A bad implementation of malloc using only mmap.

allocate_test.c

#include <sys/mman.h>
#include <stdio.h>

void* mymalloc(size_t len) {
  void* addr = mmap(0,
                    len + sizeof(size_t),
                    PROT_READ | PROT_WRITE,
                    MAP_ANON | MAP_PRIVATE,
                    -1,
                    0);
  *(size_t*)addr = len;
  return addr + sizeof(size_t);
}

int myfree(void* addr) {
  return munmap(addr - sizeof(size_t), (size_t) addr);
}

int main(int argc, char* argv[]) {
  puts("Allocating space...\n");
  int* heap_integer = mymalloc(sizeof(int));
  puts("Writing to allocated space...\n");
  *heap_integer = 5;
  puts("Reading from allocated space...");
  printf("Heap-allocated integer: %d\n", *heap_integer);
  myfree(heap_integer);
  return 0;
}