semid_ds(4)						     Kernel Interfaces Manual						       semid_ds(4)

NAME

       semid_ds - Defines a semaphore set

SYNOPSIS

       #include <sys/sem.h>

       struct  semid_ds{       struct  ipc_perm  sem_perm;	 struct  sem *sem_base;      u_short sem_nsems;      time_t sem_otime;	    time_t
       sem_ctime; };

DESCRIPTION

       The semid_ds structure defines a semaphore set associated with a semaphore ID.  There is one semaphore set per semaphore ID.

       A semaphore set is implemented as an array of sem_nsems semaphores, with sem_base pointing to the first semaphore in the set.

       The IPC permissions for a semaphore set are implemented in a separate, but associated, ipc_perm structure.

       A semaphore set is created indirectly via the semget() call.  If semget() is called with a non-existent semaphore ID, the kernel  allocates
       a new semid_ds structure, initializes it, and returns the semaphore ID that is to be associated with the semaphore set.

FIELDS

       The  ipc_perm structure that defines permissions for semaphore operations.  See NOTES.  A pointer to the first semaphore in the set.  Indi-
       vidual semaphores are defined using the sem structure.  See NOTES.  The number of semaphores in the set.  Each semaphore in the set is ref-
       erenced	by  a unique integer.  A semaphore number is sometimes referred to as sem_num, but this is not a field carried in any of the rele-
       vant data structures.  Semaphore numbers run sequentially from zero to sem_nsems-1.  The time of the last semop()  operation  on  the  set.
       The time of the last semctl() operation that changed a semaphore in the set.

NOTES

       The  sem_perm  field  identifies  the  associated ipc_perm structure that defines the permissions for operations on the semaphore set.  The
       ipc_perm structure (from the sys/ipc.h header file) is shown here.

       struct ipc_perm {
	       ushort	 uid;	     /* owner's user id 	   */
	       ushort	 gid;	     /* owner's group id	   */
	       ushort	 cuid;	     /* creator's user id	   */
	       ushort	 cgid;	     /* creator's group id	   */
	       ushort	 mode;	     /* access modes		   */
	       ushort	 seq;	     /* slot usage sequence number */
	       key_t	 key;	     /* key			   */ };

       The mode field is a 9-bit field that contains the permissions for semaphore operations.	The first three bits identify  owner  permissions;
       the  second  three bits identify group permissions; and the last three bits identify other permissions.	In each group, the first bit indi-
       cates read permission; the second bit indicates write permission; and the third bit is not used.

       Individual semaphores are implemented with the sem structure.  The sem structure (from the sys/sem.h header file) is shown here:

       struct sem {	 u_short   semval;	short	  sempid;      u_short	 semncnt;      u_short	 semzcnt; };

       The sem fields are defined as follows: A nonnegative integer that is the current value of the  semaphore.   The	process  ID  of  the  last
       process to perform an operation on the semaphore.

	      The number of processes that are currently suspended while waiting for an operation to increment the current semval value.  The num-
	      ber of processes that are currently suspended while waiting for semval to go to zero.

RELATED INFORMATION

       Functions: semctl(2), semget(2), semop(2) delim off

																       semid_ds(4)