c - how can I detect whether a specific page is mapped in memory?

Question

I would like to detect whether or not a specific page has already been mapped in memory. The goal here is to be able to perform this check before calling mmap with a fixed memory address. The following code illustrates what happens in this case by default: mmap silently remaps the original memory pages.

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

int main(int argc, char *argv[])
{
  int page_size;
  void *ptr;
  page_size = getpagesize();
  ptr = mmap(0, 10 * page_size, PROT_READ | PROT_WRITE,
             MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
  if (ptr == MAP_FAILED) {
    printf ("map1 failed
");
    return 1;
  }
  ((int *)ptr)[0] = 0xdeadbeaf;
  ptr = mmap(ptr, 2 * page_size, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, 0, 0);
  if (ptr == MAP_FAILED) {
    printf ("map2 failed
");
    return 1;
  }
  if (((int *)ptr)[0] != 0xdeadbeaf) {
    printf ("oops, data gone !
");
  }
  return 0;
}

I understand that I could open and parse /proc/self/maps to figure out which memory range has been allocated and infer from that if I can safely request a specific memory range with mmap but I am looking for a proper API: is there such a thing ?

Answer 1

msync(addr, len, 0) and checking for ENOMEM seems to work (with a fairly superficial test).