title: C/C++ - Memory map file using mmap
body: <p>I'm new to memory mapping files and am a bit confused. Is it possible to map files larger than the total amount of memory because as I understood memory mapping uses demand paging and will hold only the currently relevant pages in memory. Is this right or will my application crash when using more space than the actual memory.</p>
  
  <p>Thanks</p>
  
  <p><strong>EDIT</strong>
  OS: Ubuntu 14.04 LTS x86_64
  App-Bitness: 64bit (I guess: pointers are 8 byte)</p>
  
  <p>I'm trying to allocate memory from the mapped file in order to store the tree inside the mapped file.</p>
  
  <pre><code>#define MEMORY_SIZE 300000000
  
  unsigned char *mem_buffer;
  void *start_ptr;
  
  void *my_malloc(int size) {
      unsigned char *ptr = mem_buffer;
      mem_buffer += size;
  
      return ptr;
  }
  
  void *my_calloc(int size, int object_size) {
      unsigned char *ptr = mem_buffer;
      mem_buffer += (size * object_size);
  
      return ptr;
  }
  
  void init(const char *file_path) {
      int fd = open(file_path, O_RDWR, S_IREAD | S_IWRITE);
  
      if (fd &lt; 0) {
          perror("Could not open file for memory mapping");
          exit(1);
      }
  
      start_ptr = mmap(NULL, MEMORY_SIZE, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
      mem_buffer = (unsigned char *) start_ptr;
  
      if (mem_buffer == MAP_FAILED) {
          perror("Could not memory map file");
          exit(1);
      }
  
      printf("Successfully mapped file.\n");
  }
  
  void unmap() {
      if (munmap(start_ptr, MEMORY_SIZE) &lt; 0) {
          perror("Could not unmap file");
          exit(1);
      }
  
      printf("Successfully unmapped file.\n");
  }
  </code></pre>
  
  <p>main method:</p>
  
  <pre><code>int main(int argc, char **argv) {
  
      init(argv[1]);
  
      unsigned char *arr = (unsigned char *) my_malloc(6);
      arr[0] = 'H';
      arr[1] = 'E';
      arr[2] = 'L';
      arr[3] = 'L';
      arr[4] = 'O';
      arr[5] = '\0';
  
      unsigned char *arr2 = (unsigned char *) my_malloc(5);
      arr2[0] = 'M';
      arr2[1] = 'I';
      arr2[2] = 'A';
      arr2[3] = 'U';
      arr2[4] = '\0';
  
      printf("Memory mapped string1: %s\n", arr);
      printf("Memory mapped string2: %s\n", arr2);
  
      struct my_btree_node *root = NULL;
  
      insert(&amp;root, arr, 10);
      insert(&amp;root, arr2, 20);
  
      print_tree(root, 0, false);
  
  //  cin.ignore();
  
      unmap();
  
      return EXIT_SUCCESS;
  }
  </code></pre>