Thread: incremented pointer and free

  1. #1
    Registered User
    Join Date
    Nov 2004

    incremented pointer and free

    Hi All,

    I've got a problem with some code. It does something like this:

    - create a pointer p (char *)

    - allocate a buffer pointed to by p

    - fill buffer with data

    - assign another pointer q (char *) to the buffer

    - set p to NULL

    - use pointer q, increment it to the end of the buffer

    - free q

    My question is, what happens to the buffer, and more importantly the program? The pointer has been moved to the end of the buffer before it has been free'd. This program runs on several different unix platforms and crashes on one of them - at the free.

    cheers to anyone who can help.

    (sorry I can't list any code as it's split all over the place).

  2. #2
    Confused Magos's Avatar
    Join Date
    Sep 2001
    Well, it's obviously wrong. You should only call free if it points at the beginning of the buffer. Why not keep p and call free on p, p still points at the beginning of the buffer.

    Give a man a fish and you feed him for a day.
    Teach a man to fish and you feed him for a lifetime.

  3. #3
    Registered User
    Join Date
    Nov 2004
    I figured it was wrong. This isn't my code, it's complicated stuff and does some things I don't get - with some fairly low-level system calls to fill the buffer.

    Having run it through a memory checker (valgrind) it seems to be ok (no memory leaks!). I'm trying to make sure the free is incorrect before I have to wade in to the spaghetti to fix it (it isn't a simple job to keep the pointer p).

    I was wondering if the free of such a pointer, q, was undefined behaviour (as it works on some platforms and not others and the memory checker shows no leaks - it doesn't crash on the first free btw).

  4. #4
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    Well any code which makes such a mess of it's only pointer to allocated memory is really in deep doo doo IMO

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