Posix shared mem with mmap between unrelated processes

Question

I want to use Posix shared memory between two unrelated processes.

The below code does the following

.1) Created a shared memory segment with MAP_SHARED flag. Line 36

.2) based on command line arg aiType 1 or 2 - write some data into memory . Line 51 - read the data in memory . Line 56

.3) msync to flush data into memory . Line 60
.4) munmap and shm_unlink to clean up Line 66 and 71

The issue : Not able to read data written by another process in Shared Memory

I want to run the below code twice first run1 - ./test 1 and then as run2 - ./test 2

I was expecting to read value written by run1 in run2 console BUT THAT DOES NOT HAPPEN. Also, as soon as run2 is started the value of *d in "run1 becomes 0" Line 63

When i look at /proc/pid/maps

The Inode value is same for both the processes - 3014341

run1 7f83592ee000-7f83592ef000 rw-s 00000000 00:11 3014341 /dev/shm/test

run2 7f740650d000-7f740650e000 rw-s 00000000 00:11 3014341 /dev/shm/test

Please let me know what i am missing

Code 

  1 #include <stdio.h>
  2 #include <string.h>
  3 #include <stdlib.h>
  4 #include <unistd.h>
  5 #include <sys/file.h>
  6 #include <sys/mman.h>
  7 #include <sys/wait.h>
  8 #include <errno.h>
  9 
 10 void error_and_die(const char *msg) {
 11   perror(msg);
 12   exit(EXIT_FAILURE);
 13 }
 14 
 15 int main(int argc, char *argv[]) {
 16   int rc;
 17   int aiType = 0;
 18 
 19   const char *memname = "test1";
 20   const size_t region_size = sysconf(_SC_PAGE_SIZE);
 21   void *ptr;
 22   u_long *d;
 23 
 24   int fd = shm_open(memname, O_CREAT | O_TRUNC | O_RDWR, 0666);
 25   if (fd == -1)
 26     error_and_die("shm_open");
 27 
 28 
 29   aiType = atoi(argv[1]);
 30 
 31   rc = ftruncate(fd, region_size);
 32   printf("errno : %d \n",errno);
 33   if (rc != 0)
 34     error_and_die("ftruncate");
 35 
 36   ptr = mmap(NULL, region_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
 37   printf("ptr ret by : %p \n",ptr);
 38 
 39   if (ptr == MAP_FAILED) {
 40     error_and_die("mmap");
 41     close(fd);
 42   }
 43 
 46 
 47   d = (u_long *) ptr;
 48 
 49   if(aiType == 1){
 50 
 51     *d = 0xdeadbeef;
 52     printf(" Wrote  *d : %x  \n",*d);
 53 
 54   } else if (aiType == 2){
 55 
 56     printf(" Read  *d : %x   \n",*d);
 57 
 58   }
 59 
 60   rc = msync(ptr, region_size, MS_SYNC);
 61   getchar();
 62 
 63   printf("  *d : %x is  \n",*d);
 64 
 65 
 66   rc = munmap(ptr, region_size);
 67   if (rc != 0)
 68     error_and_die("munmap");
 69 
 70 
 71   rc = shm_unlink(memname);
 72   if (rc != 0)
 73     error_and_die("shm_unlink");
 74 
 75 
 76   printf("End getchar() \n");
 77   getchar();
 78 
 79 
 80   return 0;

Answer 1

Do not shm_unlink the shared memory object, you're deleting it and each run starts with a fresh one.

PS. And also do not truncate it via the O_TRUNC in the shm_open.