Strange qsort issue in C/GCC
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Thread: Strange qsort issue in C/GCC

  1. #1
    Join Date
    Jul 2009
    Posts
    2

    Strange qsort issue in C/GCC

    I have written the code below attempting to narrow down my problem to a small section of code to get some help with the issue. It returns the warning:
    "tmain.c:16: warning: passing argument 4 of qsort from incompatible pointer type"

    The code seems to work in that it will actually sort the array, but I cant help but wonder if something is slightly wrong and will blow up later.
    I am running Fedora 10 and compiling it with gcc -c [file name] (gcc version 4.3.2 20081105 (Red Hat 4.3.2-7) (GCC) )
    Thank you for the help.

    <code>
    #include<stdio.h>
    #include<stdlib.h>
    int compareFloat(void *vp, void *vq)
    {
    float *p = vp;
    float *q = vq;
    float x;
    x = *p - *q;
    return (int)((x==0.0) ? 0 : (x < 0.0) ? -1 : +1);
    }
    int main(void)
    {
    float meh[4] = {4,3,2,1};
    qsort(meh,4,sizeof(float),compareFloat);
    printf("%e\n",meh[0]);
    }
    </code>

  2. #2
    Join Date
    Jan 2006
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    Re: Strange qsort issue in C/GCC

    The reason is that the comparison function is supposed to have const void* parameters, but you defined your comparison function with non-const void* parameters.
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  3. #3
    Join Date
    Jul 2009
    Posts
    2

    Re: Strange qsort issue in C/GCC

    Thank you. Managed to completely miss that in the docs somehow.

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