semid_ds(4)semid_ds(4)NAMEsemid_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 sema‐
phore 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 allo‐
cates a new semid_ds structure, initializes it, and returns the sema‐
phore 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
The ipc_perm structure that defines permissions for semaphore opera‐
tions. A pointer to the first semaphore in the set. Individual sema‐
phores are defined using the sem structure. The number of semaphores
in the set. Each semaphore in the set is referenced by a unique inte‐
ger. A semaphore number is sometimes referred to as sem_num, but this
is not a field carried in any of the relevant data structures. Sema‐
phore 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 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 sema‐
phore 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: 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 pro‐
cesses that are currently suspended while waiting for an operation to
increment the current semval value. 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)semid_ds(4)