.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it) .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" .\" Modified Tue Oct 22 17:53:56 1996 by Eric S. Raymond .\" Modified Fri Jun 19 10:59:15 1998 by Andries Brouwer .\" Modified Sun Feb 18 01:59:29 2001 by Andries Brouwer .\" Modified 20 Dec 2001, Michael Kerrisk .\" Modified 21 Dec 2001, aeb .\" Modified 27 May 2004, Michael Kerrisk .\" Added notes on CAP_IPC_OWNER requirement .\" Modified 17 Jun 2004, Michael Kerrisk .\" Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID .\" .TH SEMCTL 2 2004-06-17 "Linux 2.6.7" "Linux Programmer's Manual" .SH NAME semctl \- semaphore control operations .SH SYNOPSIS .nf .B #include .B #include .B #include .sp .BI "int semctl(int " semid ", int " semnum ", int " cmd ", ...);" .fi .SH DESCRIPTION The function .B semctl performs the control operation specified by .I cmd on the semaphore set identified by .IR semid , or on the .IR semnum -th semaphore of that set. (Semaphores are numbered starting at 0.) .PP This function has three or four arguments. When there are four, the call is .BI semctl( semid , semnum , cmd , arg ); where the fourth argument .I arg has a type .B union semun defined as follows: .nf #if defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED) /* union semun is defined by including */ #else /* according to X/OPEN we have to define it ourselves */ union semun { int val; /* value for SETVAL */ struct semid_ds *buf; /* buffer for IPC_STAT, IPC_SET */ unsigned short *array; /* array for GETALL, SETALL */ /* Linux specific part: */ struct seminfo *__buf; /* buffer for IPC_INFO */ }; #endif .fi .PP Legal values for .I cmd are: .TP 12 .B IPC_STAT Copy info from the semaphore set data structure into the structure pointed to by .IB arg .buf \fR. The argument .I semnum is ignored. The calling process must have read access privileges on the semaphore set. .TP .B IPC_SET Write the values of some members of the .B semid_ds structure pointed to by .IB arg .buf to the semaphore set data structure, updating also its .B sem_ctime member. Considered members from the user supplied .B "struct semid_ds" pointed to by .IB arg .buf are .nf .sp .ft B sem_perm.uid sem_perm.gid sem_perm.mode \fR/* only lowest 9-bits */\fP .fi .ft R .sp The effective user\-ID of the calling process must be that of the super\-user, or match the creator or owner of the semaphore set. The argument .I semnum is ignored. .TP .B IPC_RMID Immediately remove the semaphore set and its data structures awakening all waiting processes (with an error return and .B errno set to .BR EIDRM ). The effective user\-ID of the calling process must be that of the super\-user, or match the creator or owner of the semaphore set. The argument .I semnum is ignored. .TP .B GETALL Return .B semval for all semaphores of the set into .IB arg .array \fR. The argument .I semnum is ignored. The calling process must have read access privileges on the semaphore set. .TP .B GETNCNT The system call returns the value of .B semncnt for the .IR semnum \-th semaphore of the set (i.e. the number of processes waiting for an increase of .B semval for the .IR semnum \-th semaphore of the set). The calling process must have read access privileges on the semaphore set. .TP .B GETPID The system call returns the value of .B sempid for the .IR semnum \-th semaphore of the set (i.e. the pid of the process that executed the last .B semop call for the .IR semnum \-th semaphore of the set). The calling process must have read access privileges on the semaphore set. .TP .B GETVAL The system call returns the value of .B semval for the .IR semnum \-th semaphore of the set. The calling process must have read access privileges on the semaphore set. .TP .B GETZCNT The system call returns the value of .B semzcnt for the .IR semnum \-th semaphore of the set (i.e. the number of processes waiting for .B semval of the .IR semnum \-th semaphore of the set to become 0). The calling process must have read access privileges on the semaphore set. .TP .B SETALL Set .B semval for all semaphores of the set using .IB arg .array , updating also the .B sem_ctime member of the .B semid_ds structure associated to the set. Undo entries are cleared for altered semaphores in all processes. Processes sleeping on the wait queue are awakened if some .B semval becomes 0 or increases. The argument .I semnum is ignored. The calling process must have alter access privileges on the semaphore set. .TP .B SETVAL Set the value of .B semval to .IB arg .val for the .IR semnum \-th semaphore of the set, updating also the .B sem_ctime member of the .B semid_ds structure associated to the set. Undo entries are cleared for altered semaphores in all processes. Processes sleeping on the wait queue are awakened if .B semval becomes 0 or increases. The calling process must have alter access privileges on the semaphore set. .SH "RETURN VALUE" On failure .B semctl returns .B \-1 with .B errno indicating the error. Otherwise the system call returns a nonnegative value depending on .I cmd as follows: .TP 11 .B GETNCNT the value of .BR semncnt . .TP .B GETPID the value of .BR sempid . .TP .B GETVAL the value of .BR semval . .TP .B GETZCNT the value of .BR semzcnt . .LP All other .I cmd values return 0 on success. .SH ERRORS On failure, .B errno will be set to one of the following: .TP 11 .B EACCES The argument .I cmd has one of the values .BR GETALL , .BR GETPID , .BR GETVAL , .BR GETNCNT , .BR GETZCNT , .BR IPC_STAT , .BR SEM_STAT , .BR SETALL , or .B SETVAL and the calling process does not have the required permissions on the semaphore set and does not have the .B CAP_IPC_OWNER capability. .TP .B EFAULT The address pointed to by .IB arg .buf or .IB arg .array isn't accessible. .TP .B EIDRM The semaphore set was removed. .TP .B EINVAL Invalid value for .I cmd or .IR semid . .TP .B EPERM The argument .I cmd has value .B IPC_SET or .B IPC_RMID but the but the effective user ID of the calling process is not the creator (as found in .IR sem_perm.cuid ) or the owner (as found in .IR sem_perm.uid ) of the semaphore set, and the process does not have the .B CAP_SYS_ADMIN capability. .TP .B ERANGE The argument .I cmd has value .B SETALL or .B SETVAL and the value to which .B semval has to be set (for some semaphore of the set) is less than 0 or greater than the implementation value .BR SEMVMX . .SH NOTES The .BR IPC_INFO , .BR SEM_STAT and .B SEM_INFO control calls are used by the .BR ipcs (8) program to provide information on allocated resources. In the future these can be modified as needed or moved to a proc file system interface. .LP Various fields in a \fIstruct semid_ds\fP were shorts under Linux 2.2 and have become longs under Linux 2.4. To take advantage of this, a recompilation under glibc-2.1.91 or later should suffice. (The kernel distinguishes old and new calls by a IPC_64 flag in .IR cmd .) .PP The following system limit on semaphore sets affects a .B semctl call: .TP 11 .B SEMVMX Maximum value for .BR semval : implementation dependent (32767). .LP For greater portability it is best to always call .B semctl with four arguments. .LP Under Linux, the function .B semctl is not a system call, but is implemented via the system call .BR ipc (2). .SH "CONFORMING TO" SVr4, SVID. SVr4 documents more error conditions EINVAL and EOVERFLOW. .SH "SEE ALSO" .BR ipc (2), .BR semget (2), .BR semop (2), .BR ipc (5), .BR capabilities (7)