semid_ds(4) — File Formats
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.