Question

I’m writing my own memory allocation program (without using malloc) and now I’m stuck with the free function (asfree in my code). I believe the functionality for the allocation is all there, the only problem lays on the free function. So by running the code below I can allocate 32 blocks: each block has a size of 48 + 16 (size of header).
So how can I deallocate/free all of them just after I have allocated them? Could you have a look at my free function and point me at the right direction?

P.S.: This is for learning purposes. I’m trying to get my head around structs, linked lists, memory allocations.
Thanks in advance.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define BUFFER_SIZE 2048

typedef struct blk_struct
{
    size_t  size_blk;
    struct  blk_struct *next;
    char    data[0];
}blk_struct;

struct blk_struct *first = NULL;
static char buffer[BUFFER_SIZE];

void *asalloc(size_t size)
{
    int nunits = (size + sizeof(blk_struct));
    static int val = 1;

    blk_struct *block, *current;

    //locate position for block
    if(first == NULL)
    {
        block = (blk_struct *)&buffer[0];

        // Sanity check size.
        if(nunits > BUFFER_SIZE)
            return NULL;

        // Initialise structure contents.
        block->size_blk = size;
        block->next     = NULL;

        // Add to linked list.
        first = block;

        // Give user their pointer.
        return block->data;
    }
    //create a free list and search for a free block
    for(current = first; current != NULL; current = current->next)
    {
        // If this element is the last element.
        if(current->next == NULL)
        {
            block = (blk_struct *) (current->data + current->size_blk);

            // Check we have space left to do this allocation
             if((blk_struct *) &block->data[size] >
                     (blk_struct *) &buffer[BUFFER_SIZE])
             {
                 printf("No more space\n");
                 return NULL;
             }

            // Initialise structure contents.
            block->size_blk = size;
            block->next     = NULL;

            // Add to linked list.
            current->next   = block;
            // Give user their pointer.
            return block->data;
        }
    }
    printf("List Error\n");
    return NULL;
}

// 'Free' function. blk_ptr = pointer to the block of memory to be released
void asfree(void *blk_ptr)
{
    struct blk_struct *ptr = first;
    struct blk_struct *tmp = NULL;

    while(ptr != NULL)
    {
        if(ptr == blk_ptr)
        {
            printf("Found your block\n");
            free(blk_ptr);
            break;
        }
        tmp = ptr;
        ptr = ptr->next;
    }
}

// Requests fixed size pointers
int test_asalloc(void)
{
    void *ptr = NULL;
    int size = 48;
    int i = 1;
    int total = 0;

    do
    {
        ptr = asalloc(size);

        if(ptr != NULL)
        {
            memset(ptr, 0xff, size);
            printf("Pointer %d = %p%s", i, ptr, (i % 4 != 0) ? ", " : "\n");
            // each header needs 16 bytes: (sizeof(blk_struct))
            total += (size + sizeof(blk_struct));
            i++;
        }
        asfree(ptr); // *** <--- Calls the 'free' function ***
    }
    while(ptr != NULL);
    printf("%d expected %zu\nTotal size: %d\n", i - 1,
            BUFFER_SIZE / (size + sizeof(blk_struct)), total);
}

int main(void)
{
    test_asalloc();
    return 0;
}

Answer 1

I can see some problems with your asfree.

You need to subtract sizeof(blk_struct) when looking for block head. As allocated data the user wants to free are right behind the header and you have pointer to that data, not the header.

The second problem is what to do when you get the header. You cannot just call free on the data. You need to have some flag in the header and mark the block as free. And next time you try to allocate a block you need to be able to reuse free blocks, not just creating new blocks at the end. It is also good to be able to split a large free block into two smaller. To avoid fragmentation is is needed to merge neighbour free blocks to one larger.

Currently I am writing an OS as a school project. I recommend you to use simple alocator working like this:

• Blocks of memory have headers containing links to neighbour blocks and free flag.
• At the beginning there is one large block with free flag covering whole memory.
• When malloc is called, blocks are searched from first to last. When large enough block is found it is split into two(allocated one and free reminder). Pointer to allocated block data is returned.
• When free is called, matching block is marked as free. If neighbour blocks are also free they are merged.

I think this is the basic to be able to do malloc and free without fragmentation and loosing memory.

This is how a structure of headers and footer can look like:

// Header of a heap block
typedef struct {
    // Size of the block including header and footer
    size_t size;

    // Indication of a free block
    bool free;

    // A magic value to detect overwrite of heap header.
    uint32_t magic;

} heap_block_head_t;


// Footer of a heap block
typedef struct {
    // A magic value to detect overwrite of heap footer.
    uint32_t magic;

    // Size of the block
    size_t size;

} heap_block_foot_t;

The heap full of blocks with headers and footers like the one above is much like a linked list. Blocks do not describe their neighbours explicitly, but as long as you now they are there you can find them easily. If you have a position of one header then you can add block size to that position and you have a position of a header of the next block.

// Get next block
heap_block_head_t *current = ....
heap_block_head_t *next = (heap_block_head_t*)(((void*) current) + current->size);

// Get previous block
heap_block_head_t *current = ....
heap_block_foot_t *prev_foot = (heap_block_foot_t*)(((void*) current) - sizeof(heap_block_foot_t));
heap_block_head_t *prev = (heap_block_head_t*)(((void*) prev_foot) + sizeof(heap_block_foot_t) - prev_foot->size);

// Not sure if this is correct. I just wanted to illustrate the idea behind.
// Some extra check for heap start and end are needed

Hope this helps.