SEMCTL(2) OpenBSD Programmer's Manual SEMCTL(2)
NAME
semctl - semaphore control operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int
semctl(int semid, int semnum, int cmd, union semun arg);
DESCRIPTION
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). arg 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 de-
fined as follows in <sys/sem.h>:
struct semid_ds {
struct ipc_perm sem_perm; /* operation permissions */
struct sem *sem_base; /* semaphore set */
u_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 looks like this:
struct ipc_perm {
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; /* r/w permission (see chmod(2)) */
u_short seq; /* sequence # (to generate unique msg/sem/shm id) */
key_t key; /* user specified msg/sem/shm key */
};
semctl() provides the following operations:
GETVAL Return the value of the semaphore.
SETVAL Set the value of the semaphore to arg.val.
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 for all the semaphores associated with
semid.
SETALL Set the values for all the semaphores that are associated with
the semaphore identifier semid to the corresponding values in
arg.array.
IPC_STAT Gather statistics about a semaphore and place the information
in the semid_ds structure pointed to by arg.buf (see above).
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. This operation can
only be executed by the superuser, 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 message
queue.
IPC_RMID Remove the semaphores associated with semid from the system
and destroy the data structures associated with it. Only the
superuser or a process with an effective UID equal to the
sem_perm.cuid or sem_perm.uid values in the data structure as-
sociated with the semaphore can do this.
The permission to read or change a message queue (see semop(2)) is deter-
mined 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.
RETURN VALUES
For the GETVAL, GETPID, GETNCNT, and GETZCNT operations, semctl() returns
one of the values described above if successful. All other operations
will make semctl() return 0 if no errors occur. Otherwise -1 is returned
and errno set to reflect the error.
ERRORS
semctl() will fail if:
[EPERM] cmd is equal to IPC_SET or IPC_RMID and the caller is not
the superuser, nor does the effective UID match either the
sem_perm.uid or sem_perm.cuid fields of the data structure
associated with the message queue.
[EACCES] The caller has no operation permission for this semaphore.
[EINVAL] semid is not a valid message semaphore identifier.
cmd is not a valid command.
[EFAULT] arg.buf specifies an invalid address.
SEE ALSO
semget(2), semop(2)
OpenBSD 3.4 August 17, 1995 2