CODEWITHSUNDEEP
cpointersmemory-managementmemory-leaksnull
sluhtala
sluhtala

Reputation: 11

Trying to understand memory leaks with NULL in c

I created a small test in order to create memory leaks and testing the leaks in terminal using leaks command. Now I encountered a strange behavior with the NULL. So any explanation why the other code leaks, and the other one doesn't? Aren't they really the same? 


int     main(void)
{
    char *ptr;
    char *btr;

    ptr = NULL;
    btr = (char*)malloc(4);
    btr = ptr;
    while (1)
        ;
    return (0);
}
    // LEAKS

int     main(void)
{
    char *btr;

    btr = (char*)malloc(4);
    btr = NULL;
    while (1)
        ;
    return (0);
}
    //NO LEAKS ?? why

Upvotes: 0

Views: 509

Answers (2)

Kim Säppi
Kim Säppi

Reputation: 1

Unfortunately I can't comment, but some people have commented that it would the result of compiler optimization.

However, if we copy the original non-leaky code and change the value to a larger number, we can clearly see that it is allocating memory:

int     main(void)
{
    char *btr;

    btr = (char*)malloc(1000000);
    btr = NULL;
    while (1)
        ;
    return (0);
}

Leaks output:

Process 9945: 162 nodes malloced for 994 KB
Process 9945: 0 leaks for 0 total leaked bytes.

If we comment the allocation out:

Process 10070: 161 nodes malloced for 14 KB
Process 10070: 0 leaks for 0 total leaked bytes.

So clearly the allocation does happen and leaks is aware of it (+1 node, +circa 1 MiB). If the compiler (clang, Hive Mac default settings, no special flags) has supposedly optimised it out, how can leaks know about the allocation?

Upvotes: 0

ChrisBD
ChrisBD

Reputation: 9209

If anything I would hazard that you have compiler optimisation turned on. In the second example this will likely ignore the line

btr = (char*)malloc(4);

during compilation as you immediately set the result to NULL.

Upvotes: 1

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