Some of my malloc implementation

meh_malloc.c

#include <unistd.h> // for sbrk()

// Store free memory.
typedef struct free_list{
    int size;
    struct free_list *next;
    struct free_list *before;
} *Free_list;

Free_list f = NULL;

void *meh_malloc1(unsigned int size)
{
    char *p;

    // FIrst time malloc called.
    if (f == NULL)
    {
        // Allocate a bunch of memory with 12 bytes for header.
        // If size is less than 3, 10*size is not enough so allocate 40.
        if (size > 3)
        {
            p = sbrk(10*size);
            f = (void *) p;
            f -> size = 10*size - 12;
        }
        else
        {
            p = sbrk(40);
            f = (void *) p;
            f -> size = 40 - 12;
        }
        f -> next = NULL;
        f -> before = NULL;

        // 8 byte header after (p + 12) which has the actual size to be used and the checksum.
        *(p + 12) = size;   // Actual size to be returned.
        *(p + 12 + 4) = 0;  // Checksum.

        // This chunk is not available any more.
        f -> size = 0;

        // Set the next attributes.
        f -> next = (void *) p + 20 + size;
        if (size > 3)
            f -> next -> size =  10*size - (12 + 8+ size + 12);
        else
            f -> next -> size =  40 - (12 + 8 + size + 12);
        f -> next -> next = NULL;
        f -> next -> before = (void *) p;
 
        // Return the space requested from user.
        return p + 20;

    }
// Not the first time malloc called.
...
}