malloc segmentation fault

This is a discussion on malloc segmentation fault within the C Programming forums, part of the General Programming Boards category; hello everybody, I have written a simple program with malloc ,and I am getting segmentation fault . Can anyone tell ...

  1. #1
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    malloc segmentation fault

    hello everybody,
    I have written a simple program with malloc ,and I am getting segmentation fault . Can anyone tell me why its so?

    Thanks in advance

    Code:
    #include<stdio.h>
    #include<malloc.h>
    
    int main()
    {
            int **a;
            a[0]=(int *)malloc(2*(sizeof(int)));
    }

  2. #2
    Deathray Engineer MacGyver's Avatar
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    Quote Originally Posted by BharathKumar View Post
    Can anyone tell me why its so?
    You're accessing memory your program does not own.

  3. #3
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    1) <malloc.h> is a nonstandard header. When using malloc(), you should #include <stdlib.h> instead.

    2) You don't have to cast malloc() - see

    http://faq.cprogramming.com/cgi-bin/...&id=1043284351

    3) You need to allocate space for a[0], a[1], etc. BEFORE using malloc() to assign memory to each of them. For example, if you wanted to allocate space for a[0], a[1], and a[2], you could write
    Code:
      int **a;
      a = malloc(3*sizeof(int *)); /* allocates space for a[0], a[1], a[2] */
      a[0] = malloc(2*sizeof(int)); /* allocates space for a[0][0], a[0][1] */
    4) Of course you should check for NULL after calling each malloc() - I neglected to do this in the example above.

  4. #4
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    Thank You Robatino .

  5. #5
    Woof, woof! zacs7's Avatar
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    And you should free the memory you've allocated, once your done with it

    Eg:
    Code:
    #include <stdlib.h>
    
    int main(void)
    {
        const size_t rows = 3, cols = 5;
        int ** a;
        size_t c;
        
        /* allocate rows */
        if((a = malloc(rows * sizeof(int *))) == NULL)
        {
            perror("Failed to allocate rows");
            return 1;
        }
    
        /* allocate cols */
        for(c = 0; c < rows; c++)
        {
            if((a[c] = malloc(cols * sizeof(int))) == NULL)
            {
                perror("Failed to allocate cols");
                return 2;   /* ideally you should free the cols and rows, but cbf :) */
            }
        }
    
        /* do whatever with rows and cols */
    
        /* free cols */
        for(c = 0; c < rows; c++)
        {
            free(a[c]);
            a[c] = NULL;
        }
    
        /* free rows */
        free(a);
        a = NULL;
        return 0;
    }
    Last edited by zacs7; 06-26-2007 at 05:25 AM.

  6. #6
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    Thank You Zacs7 ,as I had missed that part.

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