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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
  sem_perm  The ipc_perm structure that defines permissions for semaphore
	    operations.	 See NOTES.

  sem_base  A pointer to the first semaphore in the set.  Individual
	    semaphores are defined using the sem structure.  See NOTES.

  sem_nsems The number of semaphores in the set.  Each semaphore in the set
	    is referenced 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 relevant data structures.  Semaphore numbers run
	    sequentially from zero to sem_nsems-1.

  sem_otime The time of the last semop() operation on the set.

  sem_ctime 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 indicates 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:

  semval    A nonnegative integer that is the current value of the semaphore.

  sempid    The process ID of the last process to perform an operation on the
	    semaphore.



  semncnt   The number of processes that are currently suspended while
	    waiting for an operation to increment the current semval value.

  semzcnt   The number of processes that are currently suspended while
	    waiting for semval to go to zero.

RELATED INFORMATION
  Functions: semctl(2), semget(2), semop(2)