I’m trying to use mallinfo2 to get a snapshot of how many bytes of memory are allocated at a particular time for testing purposes, however it’s not behaving as I would expect. My understanidng is that mallinfo2().uordblks should reflect the current amount of memory in-use by mallocs. When a chunk of memory is freed uordblks should decrease accordingly. However, this is not what I’m seeing at all.

I wrote this simple program, malltest.c:

#include <malloc.h>

int main(void) {
    char *myptr;
    printf("Value before malloc %zun", mallinfo2().uordblks);
    myptr = malloc(sizeof(char));
    printf("Value after malloc %zun", mallinfo2().uordblks);
    free(myptr);
    printf("Value after free %zun", mallinfo2().uordblks);
    return 0;
}

Running it produces this output:

# gcc malltest.c
# ./a.out
Value before malloc 0
Value after malloc 1728
Value after free 1728

I was expecting uordblks to increase by the size of a memory chunk after the malloc, and then reduce back to 0 after the free. Clearly I am misunderstanding something here. Why does uordblks behave this way, and is there an alternative that accomplishes what I’m actually looking to do?