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From: dg@wrs.UUCP (David Goodenough)
Newsgroups: comp.lang.c
Subject: Re: Accessing argc & argv from a function
Message-ID: <289@wrs.UUCP>
Date: Tue, 4-Aug-87 00:23:17 EDT
Article-I.D.: wrs.289
Posted: Tue Aug  4 00:23:17 1987
Date-Received: Thu, 6-Aug-87 01:37:35 EDT
References: <420@sugar.UUCP> <39@flmis06.ATT.COM> <382@root44.co.uk> <5773@ut-ngp.UUCP>
Reply-To: dg@wrs.UUCP (David Goodenough)
Organization: Wind River Systems, Emeryville, CA
Lines: 94

In article <5773@ut-ngp.UUCP> ayac071@ngp.UUCP (Bill Douglass) writes:
>In article <382@root44.co.uk> njh@root44.UUCP (Nigel Horne) writes:
>>In article <39@flmis06.ATT.COM> mikel@flmis06.ATT.COM (Mikel Manitius) writes:
>>>
>>>However I am still looking for a way to get at "argc" and "argv" from
>>>a function when I don't have access to "main". getenv() and putenv()
>>>gets it from somewhere...

Technically getenv reference environ - a global variable which is a char **
type of thing. However it just so happens that environ is also the third
argument to main(), which brings me to the following little program.
I wrote this over lunch the other day, and believe it or not it works.
I'm not suggesting that you all go and replace /bin/echo with this [among
other things I haven't implemented the -n switch :-) ], but it demonstrates
that what was requested is possible - if a little messy, and about as safe
as nitroglycerine.

*NOTE* it runs on a 68K and you *WILL*HAVE* to change struct frame to reflect
what your stack frames look like on whatever machine you're using. However
it does behave as expected, for all those of you with 68Ks I suggest you try
it (if you drop the printf in getargs, and the passing of ip all the way down)
it behaves just like echo - except there ain't no argc & argv declared in
main().

If Mikel Manitius is still after something and the following is close, but
doesn't work - email me & I'll help you as much as I can.
--
		dg@wrs.UUCP - David Goodenough

					+---+
					| +-+-+
					+-+-+ |
					  +---+

--- cut here --- cut here --- cut here --- cut here --- cut here ---
extern char **environ;		/* need this to know when to stop */

main()				/* no args!! */
 {
    int i;			/* just used for address checking when we
				 * get to the bottom */
    getit(&i, 5);		/* go do the work */
 }

getit(ip, n)			/* this recursively calls itself to put
				 * some fake crud on the stack */
int *ip;
 {
    int ac;
    char **av;
    int i;

    if (n)
      getit(ip, n - 1);		/* not far enough down */
    else
     {
	findargs(&ac, &av, ip);	/* at the bottom - get argc & argv */
	for (i = 1; i < ac; i++)
	  printf("%s%c", av[i], (i == ac - 1) ? '\n' : ' ');
				/* and print args - just like echo!! */
     }
 }

struct frame			/* stack frame structure for a 68K */
 {
    struct frame *_dynamic;	/* dynamic link to previous frame */
    char *_return;		/* return address: I didn't know what type to
				 * make this, so I made it a char * */
    int _argc;			/* argc when we get there */
    char **_argv;		/* argv when we get there */
    char **_envp;		/* envp when we get there */
 } *work;

findargs(acp, avp, ip)		/* this does the work - ip is not needed,
				 * it's just printed for verification */
int *acp, *ip;
char ***avp;
 {
    int i;			/* only one local variable, used to get a
				 * reference to the current stack frame */

    work = (struct frame *) (&i + 1);
				/* ugh! - point work at the current frame */

    while (work->_envp != environ)
      work = work->_dynamic;	/* run up the stack till we hit a valid
				 * environment which we hope is main!! */
    *acp = work->_argc;		/* get argc */
    *avp = work->_argv;		/* and argv */

    printf("%x %x\n", ip, work);
				/* just to compare pointers - these two should
				 * be sizeof int apart */
 }