Thread: Segmentation fault question

  1. 02-17-2010 #1
    waterborne
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    Segmentation fault question

    I am working on some code that multiplies 2 matrices in parallel. I statically allocate memory for the matrices so that each matrix has the memory to be 2304x2304. When I compile and run I get a segmentation fault error and the program quits. However, when I switch the dimensions to 100x100 the program runs fine. Can anyone think of why this might happen?

    I first thought maybe I should dynamically allocate memory but it seems that when I need to allocate memory for a 2304x2304 matrix I will still run into this problem.

    Maybe I am looking at this from a wrong angle. Thanks in advance.
  2. 02-17-2010 #2
    quzah
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    Q. What's 2304 * 2304 * sizeof( yourdatatype )?
    A. Too big.


    Quzah.
    Hope is the first step on the road to disappointment.
  3. 02-17-2010 #3
    Dino
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    That's a ~21MB array, and you have two of them (2304 * 2304 * 4 * 2). An array this large should be gotten from the heap.

    We would be guessing on the error without seeing your code.
    Mainframe assembler programmer by trade. C coder when I can.
  4. 02-17-2010 #4
    waterborne
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    Ahh I just realized I don't need to allocate all that memory. Since I am doing this in parallel I only need to allocate memory for (2304/max_process_count)x(304/max_process_count) matrix, which in my case is bounded by 50x50.

    Briefly, what is heap? I just looked it up on Wikipedia but it didn't really make sense (I'm not a computer guy). I see that it is a tree based data structure and from your comment I take it that it is used for very large data sets. Is heap stored on the hard disk or in memory? Naively I would guess the hard disk but then again I have no clue what I am talking about.

    Thanks for the fast responses!
  5. 02-17-2010 #5
    Dino
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    You can think of Stack storage are being part of your program that is used for variables you have defined in your program.

    Heap storage (aka, main memory, RAM, dynamically obtained storage at runtime, whatever you want to call it), is storage you ask the operating system to give you at runtime via malloc() and malloc()'s sister functions.
    Mainframe assembler programmer by trade. C coder when I can.
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