linux: semget across the processes

Recently I've written an issue on whether the semaphore handle obtained with sem_open is valid in children processes.
To make the picture complete I'd like to describe is it possible to pass semaphore handle to children processes obtained by IXS's semget.

When you call semget with some key and the call was successful you get semaphore id that is valid not only within process tree but system wide.
So actually there shouldn't be any problems with the children processes that are using semaphore handle obtained in the parent process. Let's glance at the code below.

#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <errno.h>
#include <string.h>

union semun
{
 int              val;    /* Value for SETVAL */
 struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */
 unsigned short  *array;  /* Array for GETALL, SETALL */
};

int main()
{
 struct sembuf op[1];
 union semun ctrl;
 int sem = semget(0xF00B00, 1, IPC_CREAT|0600);
 if (sem == -1)
 {
  perror("semget");

  return 1;
 }

 op[0].sem_num = 0;
 op[0].sem_flg = 0;

 ctrl.val = 1;
 if (semctl(sem, 0, SETVAL, ctrl) == -1)
 {
  perror("semctl");

  return 1;
 }

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

   return 1;

  case 0:
  {
   printf("Child %u waiting for semaphore(%d)...\n",getpid(), sem);
   op[0].sem_op = 0;
   semop(sem, op, 1);
   printf("Child: Done\n");

   return 0;
  }
 }

 sleep(1);

 printf("Parent %u setting semaphore(%d)...\n",getpid(),sem);
 op[0].sem_op = -1;
 semop(sem, op, 1);
 printf("Parent: Set\n");

 return 0;
}

Here a semaphore was created and its initial value was 1. Next child process was created which waits on the semaphore until its value becomes zero. It finishes as soon as parent process decrements the value of the semaphore.The output of this test application would be

$ ./test 
Child 3353 waiting for semaphore(0)...
Parent 3352 setting semaphore(0)...
Child: Done
Parent: Set

With ipcs we can look at the system semaphores:

$ ipcs -s

------ Semaphore Arrays --------
key        semid      owner      perms      nsems     
0x00f00b00 0          niam      600        1 

You can see that semaphore with key 0x00f00b00 and id 0 is present in the system.
Since semaphore is system wide and could be accessed by its id the call to sem get may be omitted is we know that semaphore is already available and the id is known. The following code snippet is the same as previous one except there's no call to semget but value of sem is hardcoded with value 0(the id of that semaphore in our case):

int main()
{
 struct sembuf op[1];
 union semun ctrl;
 int sem = 0;

 op[0].sem_num = 0;
 op[0].sem_flg = 0;

 ctrl.val = 1;
 if (semctl(sem, 0, SETVAL, ctrl) == -1)
 {
  perror("semctl");

  return 1;
 }

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

   return 1;

  case 0:
  {
   printf("Child %u waiting for semaphore(%d)...\n",getpid(), sem);
   op[0].sem_op = 0;
   semop(sem, op, 1);
   printf("Child: Done\n");

   return 0;
  }
 }

 sleep(1);

 printf("Parent %u setting semaphore(%d)...\n",getpid(),sem);
 op[0].sem_op = -1;
 semop(sem, op, 1);
 printf("Parent: Set\n");

 return 0;
}

The output is

$ ./test 
Child 3366 waiting for semaphore(0)...
Parent 3365 setting semaphore(0)...
Child: Done
Parent: Set

Here as before the child process waits for the semaphore until the parent process decrements its value to become 0.

If you'll look into the code of semget you'll find that it does semget syscall. In ipc/util.c in the linux kernel tree you should be able to find ipcget function which calls ipcget_public routine that makes key checks and in under certain circumstances creates new semaphore with new id and adds to semaphore set. The value of id is system wide, so after the semaphore had been created you will be able to get access to it if you have appropriate permissions.