gripe: variable arg lists

hamilton at uiucuxc.UUCP hamilton at uiucuxc.UUCP
Wed Dec 12 09:12:00 AEST 1984

>> ... The secret is a little subroutine that knows where arguments are to be
>> found and ... builds a contiguous memory array of them. ... I think
>> that with sufficient "cleverness", one can always get away with this
>> type of trick.
>Does the magic routine parse the printf string to find out how many
>arguments there are?  If not, how does it determine this?

i think the "secret" subroutine he's referring to is _vaarg(), which gets
called by the va_arg macro in <varargs.h>.  however, it doesn't really
build up a list; since it's called in a sequential-access mode, it only
scans a pointer across the places where args are stashed.  it's main
job is figuring when that pointer needs to jump from the register frame
to the stack.

other lovers of _doprnt() who have migrated to a pyramid 90x, try this:

	#include <stdio.h>
	main ()
	    foo ("%d %.0f %s\n", 1, 2., "3");

	foo (fmt, args)
	    char *fmt;
	    int args;
	    int varargs_block[3];

	    _doprnt (fmt, _vastart (varargs_block, &args), stdout);

wayne ({decvax,ucbvax}!pur-ee!uiucdcs!uiucuxc!)hamilton

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