Segmentation Fault when using array of pointers

Question

I am working on a program where a block of memory is allocated using malloc and pointers are used to add information to the block of memory. I am using an array of pointers since the number of pointers is dependent on the size of the block but I am running into some issues whenever I'm adding the information to the block. I had to shorten my code but basically this is what it looks like

struct Header{
    int free;
    int size;
};

void* memory_allocator(int length){

    void* memoryBlock = malloc(length);

    //assuming that length is a multiple of 2
    int index = log2(length);

    Header** freeList = (Header**)malloc(sizeof(Header)*(index+1));
    freeList[index] = (Header*) memoryBlock;
    freeList[index]->size = length;
    freeList[index]->free = 1;

//Divide the memory block into chunks... This is where the problem happens
    for(int j=1; j <= index; j++){

        length = length/2;

        freeList[index-j] = (Header*)(((char*)freeList[index])+length);
        freeList[index-j]->size = length;
        freeList[index-j]->free = 1;
    }
}

My problem starts to happen at the for loop; it works fine at the first iteration but whenever it gets to the second it shoots a segmentation fault. The number I have been using to test this is 512 if it helps. Anybody can point out what I'm doing wrong please?

Answer 1

The block you malloc is not initialized with NULL. You probably try to dereference an unititialized pointer.

Use calloc instead. The block will be initialized to NULL.

Header** freeList = calloc(index+1, sizeof(Header*));

You are also realocating the freelist for every block insertion. Is this really what you want ?