C Shared memory existing flag?

Question

I have seen that question on SO already but it wasn't clear to me the following case:

A shm has been created. So if I call in my case:

int shmid = shmget(key, sizeof(struct messageQueue), IPC_CREAT | S_IRWXU | IPC_EXCL);

shmid turns -1 if the shm already exists. But can i somewhere get it's ID? Or do I need to call shmget(...) without EXCL flag again in order to get the ID?

Thanks in advance

• Barmar: I think you have to call it again. This sounds like an XY problem, what is the overall goal?
• RKou: When it returns error with EXCL and CREAT, just call it again without EXCL and CREAT to get an id.

Answer 1

Normally, IPC_CREAT | IPC_EXCL is used if you want to create and initialize a new memory block. E.g.:

int shmid = shmget(key, sizeof(struct messageQueue), IPC_CREAT | S_IRWXU | IPC_EXCL);
if( shmid != -1 )
{
 /* initialization code */
}
/* if it already exists, open it: */
if( shmid == -1 && errno == EEXIST ) 
 shmid = shmget(key, sizeof(struct messageQueue), S_IRWXU );
if( shmid == -1 ) {
 perror("shmget");
}

If you don't need to initialize it, you can skip the IPC_EXCL:

int shmid = shmget(key, sizeof(struct messageQueue), IPC_CREAT | S_IRWXU );

and if you don't need to create it, you can skip the IPC_CREAT:

int shmid = shmget(key, sizeof(struct messageQueue), S_IRWXU );