Seg faulting when assigning values to a struct field

This is a discussion on Seg faulting when assigning values to a struct field within the C Programming forums, part of the General Programming Boards category; I am still fairly new to C, under 3 months experience. This is part of an assignment, just looking for ...

  1. #1
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    May 2009
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    Seg faulting when assigning values to a struct field

    I am still fairly new to C, under 3 months experience.

    This is part of an assignment, just looking for some hints because I am not sure whats going wrong.

    I have a file that I am sending to my readFile function. This function will transfer all the contents of the file into an int array.

    The file contains values like this.
    2 4
    1 2 3 4
    5 6 7 8
    3 3
    9 8 7
    6 5 4
    3 2 1
    2 4
    5 5 5 5
    6 6 6 6

    where 2 is the row count, and 4 is the columns, followed by the matrix of size 2x4

    after transferred to the array, it looks like this:
    data[2 4 1 2 3 4 5 6 7 8 3 3 9 8 7 6 5 4 3 2 1 2 4 5 5 5 5 6 6 6 6]

    I am using ctr as my pointer, i pull off my row and col values, then travers the array row*col times and populate my Matrix field. It runs the first pass fine, but seg faults the 3rd time around.

    The count variable is just from my matrixTester which just holds the total number of matrices in the file.
    Code:
    extern int count;
    my readFile function, i bolded the loop that i think is causing the seg fault

    Code:
    void readFile(FILE *fin, Matrix **m){
      int data[MAX*MAX];
      //reads entire file into int array
      int i = 0;
      while(!feof(fin)){
        fscanf(fin, "%i", &data[i]);
        i++;
      }
      //ctr points to current position of data array
      int ctr = 0;
      for(i = 0; i<count; i++){
        //assign row
        m[i]->row = data[ctr];
        //increment pointer
        ctr++;
        //assign col
        m[i]->col = data[ctr];
        //increment pointer
        ctr++;
        //DEBUG INFO
        printf("m[%i].row: %i  m[%i].col: %i\n", i, m[i]->row, i, m[i]->col);
        printf("counter position: %i\n", ctr);
        //DEBUG END
        //gets size of array needed
        int size = m[i]->row * m[i]->col;
        //create memory for int array[size]
        m[i]->loc = (int *)malloc(size * sizeof(int));
    
        //DEBUG INFO
        printf("created array size: %i\n", (m[i]->row * m[i]->col));
        printf("data[%i]: %i \n", ctr, data[13]);
        //DEBUG END
        int j = 0;
        for(j = 0; j < size; j++){
          m[i]->loc[j] = data[ctr];
          //DEBUG
          printf("%i ", m[i]->loc[j]);
          //DEBUG END
          //increment pointer
          ctr++;
        }
        //DEBUG INFO
        printf("\n");
        //DEBUG END
      }
    }

    Here is my output

    Code:
    input.txt found.                                                                   
    m[0].row: 2  m[0].col: 4                                                           
    counter position: 2                                                                
    created array size: 8                                                              
    data[2]: 1                                                                         
    1 2 3 4 5 6 7 8
    m[1].row: 3  m[1].col: 3
    counter position: 12
    created array size: 9
    data[12]: 9
    9 8 7 6 5 4 3 2 1
    Segmentation fault

    I appreciate any help before hand.

    Thanks
    Last edited by klaw; 05-31-2009 at 03:18 PM. Reason: explanation of count

  2. #2
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    May 2009
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    2
    I solved it.

    I am not sure why it was a problem, but ill go over it anyway.

    In my main i allocated space for my Matrix like this, Matrix *m = (Matrix *)malloc(count*sizeof(Matrix));

    what i changed was the way i declared m, to Matrix m[count]. Then i changed my parameters to accept [] rather than pointers, and now all is well.

    Good luck!

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