Segmentation Fault: 11 when allocating a matrix

Question

I get segmentation fault when using an allocated matrix and I don't understand why. This code currently works and doesn't work dependently from the computer

#include <stdlib.h>

void allocMatrix(int ***M, int n, int m) {
    *M = (int**)malloc(n * sizeof(int));
    int i = 0;
    while(i<n) {
        (*M)[i] = malloc(m * sizeof(int));
        i++;
    }
}


int main(void) {
    int **mat;

    int R, C;
    R = 15;
    C = 10;
    allocMatrix(&mat, R, C);
    int i,j;
    for(i = 0; i < R; i++) {
        for( j = 0; j < C; j++) {
             *(*(mat+i)+j) = j+i*R;
        }
    }
#ifdef WIN32
    system("pause");
#endif
}

I get segmentation fault: 11 or EXC BAD ACCESS in Xcode. As said, happens only to some computers

`*M = (int**)malloc(n * sizeof(int));` obviously wrong, compare the types. — Sourav Ghosh, Mar 16 '16 at 19:53
M passed as reference is a int *** and I have to get its original value. This currently works on my machine. I found out this because another PC having troubles plus my mac. What would be the right line anyway? — BlackBox, Mar 16 '16 at 19:56
I think Sourav is talking about `sizeof(int)` versus `sizeof(int *)` in that line. — Jim Lewis, Mar 16 '16 at 19:58
I'm wondering why many beginners shy away from using the possibility to return values from their functions ... — Daniel Jour, Mar 16 '16 at 20:00
But returning the matrix directly from the function currently works (so declaring it in the function) — BlackBox, Mar 16 '16 at 20:05
Change `*(*(mat+i)+j) = j+i*R` to `mat[i][j] = j+i*R` More readable, and more likely to work. — Steve Summit, Mar 16 '16 at 20:07
@BlackBox That line does no occur in the code you've posted. — EOF, Mar 16 '16 at 20:19
@BlackBox: Neither does *that* line. Also, it's *extremely* for the crash to occur on that line. — EOF, Mar 16 '16 at 20:25
@BlackBox have you checked that none of your `malloc`s return `NULL`? — yano, Mar 16 '16 at 20:36
Are you *sure* you changed `*M = (int**)malloc(n * sizeof(int))` to `*M = (int**)malloc(n * sizeof(int *))` as Sourav Ghosh originally suggested? When I run your original code under Xcode I get a segmentation fault (signal 11) just like you did, and when I fix that bug the crash goes away. — Steve Summit, Mar 16 '16 at 20:47
My final comment is in the answer below. Thanks to everybody for your time guys! — BlackBox, Mar 16 '16 at 21:46

score 4 · Accepted Answer · answered Mar 16 '16 at 20:48

4

sizeof(int) is not guaranteed to be equal to sizeof(int*), so your allocated memory block is very likely to be too small. Writing to unallocated memory is undefined behaviour, sometimes it may work as expected, sometimes not.

answered Mar 16 '16 at 20:48

Vasfed

18,013
10
47
53

he said that doesn't work in the comments, but I completely agree. On my 64-bit Mac sizeof pointers are 8 and sizeof ints are 4 – yano Mar 16 '16 at 21:05
@yano "it doesn't work" is not a useful problem description – M.M Mar 16 '16 at 21:23
I don't know exactly what I was doing wrong earlier not to make work that either, but now is going perfect. I realized the mistake. Unfortunately I was also issued by some recommendation of passing the matrix as return type and not as parameter. Even being (maybe) a better programming style for this job, that wasn't helping me to resolve this problem. Ty and cheers! – BlackBox Mar 16 '16 at 21:41
@M.M I agree... I assumed he meant he was still getting the seg fault with the `sizeof int` to `sizeof int*` change. No matter now, he only thought he was. – yano Mar 16 '16 at 23:05

Segmentation Fault: 11 when allocating a matrix

1 Answers1