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Path: news.cern.ch!danpop
From: [e-mail] (Dan Pop)
Newsgroups: comp.lang.c
Subject: Re: char* still alive after free ???
Date: 15 Apr 96 17:33:20 GMT
Organization: CERN European Lab for Particle Physics
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In <[e-mail]> Markus Heller <[e-mail]> writes:

>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));

1. 10*sizeof(char) is overkill, plain 10 would do as well.

2. If you want to store 10 charaters as a C string, allocate space for
   11 characters.  C strings are null-terminated.

3. The cast is a bad idea.  Don't use it.

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

Why not?  The point is that, after text = malloc(3), it is _illegal_
to attempt to access the 4th to 10th characters.  Your program could
simply crash and burn with a segmentation fault.  text is now pointing
to the first character of an array of 3 characters, period.

>This happens under linux, but why ?

For the simple reason that malloc is returning the same address as in
the previous call and the old characters have retained their values.
Calling free() isn't likely to alter the contents of the memory block
being returned to the memory allocator.

Dan
--
Dan Pop
CERN, CN Division
Email: [e-mail]
Mail:  CERN - PPE, Bat. 31 R-004, CH-1211 Geneve 23, Switzerland