Using a pointer and did not use `free()` and no memory leaks

Question

In my program I have used malloc() and pointer but I did not use free() to free those memory, I have compiled with -fsanitize=address flag but it is saying there is no memory leaks. As far as I know if I allocate memory I also have to free the memory in the end of the program otherwise there will be memory leaks.

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


void print2D(int rowSize, int **cols_size, int **arr) {

    for(int i = 0; i < rowSize; i++) {
        for(int j = 0; j < (*cols_size)[i]; j++) {
            printf("%d ", arr[i][j]);
        }
        printf("\n");
    }
}

int** make2D(int rowSize, int **colSize) {

    int** arr = (int**) malloc(sizeof(int*) * rowSize);

    for(int i=0; i < rowSize; i++) {
        arr[i] = (int*) malloc(sizeof(int) * 2);
    }

    *colSize = (int*) malloc(sizeof(int*) * 2);

    (*colSize)[0] = 2;
    (*colSize)[1] = 2;

    arr[0][0] = 1;
    arr[0][1] = 2;

    arr[1][0] = 3;
    arr[1][1] = 4;

    return arr;
}

int main() {

    int **colSize;
    int rowSize = 2;

    int** arr = make2D(rowSize, colSize);

    print2D(rowSize, colSize, arr);

    return 0;
}

I compiled and ran it with gcc -o wow -fsanitize=address wow.c &&./wow

How so there is no memory leaks? Am I missing something?

update

first time I compiled and ran using wsl (debian 10) and fsanitize=address failed to detect memory leaks

But when I compiled and ran with Arch 5.19 it detected DEADLYSIGNAL . Now why is that?

Answer 1

The compiler not only reports the leak (if the program manages to avoid a SIGSEGV) but that your program exhibits undefined behaviour.

$ gcc -Wall -Wextra -pedantic -fsanitize=address a.c -o a && ./a
a.c: In function ‘main’:
a.c:42:17: warning: ‘colSize’ is used uninitialized in this function [-Wuninitialized]
   42 |     int** arr = make2D(rowSize, colSize);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~
1 2
3 4

=================================================================
==187==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x7fda63d72808 in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:144
    #1 0x5594afb0c407 in make2D (/tmp/ikegami/a/a+0x1407)
    #2 0x5594afb0c6d6 in main (/tmp/ikegami/a/a+0x16d6)
    #3 0x7fda63a97082 in __libc_start_main ../csu/libc-start.c:308

Indirect leak of 16 byte(s) in 2 object(s) allocated from:
    #0 0x7fda63d72808 in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:144
    #1 0x5594afb0c433 in make2D (/tmp/ikegami/a/a+0x1433)
    #2 0x5594afb0c6d6 in main (/tmp/ikegami/a/a+0x16d6)
    #3 0x7fda63a97082 in __libc_start_main ../csu/libc-start.c:308

SUMMARY: AddressSanitizer: 32 byte(s) leaked in 3 allocation(s).

To fix the undefined behaviour, replace

int **colSize;
int rowSize = 2;

int** arr = make2D(rowSize, colSize);

print2D(rowSize, colSize, arr);

with

int *colSize;
int rowSize = 2;

int** arr = make2D(rowSize, &colSize);

print2D(rowSize, &colSize, arr);

Then you get

$ gcc -Wall -Wextra -pedantic -fsanitize=address a.c -o a && ./a
1 2
3 4

=================================================================
==249==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x7fd485c86808 in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:144
    #1 0x55e3c49a8447 in make2D (/tmp/ikegami/a/a+0x1447)
    #2 0x55e3c49a8797 in main (/tmp/ikegami/a/a+0x1797)
    #3 0x7fd4859ab082 in __libc_start_main ../csu/libc-start.c:308

Direct leak of 16 byte(s) in 1 object(s) allocated from:
    #0 0x7fd485c86808 in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:144
    #1 0x55e3c49a84af in make2D (/tmp/ikegami/a/a+0x14af)
    #2 0x55e3c49a8797 in main (/tmp/ikegami/a/a+0x1797)
    #3 0x7fd4859ab082 in __libc_start_main ../csu/libc-start.c:308

Indirect leak of 16 byte(s) in 2 object(s) allocated from:
    #0 0x7fd485c86808 in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cc:144
    #1 0x55e3c49a8473 in make2D (/tmp/ikegami/a/a+0x1473)
    #2 0x55e3c49a8797 in main (/tmp/ikegami/a/a+0x1797)
    #3 0x7fd4859ab082 in __libc_start_main ../csu/libc-start.c:308

SUMMARY: AddressSanitizer: 48 byte(s) leaked in 4 allocation(s).

---

You should change print2D to take const int *cols_size instead of int **cols_size . There's no need for the indirection.