free pointer to chars

This is a discussion on free pointer to chars within the C Programming forums, part of the General Programming Boards category; Hi all, I am having a problem I never had before when deallocating a pointer to char. I am not ...

  1. #1
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    free pointer to chars

    Hi all,

    I am having a problem I never had before when deallocating a pointer to char. I am not sure what I am doing wrong because I am using the same standard I have used other times.

    I am declaring and allocating the following variable:

    Code:
    char   *problem[10];
    
    //Allocation of the strings of the above lines:
      for(i=0; i<10; i++)
        problem[i] = (char *) malloc(32 * sizeof(char *));
    and after it use, I free it by:

    Code:
      for(i=0; i<10; i++)
        free(problem[i]);
    but I get the following error:

    malloc: *** error for object 0x200000: pointer being freed was not allocated
    *** set a breakpoint in malloc_error_break to debug

    Could anyone help me with this?

    thank you in advance,
    cfd

  2. #2
    C++ Witch laserlight's Avatar
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    This is wrong:
    Code:
    problem[i] = (char *) malloc(32 * sizeof(char *));
    It should be:
    Code:
    problem[i] = malloc(32 * sizeof(char));
    or better yet:
    Code:
    problem[i] = malloc(32 * sizeof(problem[i][0]));
    or since sizeof(char) == 1:
    Code:
    problem[i] = malloc(32);
    though you probably should use a named constant instead of directly using 32.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
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    Hi, thanks for replying, but although I change the allocation following your suggestion, I am still getting the error the the pointer I am freeing was not allocated to begin with.

    I don't know where the actual problem would be since I am not freeing it twice throughout the code.

    Thanks again

  4. #4
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    Apologies,

    I was deallocating another array within another function. That is what was causing the problem.

    Thanks again for helping.
    cfd

  5. #5
    C++ Witch laserlight's Avatar
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    So, this results in the error that you were talking about?
    Code:
    #include <stdlib.h>
    
    int main(void)
    {
        char *problem[10];
        int i;
    
        for (i = 0; i < 10; i++)
            problem[i] = malloc(32 * sizeof(problem[i][0]));
    
        for (i = 0; i < 10; i++)
            free(problem[i]);
    
        return 0;
    }
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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