Freeing dynamically allocated matrix

Question

I have this code for allocating a float matrix on runtime:

float **create_matrix(int w, int h) {
    // alocates the matrix rows first
    float **matrix = (float**)malloc(h * sizeof(float*));

    // now allocates and populates each line
    for (int i = 0; i < h; i++) {
        matrix[i] = (float*) malloc(w * sizeof(float));

    // sample matrix filling
    for (int j = 0; j < w; j++)
        matrix[i][j] = (i + 1) * (j + 1);
    }

    return matrix;
}

It seems to be working fine, since it doesn't crash and when I display the matrix I have the very same values that I initialized it. Although when I try to free the matrix, iff the number of rows is lesser than the number of columns, I receive the following error message:

a.out(14284,0x7fff73cc9300) malloc: * error for object 0x9000000000000000: pointer being freed was not allocated * set a breakpoint in malloc_error_break to debug. Abort trap: 6"

My freeing routine follows:

void free_matrix(float **matrix, int h) {
    for (int i = 0; i < h; i++)
        free(matrix[i]);
    free(matrix);
}

FYI: [don't cast malloc](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — Barmar, Jul 24 '15 at 06:06
Shame, shame, shame. Spent a lot of time reading the code and looking for the bug that I overlooked that. Should I delete the question? And why not, @Barmar? — , Jul 24 '15 at 06:06
@Barmar Sir, beware, [people may accuse you for posting "debatable and irrelevant" comment.](http://stackoverflow.com/questions/31536174/buffer-size-for-converting-unsigned-long-to-string#comment51033100_31536174) :-) — Sourav Ghosh, Jul 24 '15 at 06:12

Atul Shanbhag · Answer 1 · 2015-07-24T06:38:13.493

-3

Try this

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

float **create_matrix(int h, int w)
{
    float **matrix = (float **)malloc(h*sizeof(float *));
    int i,j;
    for(i=0;i<h;i++){
       matrix[i] = (float *)malloc(w*sizeof(float));
    }
    for(i=0;i<h;i++){
        for(j=0;j<w;j++){
            matrix[i][j] = (i+1)*(j+1);
        }
    }
    return matrix;
}

int main()
{
    int h = 2, w = 3;
    float **matrix = create_matrix(h,w);
    int i,j;
    for(i=0;i<h;i++){
        for(j=0;j<w;j++){
            printf("%.2f ",matrix[i][j]);
        }
        printf("\n");
    }
    return 0;
}

You need to allocate memory before you start making operations.

edited Jul 24 '15 at 06:38

answered Jul 24 '15 at 06:14

Atul Shanbhag

636
5
13

@lfelix please check this – Atul Shanbhag Jul 24 '15 at 08:57
Actually the original code is working fine. As pointed out by @SouravGhosh I was using a wrong parameter for `h` on the free_matrix() call. – Jul 25 '15 at 03:44

Freeing dynamically allocated matrix

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