In C89, sizeof operator only finds the size of a variable in bytes at compile time (in this case a void pointer of 8 bytes). It works the way you'd expect it to work on plain arrays, because their size is known at compile time.
char arr[100]; // sizeof arr == 100
char *p = arr; // sizeof p == 4 (or 8 on 64-bit architectures)
char *p = malloc(100); // sizeof p == 4 (or 8). Still!
To know the size of heap-allocated memory, you need to keep track of it manually, sizeof won't help you.
