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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 by 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.

  The following sections describe the fields in the semid_ds structure and
  show the associated ipc_perm and sem_perm structures.

FIELDS
  sem_perm
      The ipc_perm structure that defines permissions for semaphore
      operations.

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

  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 member of the
      ipc_perm structure specified by the sem_perm field.

ASSOCIATED STRUCTURES
  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 as follows:

       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 following sem structure
  (defined in the sys/sem.h header file):

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

  The fields in the sem structure are 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.

SEE ALSO
  Functions: semctl(2), semget(2), semop(2)

Index for Section 4

Alphabetical listing for S

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