semid_ds - Defines a semaphore set
#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 );
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.
The ipc_perm structure that defines permissions for
semaphore operations. A pointer to the first semaphore in
the set. Individual semaphores are defined using the sem
structure. 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. 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 [Toc] [Back] 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: 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 number of processes
that are currently suspended while waiting for semval to
go to zero.
Functions: semctl(2), semget(2), semop(2)
semid_ds(4)
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