Mutex in mmap memory shared between processes.

shared_mutex.c

/*
 * shared_mutex: Mutex in mmap memory shared between processes.
 *
 * Ben Cohen, July 2017.
 *
 * Compile using:
 *    gcc -o shared_mutex shared_mutex.c -ggdb -Wall -std=c99 -pthread
 *
 * Two processes are created with an mmap shared memory region
 * containing an array of chars and a mutex.  The mutex is created with
 * the attribute PTHREAD_PROCESS_SHARED.
 *
 * One process writes increasing numbers into an array and the other
 * prints the contents once a second.
 *
 * By default the mutex will prevent the first process updating while
 * the second is reading so all the value in each row will be the same,
 * for example:
 *
 *       0   0   0   0   0   0   0   0   0   0
 *      79  79  79  79  79  79  79  79  79  79
 *     209 209 209 209 209 209 209 209 209 209
 *     122 122 122 122 122 122 122 122 122 122
 *     106 106 106 106 106 106 106 106 106 106
 *     ...
 *
 * If invoked with the argument --no-mutex then it does not obtain the
 * mutex so the values are modified while the second process reads the
 * array and the values in each row will not all be the same:
 *
 *     130 206 230 236 241 247 253   3  12  20
 *      16  22  28  34  39  45  51  57  63  69
 *     196 186 193 200 206 211 217 223 228 234
 *     133 104 112 118 123 128 133 138 144 150
 *     136 111 116 122 128 133 138 143 149 154
 *     ...
 */

#define _BSD_SOURCE
#include <sys/mman.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <pthread.h>
#include <string.h>

#define MMAP_NAME         "/tmp/mmaptest"
#define LENGTH            sizeof(struct mapped)
#define ARRAY_ELEMENTS    10

struct mapped
{
  unsigned char    array[ARRAY_ELEMENTS];
  int              use_mutex;
  pthread_mutex_t  mutex;
};

void die(char *msg)
{
  perror(msg);
  exit(1);
}

void child(struct mapped *mapping)
{
  unsigned int n = 0;
  while (1)
  {
    if (mapping->use_mutex)
      pthread_mutex_lock(&mapping->mutex);
    for (int i = 0; i < 10; i ++)
    {
      mapping->array[i] = n;
    }
    n ++;
    if (mapping->use_mutex)
      pthread_mutex_unlock(&mapping->mutex);
  }
}

void parent(struct mapped *mapping)
{
  while (1)
  {
    if (mapping->use_mutex)
      pthread_mutex_lock(&mapping->mutex);
    for (int i = 0; i < 10; i ++)
    {
      printf("%4d", mapping->array[i]);
    }
    printf("\n");
    if (mapping->use_mutex)
      pthread_mutex_unlock(&mapping->mutex);
    sleep(1);
  }
}

int main(int argc, char **argv)
{
  int rc;
  unlink(MMAP_NAME);
  int fd = open(MMAP_NAME, O_CREAT|O_RDWR, 00600);   
  if (fd == -1)
    die("open");
  rc = ftruncate(fd, LENGTH);
  if (rc != 0)
    die("ftruncate");

  struct mapped *mapping = (struct mapped *)mmap(NULL,
                                                 LENGTH,
                                                 PROT_READ|PROT_WRITE,
                                                 MAP_SHARED,
                                                 fd,
                                                 0);
  if (mapping == MAP_FAILED)
    die("mmap");
  close(fd);

  pthread_mutexattr_t mutexattr;
  rc = pthread_mutexattr_init(&mutexattr);
  if (rc != 0)
    die("pthread_mutexattr_init");
  rc = pthread_mutexattr_setpshared(&mutexattr, PTHREAD_PROCESS_SHARED);
  if (rc != 0)
    die("pthread_mutexattr_setpshared");
  pthread_mutex_init(&mapping->mutex, &mutexattr);
  if (rc != 0)
    die("pthread_mutex_init");

  if (argc >= 2
      && strcmp(argv[1], "--no-mutex") == 0)
    mapping->use_mutex = 0;
  else
    mapping->use_mutex = 1;

  switch (fork())
  {
    case -1:
      die("fork");

    case 0:
      child(mapping);
      break;

    default:
      parent(mapping);
      break;
  }

  rc = munmap(mapping, LENGTH);
  if (rc != 0)
    die("munmap");

  return 0;
}