Shrink matrix with realloc

Question

I am currently trying to understand how dynamic memory allocation works. I have this code:

int main()
{
    int **mat, i;

    mat = calloc(3, sizeof(int*));
    for(i = 0; i < 3; i++)
        mat[i] = calloc(3, sizeof(int));

    mat = realloc(mat, 1*sizeof(int*));
    for(i = 0; i < 1; i++)
        mat[i] = realloc(mat[i], 1*sizeof(int));

    for(i = 0; i < 1; i++)
        free(mat[i]);
    free(mat);

    return 0;
} 

I have checked with valgrind for memory leaks:

 24 bytes in 2 blocks are definitely lost in loss record 1 of 1
 at 0x4C2C975: calloc (vg_replace_malloc.c:711)
 by 0x400605: main (main.c:10)

LEAK SUMMARY:
  definitely lost: 24 bytes in 2 blocks
  indirectly lost: 0 bytes in 0 blocks
    possibly lost: 0 bytes in 0 blocks
  still reachable: 0 bytes in 0 blocks
     suppressed: 0 bytes in 0 blocks

The leaks don't occur if I reallocate to n*sizeof(int) where n > 1. Why is this happening?

Answer 1

When you reduce the mat array size you have lost some of the mat[i] pointers. Namely mat[1] and mat[2]. You need to manually free any pointers that will be lost during the shrink realloc before doing the realloc.