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POMemory allocator in C -- how to utilize sbrk()'ed space
 

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  1. POMemory allocator in C -- how to utilize sbrk()'ed space
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    <p>I've been writing an implementation of malloc and was wondering if someone could help me with this problem.</p> <p>Basically, I would like to reuse memory after allocating it using sbrk(), and having made certain that the memory is free.</p> <p>So essentially, imagine my memory is like this</p> <pre><code>|------------------------------| </code></pre> <p>...and I do some allocations. When I allocate memory, each bit has a head (h) and data (d).</p> <pre><code>|hddddddhddd---hdd--hddd-------| </code></pre> <p>Now I've got these holes, and if I want to use say, the first gap in my diagram, how do I set it up so that it's got a head (h) and a body (dd) also?</p> <p>I've gotten to the point where now I've got a pointer to the memory location I want. In C, its pointed to by a pointer. The pointer has a custom type, where "meta" is a struct I defined. So now I have</p> <pre><code>metaStruct * mypointer = the memory address. </code></pre> <p>But when I try to do</p> <pre><code>mypointer-&gt;size = 30; </code></pre> <p>Or</p> <pre><code>mypointer-&gt;buddy = 1; </code></pre> <p>I get a segfault.</p> <p>The question: how do I set it up so that the memory address, which has been allocated via sbrk(), will have the form of my struct? Obviously I can't just go myPointer = malloc(sizeof(metaStruct)), because I am writing malloc itself. I'm also not interested in sbrk()'ing more space, but rather, utilizing the existing space that I'm pointing to (I want to disregard its junk data and use the space).</p> <p>How do I go about doing that?</p>
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    1. COIncidentally, many newer `malloc(3)` implementations call `mmap(2)` with `MAP_ANONYMOUS`, to allocate new zeroed pages for a process. Because the allocations can be done as small as page-sizes, and can be released back to the OS in any order, it is easier to return memory to the OS from a process. And might be easier to allocate new pages for storage of your own internal datastructures.
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    2. COThanks sarnold -- I'm aware of mmap but I'm doing this for learning purposes. With my sbrk() implementation I was only wondering if I needed to explicitly set something up to use the memory that the pointer referred to? Like, memset or something. I just want to solve this segfault, haha. I've already gotten so far it doesn't make sense to start over :D
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    3. COI can't argue against that! :) Happy hacking.
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