Segmentation code when accessing allocated memory

Question

I need to fill sparse matrix with random elemnts. I am trying to get random elements and write them as value, column and row elements of their arrays but I keep running into segmentation faults.

This only happens if I set N=1000 then SIZE=10000 (because if I set SIZE=1000 or less, it works).

Does that mean I can't allocate all this memory or access it after I allocated it?

What should I do if I really need to get all this memory (SIZE=10'000) allocated? can someone please help me?

#include <stdio.h>
#include <stdlib.h>

typedef struct _matrix {
    int size;       //number of not-null elements
    int ord;       //order of matrix
    int* val;
    int* col;
    int* row;
} matrix;

matrix init (int ord, int size)
{
    matrix m;
    m.ord = ord;
    m.size = size;

    m.val = malloc(sizeof(int) * size);
    m.val = malloc(sizeof(int) * size);
    m.val = malloc(sizeof(int) * size);
    return m;
}

matrix fill_matrix (int ord)
{
    int i, j, gap, size = ord * ord / 100;
    matrix new_matrix = init(ord, size);

    j = 1;
    for (i = 0; i < size; i++) {
        new_matrix.val[i] = rand() % 9 + 1;
        new_matrix.col[i] = rand() % ord + 1;        //<------SEGFAULT
        new_matrix.row[i] = rand() % ord + 1;
        j++;
    }
    return new_matrix;
}

int main()
{
    matrix A;
    int n = 1000;
    A = fill_matrix(n);
    return 0;
}

Alexis King · Accepted Answer · 2015-01-28 20:49:50Z

2

You assign to val three times, but never initialize the other two pointers.

m.val = malloc(sizeof(int) * size);
m.val = malloc(sizeof(int) * size);
m.val = malloc(sizeof(int) * size);

I'm guessing you meant this:

m.val = malloc(sizeof(int) * size);
m.col = malloc(sizeof(int) * size);
m.row = malloc(sizeof(int) * size);

answered Jan 28, 2015 at 20:49

Alexis King

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Jens · Accepted Answer · 2015-01-28 20:49:42Z

2

m.val = malloc(sizeof(int) * size);
m.val = malloc(sizeof(int) * size);
m.val = malloc(sizeof(int) * size);

You never malloc m.col and m.row :-) Cut & Paste programming is evil...

answered Jan 28, 2015 at 20:49

Jens

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