How to free a part of contiguous allocated memory in c?

Question

I am new to C. I have a program that allocates some memory using calloc() that store pointers. Those pointers point to places in memory where I store data of people. one pointer for each person. So when I need to delete the data, I free() the pointer to these data. But the memory with the pointers still has that one. I want to free that place too. As I know, I can't say free(ptr+5) for example. How can I do that?

Answer 1

You can't free the start or middle of an allocated block and leave the end of the allocated block alone; so (for deleting an entry in an array):

• move everything after the entry you're removing up to the end of the allocated block towards the start of the allocated block; so that the piece to be removed is at the end

• use realloc() to reduce the size of the allocated block (to free the piece at the end).

Dodgy example (assuming that i is the entry you want to free):

    memmove(&myArray[i], &myArray[i+1], (total_entries - 1 - i) * sizeof(myArray[0]) );
    temp = realloc(myArray, total_entries - 1 * sizeof(myArray[0]));
    if(temp == NULL) {
        /* Ooops. Need to handle the error somehow */
    } else {
        myArray = temp;
        total_entries--;
    }