output

# 'make' doesn't need a makefile to compile basic C programs
$ make shr && ./shr  | grep 'Vm\|\[\|Rss'
[P] Parent, pre munmap
VmPeak:	   23708 kB
VmSize:	   23708 kB
VmLck:	       0 kB
VmPin:	       0 kB
VmHWM:	   20656 kB
VmRSS:	   20656 kB
RssAnon:	      88 kB
RssFile:	    1076 kB
RssShmem:	   19492 kB
VmData:	      56 kB
VmStk:	     140 kB
VmExe:	       8 kB
VmLib:	    1888 kB
VmPTE:	      64 kB
VmPMD:	      12 kB
VmSwap:	       0 kB
[C] In child 2280363, parent 2280361
[C] Child sees map = AAAAAAAAAA...
[C] Child status:
VmPeak:	   23708 kB
VmSize:	   23708 kB
VmLck:	       0 kB
VmPin:	       0 kB
VmHWM:	      88 kB
VmRSS:	      88 kB
RssAnon:	      88 kB
RssFile:	       0 kB
RssShmem:	       0 kB
VmData:	      56 kB
VmStk:	     140 kB
VmExe:	       8 kB
VmLib:	    1888 kB
VmPTE:	      28 kB
VmPMD:	      12 kB
VmSwap:	       0 kB
[C] Child waiting for parent unmap...
[P] Parent, post munmap
VmPeak:	   23708 kB
VmSize:	    4176 kB
VmLck:	       0 kB
VmPin:	       0 kB
VmHWM:	   20656 kB
VmRSS:	    1164 kB
RssAnon:	      88 kB
RssFile:	    1076 kB
RssShmem:	       0 kB
VmData:	      56 kB
VmStk:	     140 kB
VmExe:	       8 kB
VmLib:	    1888 kB
VmPTE:	      24 kB
VmPMD:	      12 kB
VmSwap:	       0 kB
[C] Child (post parent munmap) sees map = AAAAAAAAAA...
[C] Child status:
VmPeak:	   23708 kB
VmSize:	   23708 kB
VmLck:	       0 kB
VmPin:	       0 kB
VmHWM:	      88 kB
VmRSS:	      88 kB
RssAnon:	      88 kB
RssFile:	       0 kB
RssShmem:	       0 kB
VmData:	      56 kB
VmStk:	     140 kB
VmExe:	       8 kB
VmLib:	    1888 kB
VmPTE:	      28 kB
VmPMD:	      12 kB
VmSwap:	       0 kB
[P] waited: 0

shr.c

#include <sys/mman.h>
#include <sys/wait.h>
#include <sys/sendfile.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>

#define MAP_SIZE 1000000 * 20

// just bail hard if *anything* goes wrong
#define CHECK(x) ({ \
  int _res = (x); \
  if(_res == -1) { perror(#x); exit(1); } \
  _res; })

void dumpfile(const char* path) {
  int fd = CHECK(open(path, O_RDONLY));
  while(CHECK(sendfile(STDOUT_FILENO, fd, NULL, 4096)) > 0);
  close(fd);
}

int main(int argc, char **argv) {
  int parent = getpid();
  int child;
  int childstatus;
  int p[2];
  char buf[1];
  CHECK(pipe(p));
  setvbuf(stdout, NULL, _IONBF, 0);

  // get some memory
  void *map = mmap(
      NULL, MAP_SIZE, PROT_READ | PROT_WRITE,
      MAP_ANONYMOUS | MAP_SHARED, -1, 0);

  if (map == NULL) {
    perror("mmap");
    exit(1);
  }

  // touch it
  memset(map, 0x41, MAP_SIZE);

  child = CHECK(fork());
  if (child == 0) {
    close(p[1]);
    child = getpid();
    printf("[C] In child %d, parent %d\n", child, parent);
    CHECK(read(p[0], buf, 1));
    printf("[C] Child sees map = %.10s...\n", map);
    printf("[C] Child status:\n", map);
    dumpfile("/proc/self/status");
    printf("[C] Child waiting for parent unmap...\n");
    CHECK(read(p[0], buf, 1));
    printf("[C] Child (post parent munmap) sees map = %.10s...\n", map);
    printf("[C] Child status:\n", map);
    dumpfile("/proc/self/status");
    exit(0);
  }
  close(p[0]);

  printf("[P] Parent, pre munmap\n");
  dumpfile("/proc/self/status");
  write(p[1], "_", 1);
  sleep(1);
  munmap(map, MAP_SIZE);
  printf("\n[P] Parent, post munmap\n");
  dumpfile("/proc/self/status");
  write(p[1], "_", 1);
  waitpid(child, &childstatus, 0);
  printf("[P] waited: %d\n", childstatus);
  return 0;
}