Kernel semid_ds structure

As with message queues, the kernel maintains a special internal data structure for each semaphore set which exists within its addressing space. This structure is of type semid_ds, and is defined in linux/sem.h as follows:

/* One semid data structure for each
set of semaphores in the system. */
struct semid_ds {
struct ipc_perm sem_perm;   /* permissions .. see ipc.h */
time_t  sem_otime;   /* last semop time */
time_t  sem_ctime;   /* last change time */
struct sem      *sem_base;   /* ptr to first semaphore in array */
struct wait_queue *eventn;
struct wait_queue *eventz;
struct sem_undo  *undo; /* undo requests on this array */
ushort  sem_nsems;   /* no. of semaphores in array */
};

sem_perm

This is an instance of the ipc_perm structure, which is defined for us in linux/ipc.h. This holds the permission information for the semaphore set, including the access permissions, and information about the creator of the set (uid, etc).

sem_otime

Time of the last semop() operation (more on this in a moment)

sem_ctime

Time of the last change to this structure (mode change, etc)

sem_base

Pointer to the first semaphore in the array (see next structure)

sem_undo

Number of undo requests in this array (more on this in a moment)

sem_nsems

Number of semaphores in the semaphore set (the array)