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varargs(5)                                                       varargs(5)

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)
        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);
         }

NOTES
     It is up to the calling routine to specify how many arguments there
     are, since it is not always possible to determine the number of argu-
     ments from the stack frame. For example, execl() is passed a null
     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.

     varargs.h is provided for reasons of compatibility with earlier ver-
     sions. You are recommended to use stdarg.h instead.

SEE ALSO
     exec(2), printf(3S), vprintf(3S), stdarg(5).















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