malloc (0)

This is a discussion on malloc (0) within the C Programming forums, part of the General Programming Boards category; Hey, I'm doing some weird things (mix of C/C++), and I'd like to know if I was allowed to use ...

  1. #1
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    malloc (0)

    Hey,

    I'm doing some weird things (mix of C/C++), and I'd like to know if I was allowed to use malloc(0):

    Code:
    #include <stdlib.h>
     
    int main()
    {
         int *i = malloc (0);
         free(i);
     
         return 0;
    }
    Or if Windows may crash after that...
    I succeeded making Windows crash with my weird programs once

  2. #2
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    You can't crash Windows with non-kernel mode apps... It will crash your application, sure, but nothing else.
    Malloc(0) if I'm not mistaken is allowed, but may give unexpected results (such as an allocation of 0 bytes). Best if you don't do it.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    I did it a few times (crashed windows), running some SDL application of my own right after windows started. (blue screen of death... I saw it on Vista!)

    I'm totally OK with allowing 0 bytes, i'm just wondering if free(ptr) will behave normally on a 0-byte allocated pointer, and if i may have a memory leak.

    Sorry for my bad english :-/
    Last edited by coyotte508; 05-11-2008 at 09:21 AM.

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    Quote Originally Posted by coyotte508 View Post
    I did it a few times (crashed windows), running some SDL application of my own right after windows started.
    Probably bad drivers or you did something wrong that confused the drivers.

    I'm totally OK with allowing 0 bytes, i'm just wondering if free(ptr) will behave normally on a 0-byte allocated pointer, and if i may have a memory leak.
    Yes, free will work, but you probably won't be able to use your allocated memory for anything.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    ok thanks a lot.

    It'd be long to explain why in some cases i may allocate 0 bytes.

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    I don't believe there's any good reason for it.
    Perhaps you should explain your situation.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  7. #7
    and the hat of int overfl Salem's Avatar
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    As it stands, there is nothing wrong with your initial program.

    Dereferencing the pointer would be a bad thing to do (malloc(0) is allowed to return NULL).

    If there is any crashing in the program, then it's more likely to be some other problem unrelated to this. Cause and effect are separate things when it comes to memory corruption, and all you've seen so far is an effect (not a cause).
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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    Edit:
    Thanks Salem for your answer, thanks to you I'm sure there are no probs.
    There is no crashing, but i needed to be sure.
    Last edited by coyotte508; 05-11-2008 at 09:46 AM. Reason: Thanking salem

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    also if you don't free, you'll probably have a memory leak due to the record keeping in the CRT.

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    Quote Originally Posted by Elysia View Post
    You can't crash Windows with non-kernel mode apps... It will crash your application, sure, but nothing else.
    Oh I wouldn't be too sure of that... there's always a way.

    Hell I even managed to get a complete crash on Linux once, and I wasn't doing anything out of the ordinary.
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  11. #11
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    That seems kind of weird.
    What exactly did you do?
    Operating systems don't crash right out of the blur from a user mode application...
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    The easiest way to crash Linux is to use Wine to execute a buggy Windows application.
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    Wine seems to be doing a little more than just user mode stuff...
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  14. #14
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    Quote Originally Posted by iMalc View Post
    Hell I even managed to get a complete crash on Linux once, and I wasn't doing anything out of the ordinary.
    Were you running as root?

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    Frequently Quite Prolix dwks's Avatar
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    Wine seems to be doing a little more than just user mode stuff...
    Yes, I'm sure it does.

    I just wouldn't say that you can't crash Windows with a "user-mode" application. You never know what could happen.
    dwk

    Seek and ye shall find. quaere et invenies.

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