NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part2/14561

Internet Message Format | 1996-08-05 | 2.3 KB

Path: mail2news.demon.co.uk!genesis.demon.co.uk
From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: char* still alive after free ???
Date: Mon, 15 Apr 96 18:12:11 GMT
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In article <[e-mail]>
           [e-mail] "Markus Heller" writes:

>Hi,
>
>I have a global variable :
>
>char *text=NULL;
>
>I want to use it to store different "strings" (at diffreent times).
>E.g., if I want to sore 10 characters in text, I do a
>text=(char *) malloc(10*sizeof(char));

That's OK but why not write it simply as:

    text = malloc(10);

which is much clearer IMHO.

>When I want to use text to store 3 other characters, I first do a
>free(text); text=NULL; and finally a

The text = NULL; is redundant.

>text=(char *) malloc(3*sizeof(char));
>But to my surprise there are still the 4thh to 10th charcter of
>text contained before the free(text)/malloc... ???

How can you tell? It is illegal to access beyond text[2] but clearly you
have done so and have as a consequence invoked undefined behaviour. That
means anything can happen including what you saw.

By calling free() you're telling the system that it can reuse the memory
occupied by the object you passed a pointer to. It doesn't have to
wipe that memory or make it inaccessible so you're just reading the
last data that was put in it. You've no guarantees that that data won't have
changed, it just hadn't in your case.

>This happens under linux, but why ?

That's just the way the Linux malloc functions worked in that case. When you
malloc an object (such as an array of char in this case) its contents
are indeterminate. You must write some data to it before you can legally
read it.

>(what I want to to is to get filenames from the user and then open those
> files by a function to which I pass the file name..)

There's nothing here that stops you doing that.

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
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