.\" $OpenBSD: semctl.2,v 1.14 2011/02/02 09:37:09 fgsch Exp $ .\" $NetBSD: semctl.2,v 1.2 1997/03/27 08:20:40 mikel Exp $ .\" .\" Copyright (c) 1995 Frank van der Linden .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed for the NetBSD Project .\" by Frank van der Linden .\" 4. The name of the author may not be used to endorse or promote products .\" derived from this software without specific prior written permission .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\"/ .Dd $Mdocdate: February 2 2011 $ .Dt SEMCTL 2 .Os .Sh NAME .Nm semctl .Nd semaphore control operations .Sh SYNOPSIS .Fd #include .Fd #include .Fd #include .Ft int .Fn semctl "int semid" "int semnum" "int cmd" "union semun arg" .Sh DESCRIPTION The .Fn semctl system call provides a number of control operations on the semaphore specified by .Fa semnum and .Fa semid . The operation to be performed is specified in .Fa cmd (see below). .Fa arg is a union of the following fields: .Bd -literal int val; /* value for SETVAL */ struct semid_ds *buf; /* buffer for IPC_{STAT,SET} */ u_short *array; /* array for GETALL & SETALL */ .Ed .Pp The .Bf -literal semid_ds .Ef structure used in the .Dv IPC_SET and .Dv IPC_STAT commands is defined as follows in .Aq Pa sys/sem.h : .Bd -literal 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 */ }; .Ed .Pp The .Bf -literal ipc_perm .Ef structure used inside the .Bf -literal semid_ds .Ef structure is defined in .Aq Pa sys/ipc.h and looks like this: .Bd -literal 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 */ }; .Ed .Pp .Fn semctl provides the following operations: .Bl -tag -width IPC_RMIDX .It Dv GETVAL Return the value of the semaphore. .It Dv SETVAL Set the value of the semaphore to .Va arg.val . .It Dv GETPID Return the pid of the last process that did an operation on this semaphore. .It Dv GETNCNT Return the number of processes waiting to acquire the semaphore. .It Dv GETZCNT Return the number of processes waiting for the value of the semaphore to reach 0. .It Dv GETALL Return the values for all the semaphores associated with .Fa semid . .It Dv SETALL Set the values for all the semaphores that are associated with the semaphore identifier .Fa semid to the corresponding values in .Va arg.array . .It Dv IPC_STAT Gather statistics about a semaphore and place the information in the .Bf -literal semid_ds .Ef structure pointed to by .Fa arg.buf (see above). .It Dv IPC_SET Set the value of the .Va sem_perm.uid , .Va sem_perm.gid and .Va 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 .Fa arg.buf . This operation can only be executed by the superuser, or a process that has an effective user ID equal to either .Va sem_perm.cuid or .Va sem_perm.uid in the data structure associated with the message queue. .It Dv IPC_RMID Remove the semaphores associated with .Fa 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 .Va sem_perm.cuid or .Va sem_perm.uid values in the data structure associated with the semaphore can do this. .El .Pp The permission to read or change a message queue (see .Xr semop 2 ) is determined by the .Va sem_perm.mode field in the same way as is done with files (see .Xr chmod 2 ) , but the effective UID can match either the .Va sem_perm.cuid field or the .Va sem_perm.uid field, and the effective GID can match either .Va sem_perm.cgid or .Va sem_perm.gid . .Sh RETURN VALUES For the .Dv GETVAL , .Dv GETPID , .Dv GETNCNT , and .Dv GETZCNT operations, .Fn semctl returns one of the values described above if successful. All other operations will make .Fn semctl return 0 if no errors occur. Otherwise \-1 is returned and .Va errno set to reflect the error. .Sh ERRORS .Fn semctl will fail if: .Bl -tag -width Er .It Bq Er EPERM .Fa cmd is equal to .Dv IPC_SET or .Dv IPC_RMID and the caller is not the superuser, nor does the effective UID match either the .Va sem_perm.uid or .Va sem_perm.cuid fields of the data structure associated with the message queue. .It Bq Er EACCES The caller has no operation permission for this semaphore. .It Bq Er EINVAL .Fa semid is not a valid message semaphore identifier. .Pp .Va cmd is not a valid command. .It Bq Er EFAULT .Fa arg.buf or .Fa arg.array specify an invalid address. .It Bq Er ERANGE .Fa cmd is equal to .Dv SETVAL or .Dv SETALL and .Va arg.val or the values in .Va arg.array are greater than the system-imposed limit. .El .Sh SEE ALSO .Xr semget 2 , .Xr semop 2