mmaptest.cpp

// g++ -Wall -o mmaptest mmaptest.cpp

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <assert.h>

int main() {
  if (system("dd if=/dev/urandom of=mmaptest.tmp bs=4096 count=6")) {
    perror("system failed");
    return 1;
  }

  // just larger than three pages
  if (system("dd if=/dev/urandom of=mmaptest.tmp bs=12294 count=1")) {
    perror("system failed");
    return 1;
  }

  int fd = open("mmaptest.tmp", O_RDONLY, 0);
  if (fd == -1) {
    perror("open failed");
    return 1;
  }

  // map all four pages
  void* addr = mmap(NULL, 4096*4, PROT_READ, MAP_PRIVATE, fd, 0);
  if (!addr) {
    perror("mmap failed");
    return 1;
  }
  char* map = (char*)addr;
  for (int i = 12294; i < 4096*4; ++i) {
    assert(0 == map[i]);
  }
}