SYSTEM CALL: semget()

In order to create a new semaphore set, or access an existing set, the semget() system call is used.

  SYSTEM CALL: semget();

PROTOTYPE: int semget ( key_t key, int nsems, int semflg );
RETURNS: semaphore set IPC identifier on success
-1 on error: errno = EACCESS (permission denied)
EEXIST (set exists, cannot create (IPC_EXCL))
EIDRM (set is marked for deletion)
ENOENT (set does not exist, no IPC_CREAT was used)
ENOMEM (Not enough memory to create new set)
ENOSPC (Maximum set limit exceeded)
NOTES:

IPC_CREAT

Create the semaphore set if it doesn't already exist in the kernel.

IPC_EXCL

When used with IPC_CREAT, fail if semaphore set already exists.

If IPC_CREAT is used alone, semget() either returns the semaphore set identifier for a newly created set, or returns the identifier for a set which exists with the same key value. If IPC_EXCL is used along with IPC_CREAT, then either a new set is created, or if the set exists, the call fails with -1. IPC_EXCL is useless by itself, but when combined with IPC_CREAT, it can be used as a facility to guarantee that no existing semaphore set is opened for access.

As with the other forms of System V IPC, an optional octal mode may be OR'd into the mask to form the permissions on the semaphore set.

The nsems argument specifies the number of semaphores that should be created in a new set. This represents the number of printers in our fictional print room described earlier. The maximum number of semaphores in a set is defined in ``linux/sem.h'' as:

#define SEMMSL  32
/* <=512 max num of semaphores per id */

Note that the nsems argument is ignored if you are explicitly opening an existing set.

Let's create a wrapper function for opening or creating semaphore sets:

int open_semaphore_set( key_t keyval, int numsems )
{
int     sid;

if ( ! numsems )
return(-1);

if((sid = semget( mykey, numsems, IPC_CREAT | 0660 )) == -1)
{
return(-1);
}

return(sid);
}