is there any performance difference between using malloc versus realloc?

Question

I have this function:

int program_inc_capacity(Program *program)
{
    Instruction *v_old = program->v;

    size_t capacity_new = 2 * program->capacity;

    program->v = (Instruction *)malloc(capacity_new * sizeof(Instruction));
    if (program->v == NULL)
        return 0;

    for (size_t i = 0; i < program->size; i++)
    {
        program->v[i] = v_old[i];
    }

    free(v_old);

    program->capacity = capacity_new;

    return 1;
}

I call the above function when the memory previously allocated for program->v is no more enough. So I double it by reallocating the entire memory block through malloc.

Instead of using this approach I was thinking about using realloc(). So the code would look something like this:

int program_inc_capacity(Program *program)
{
    Instruction *v_old = program->v;

    size_t capacity_new = 2 * program->capacity;

    Instruction *temp = (Instruction *)realloc(v_old, capacity_new * sizeof(Instruction));
    if (temp == NULL) {
        printf("Memory reallocation failed!\n");
        free(v_old);  // Free the original memory
        return 1;
    } else {
        v_old = temp;  // Update the pointer with the new memory location
    }

    for (size_t i = 0; i < program->size; i++)
    {
        program->v[i] = v_old[i];
    }

    free(v_old);

    program->capacity = capacity_new;

    return 1;
}

So, my question is: is there any performance difference? In which case is better the first approach over the second one and viceversa? I also read that using realloc many times can lead to memory fragmentation.

Answer 1

There seems to be quite a lot wrong with your use of realloc()

• Why do you free(v_old); on success?

• You have not replaced program->v

• realloc() copies the data for you - if the reallocation was successful, you must not access the old pointer value.

• The function always returns 1.

I suggest this simple re-write:

int program_inc_capacity(Program *program)
{
    size_t capacity_new = 2 * program->capacity;
    Instruction *temp = realloc(program->v, capacity_new * sizeof(Instruction));
    if (temp == NULL) {
        printf("Memory reallocation failed!\n");
        return 1;
    }
    program->v = temp;
    program->capacity = capacity_new;
    return 0;
}