char *ptr = (char *) malloc(len * sizeof(char));

missing 1 byte for the NUL terminator, and ptr is not nul-terminated after the copy

yes

try a different language??

For JS it 7 or lower

Basically fizzbuzz in another format

Because the global var isn't reset between your function calls and keeps its value.

tried global counter; it was working on my IDE...

Only because you called your function once only. Call it a second time and you'll see why it doesn't work here.

Agreed!