Void * space in a struct instead of malloc
typedef struct Entry {
    int counter;
    void *block;
} Entry;

void *memPtr = mmap(NULL, someSize*1024, PROT_READ|PROT_WRITE,
                        MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);

int AddEntry(void *data) {
   Entry entry;
   entry.counter = 1;
   entry.block = malloc(sizeof(char *) * SECTOR_SIZE);
   memcpy(entry.block, data, SECTOR_SIZE);
   memcpy(&memPtr[0], &entry, sizeof(Entry));
   return 0;
}

int AddEntry(void *data) {
   Entry *entry;
   entry = malloc(sizeof(Entry) + ((SECTOR_SIZE-1) * sizeof(unsigned char));
   // obligitory NULL checks assumed here

// fill in structure
   entry->counter = 1;
   memcpy(entry->block, data, SECTOR_SIZE);
   memcpy(&memPtr[0], &entry, sizeof(Entry) + ((SECTOR_SIZE-1) * sizeof(unsigned char));
   return 0;
}

block