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From: ok@quintus.UUCP (Richard A. O'Keefe)
Newsgroups: comp.unix.questions
Subject: Re: rsh, rcp and the message "Where are you?"
Message-ID: <186@cresswell.quintus.UUCP>
Date: Sun, 15-Nov-87 19:12:38 EST
Article-I.D.: cresswel.186
Posted: Sun Nov 15 19:12:38 1987
Date-Received: Tue, 17-Nov-87 01:53:58 EST
References: <173@tarski.quintus.UUCP> <9312@mimsy.UUCP> <365@minya.UUCP>
Organization: Quintus Computer Systems, Mountain View, CA
Lines: 58
Keywords: UNIX exec arguments
Summary: finding argv[0]

In article <365@minya.UUCP>, jc@minya.UUCP (John Chambers) writes:
> a global environ[] so we can check the environment without permission
> from main(), but I have never seen mention of a similar global pointer
> that leads to the argv[] array.

In every UNIX system I've used (that only includes ones where
sizeof (int) == sizeof (char*), but it includes PDP-11s, VAXen,
a couple of 68000s, and an 80386), there is a a chunk of memory
that looks like this:
		argc		: int		|  addresses
		argv[0]		: char*		|  increase
		...				:  going
		argv[argc-1]	: char*		|  down the
		NULL		: char*	       \|/ page
*environ -----> envv[0]		: char*
		...
		envv[last]	: char*
		NULL		: char*
I believe this was actually documented in one of the Version 7 manuals.
If you can't get at the argument list any other way, the following
function may be of use:

/*  arguments(int *pargc, char ***pargv)
    If this function can find the program's argv[] vector -- see
    execl(3) in the UNIX manual -- it sets *pargc to the number
    of arguments and *pargv to a pointer to the argv[] vector, and
    then returns 1.  If it cannot find the argument vector, it
    returns 0 without changing either argument.  The limit ARGMAX is
    a typical limit on the number of arguments which could have been
    passed.  If either argument is NULL that argument is not set.
    For example, to find the name the program was invoked as, do
	char **argv;
	if (arguments(NULL, &argv)) {
	    program_name = argv[0];
	}
*/

#define	ARGMAX 1024

int arguments(pargc, pargv)
    int *pargc;
    char ***pargv;
    {
	extern char **environ;
	char **p;
	int n;

	p = environ;
	if (!*--p) {
	    for (n = 0; n < ARGMAX; n++) {
		if ((int)*--p != n) continue;
		if (pargc) *pargc = n;
		if (pargv) *pargv = p+1;
		return 1;
	    }
	}
	return 0;
    }