The
semctl() system call provides a number of control operations on the semaphore specified by
semnum and
semid. The operation to be performed is specified in
cmd (see below). The fourth argument is optional and depends upon the operation requested. If required, it is a union of the following fields:
int val; /* value for SETVAL */
struct semid_ds *buf; /* buffer for IPC_{STAT,SET} */
u_short *array; /* array for GETALL & SETALL */
The
semid_ds structure used in the
IPC_SET and
IPC_STAT commands is defined in
<sys/sem.h> and contains the following members:
struct ipc_perm sem_perm; /* operation permissions */
unsigned short sem_nsems; /* number of sems in set */
time_t sem_otime; /* last operation time */
time_t sem_ctime; /* last change time */
The
ipc_perm structure used inside the
semid_ds structure is defined in
<sys/ipc.h> and contains the following members:
uid_t cuid; /* creator user id */
gid_t cgid; /* creator group id */
uid_t uid; /* user id */
gid_t gid; /* group id */
mode_t mode; /* permission (lower 9 bits) */
semctl() provides the following operations:
GETVAL
Return the value of the semaphore.
SETVAL
Set the value of the semaphore to arg.val, where arg is the fourth argument to semctl().
GETPID
Return the pid of the last process that did an operation on this semaphore.
GETNCNT
Return the number of processes waiting to acquire the semaphore.
GETZCNT
Return the number of processes waiting for the value of the semaphore to reach 0.
GETALL
Return the values of all the semaphores associated with semid.
SETALL
Set the values of all the semaphores that are associated with the semaphore identifier semid to the corresponding values in arg.array, where arg is the fourth argument to semctl().
IPC_STAT
Gather information about a semaphore and place the information in the structure pointed to by arg.buf, where arg is the fourth argument to semctl().
IPC_SET
Set the value of the sem_perm.uid, sem_perm.gid and sem_perm.mode fields in the structure associated with the semaphore. The values are taken from the corresponding fields in the structure pointed to by arg.buf, there arg is the fourth argument to semctl(). This operation can only be executed by the super-user, or a process that has an effective user id equal to either sem_perm.cuid or sem_perm.uid in the data structure associated with the semaphore.
IPC_RMID
Remove the semaphores associated with semid from the system and destroy the data structures associated with it. Only the super-user or a process with an effective uid equal to the sem_perm.cuid or sem_perm.uid values in the data structure associated with the semaphore can do this.
The permission to read or change a semaphore (see
semop(2)) is determined by the
sem_perm.mode field in the same way as is done with files (see
chmod(2)), but the effective uid can match either the
sem_perm.cuid field or the
sem_perm.uid field, and the effective gid can match either
sem_perm.cgid or
sem_perm.gid.