gistfile1.c

#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/user.h>
#include <string.h>
#include <errno.h>
#include <assert.h>

/* We're mmapping 10GB of data (privately, COW-style),
 * then getting another 10GB after that for extra writes.
 * To make this work, we have to actually tell the first
 * mmap() call that we want the full amount of memory in a
 * contiguous block, then munmap() the latter region.
 * Then we "reallocate" this as a MAP_FIXED|MAP_ANONYMOUS
 * region instead. Profit!
 */

int main() {
  int fd = open("10GB", O_RDWR|O_APPEND);

  const size_t sz = 10 * (uint64_t)1024 * (uint64_t)1024 * (uint64_t)1024;
  void *mem = mmap(NULL, sz*2, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
  munmap(mem+sz, sz);
  void *mem2 = mmap(mem+sz, sz, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED, 0, 0);
  if ((int64_t) mem == -1) {
    fprintf(stderr, "ouch: %s\n", strerror(errno));
    exit(255);
  }
  if ((int64_t) mem2 == -1) {
    fprintf(stderr, "ouch2: %s\n", strerror(errno));
    exit(255);
  }
  assert(mem2 == mem+sz);

  sprintf((char *)(mem+sz), "hello mike\n");
  munmap(mem,sz);
  munmap(mem2, sz);
  return 0;
}