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From: robert@sri-spam.istc.sri.com (Robert Allen)
Newsgroups: comp.lang.c
Subject: Buffer referencing between functions.
Message-ID: <10756@sri-spam.istc.sri.com>
Date: Mon, 19-Oct-87 17:32:02 EDT
Article-I.D.: sri-spam.10756
Posted: Mon Oct 19 17:32:02 1987
Date-Received: Tue, 20-Oct-87 22:44:17 EDT
Organization: SRI International, Menlo Park, CA
Lines: 65
Keywords: *(auto_variables) in calling functions?


    Pardon me if this question seems naive, but I've
    been thinking about it and need some authoritative
    answers.

    Below is a code fragment to illustrate my problem;

    struct stack_vars {
	char *frombuf;
	int frombuf_index;	/* 0-2000 */

	char *tobuf;
	int frombuf_index;	/* 0-2000 */
    }

    void
    calling(inbuf,inbuf_index)
    char inbuf[];
    int inbuf_index;
    {
	struct stack_vars data;
	char dbuf[2000];

	data.frombuf = inbuf;
	data.frombuf_index = inbuf_index;

	data.tobuf = dbuf;
	data.tobuf_index = 0;

	called(&data);
    }

    void
    called(sv)
    struct stack_vars *sv;
    {
	/* do some buffer processing */
	bcopy(sv->frombuf,sv->tobuf,sv->frombuf_index);
	sv->tobuf_index = sv->frombuf_index;
    }


    In the example above "called()" depends on the fact that the
    pointer passed to the automatic variable 'dbuf' in "calling()"
    is correct.
    
    Based on my experience the above code fragement should work,
    since the stuff is passed on the stack and the calling routine
    is not "temporarily deallocated".  My question is, is it
    technically correct to reference an 'external' variable via a
    pointer in this manner?  If the allocated space in dbuf were
    global to the file or the program I wouldn't be suspicious, but
    since it is an automatic variable in a calling function I'm a bit
    concerned with the correctness of it.

    Email or posting of information will be greatly appreciated.

-- 
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Robert Allen,
robert@spam.istc.sri.com
415-859-2143 (work phone, days)

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