Shared Memory between two processes after exec

Question

Parent:

shm_id = shmget(IPC_PRIVATE, (1 << 16), IPC_CREAT | IPC_EXCL | 0777);
setenv("SOME_ENV_VAR",stringof(shm_id);
if(fork()=0){
    execve(some_path,argv);
}

Child:

int shm_id = atoi(getenv("SOME_ENV_VAR"));
int *shared_mem = (int*)shmat(shm_id,0,NULL);
if(!shared_mem)
  return;
shared_mem[0]++;

I want to edit the shared memory in the child. Any reasons why this should not work? I am allocating the shared mem block via shmget in the Parent.Im placing the shm_id as an env variable for the child to read it after the fork and exec.

In the child, I am reading the proper shm_id then trying to get a pointer to the shared memory via shmat. In my code I have verified the shm_id in Parent and Child are the same... Any ideas?

Answer 1

The key_t argument to shmget is not the same as the identifier that that function returns. It’s not sensible to substitute one for the other.

However, if you change that and communicate the shmid instead of the key, your basic approach will work.

The shmid is a system-wide global identifier, and shmat will succeed if you have the appropriate process permissions, even if you are an unrelated process. (And, even if you are related, an execve will detach any shared memory segments, requiring an explicit re-attach.)

Note that the spec is not terribly explicit about this, saying that "[e]ach individual shared memory segment ... shall be identified by a unique positive integer, called ... a shared memory identifier, shmid.".