Why does my program crash at the end even if it executes all the operations just fine?

Question

I have the next code which I want to read an 2D array. It does this, but after the program ends the program crashes every time. I have a bigger program, but here is the only problem. I have also read Correctly allocating multi-dimensional arrays and I debugged the program.

The question is: why does the program crash at the end even it throughout the expected operation procedure it works fine?

void dynamicMemAlloc(int n, int (**arrayPointer)[n][n]){
    *arrayPointer = malloc(sizeof(int[n][n]));
    assert(*arrayPointer != NULL);
}

void readMy2Darray(int n, int array[n][n]){
    int i, j;
    for(i = 1; i <= n; i++)
        for(j = 1; j <= n; j++)
            scanf("%d", &array[i][j]);
}
void printMy2Darray(int n, int array[n][n]){
    int i, j;
    for(i = 1; i <= n; i++){
        for(j = 1; j <= n; j++)
            printf("%d ", array[i][j]);
        printf("\n");
    }
}
int main(void){
    int n;
    printf("Read the dimension(n): ");
    scanf("%d", &n);
    int (*arrayPointer)[n][n];
    dynamicMemAlloc(n, &arrayPointer);
    printf("Read the 2D array:\n");
    readMy2Darray(n, *arrayPointer);
    printf("Printed array:\n");
    printMy2Darray(n, *arrayPointer);
    free(arrayPointer);
    return 0;
}

`Why does my program crash even if it works fine?`...what kind of a statement is this? — Sourav Ghosh, May 16 '17 at 10:06
you try to free memory that you did not allocate from the heap. you should read up on how memory management works in c. — Andreas Grapentin, May 16 '17 at 10:07
It's quite easy to do - I've done it lots. Just corrupt the main() return address on the stack. Result - 'correct' operation and segfault on main() return. — ThingyWotsit, May 16 '17 at 11:02

Sourav Ghosh · Accepted Answer · 2017-05-16T10:10:53.207

6

In your code

for(i = 1; i <= n; i++)
        for(j = 1; j <= n; j++)

goes off by one. C arrays are 0-based indexed. You should have the loop condition set as

for(i = 0; i < n; i++)
        for(j = 0; j < n; j++)

Otherwise, it causes undefined behavior.

edited May 16 '17 at 10:10

answered May 16 '17 at 10:08

Sourav Ghosh

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I understand that, but the print function still works fine even if it starts from 1. – Timʘtei May 16 '17 at 10:10
6

@Timʘɫei read about [undefined behavior](https://en.wikipedia.org/wiki/Undefined_behavior). – Sourav Ghosh May 16 '17 at 10:11
I used to use the version you suggested until about a week. That was the problem. Thanks. – Timʘtei May 16 '17 at 10:23

score 1 · Answer 2 · answered May 16 '17 at 10:20

In this case, the fourth scanf will write past the end of the malloc()d array and likely overwrite the malloc() bookkeeping, making the free() call run into trouble.

But as specified in the other answer, all of this depends on undefined behaviour and will be implementation-specific.

Why does my program crash at the end even if it executes all the operations just fine?

2 Answers2