'\" t .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it) .\" and Copyright 2004, 2005 Michael Kerrisk .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .\" 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 .\" Modified, 11 Nov 2004, Michael Kerrisk .\" Language and formatting clean-ups .\" Rewrote semun text .\" Added semid_ds and ipc_perm structure definitions .\" 2005-08-02, mtk: Added IPC_INFO, SEM_INFO, SEM_STAT descriptions. .\" 2018-03-20, dbueso: Added SEM_STAT_ANY description. .\" .TH semctl 2 2024-05-02 "Linux man-pages 6.8" .SH NAME semctl \- System V semaphore control operations .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .B #include .P .BI "int semctl(int " semid ", int " semnum ", int " op ", ...);" .fi .SH DESCRIPTION .BR semctl () performs the control operation specified by .I op on the System\ V semaphore set identified by .IR semid , or on the .IR semnum -th semaphore of that set. (The semaphores in a set are numbered starting at 0.) .P This function has three or four arguments, depending on .IR op . When there are four, the fourth has the type .IR "union semun" . The \fIcalling program\fP must define this union as follows: .P .in +4n .EX union semun { int val; /* Value for SETVAL */ struct semid_ds *buf; /* Buffer for IPC_STAT, IPC_SET */ unsigned short *array; /* Array for GETALL, SETALL */ struct seminfo *__buf; /* Buffer for IPC_INFO (Linux\-specific) */ }; .EE .in .P The .I semid_ds data structure is defined in \fI\fP as follows: .P .in +4n .EX struct semid_ds { struct ipc_perm sem_perm; /* Ownership and permissions */ time_t sem_otime; /* Last semop time */ time_t sem_ctime; /* Creation time/time of last modification via semctl() */ unsigned long sem_nsems; /* No. of semaphores in set */ }; .EE .in .P The fields of the .I semid_ds structure are as follows: .TP 11 .I sem_perm This is an .I ipc_perm structure (see below) that specifies the access permissions on the semaphore set. .TP .I sem_otime Time of last .BR semop (2) system call. .TP .I sem_ctime Time of creation of semaphore set or time of last .BR semctl () .BR IPCSET , .BR SETVAL , or .B SETALL operation. .TP .I sem_nsems Number of semaphores in the set. Each semaphore of the set is referenced by a nonnegative integer ranging from .B 0 to .IR sem_nsems\-1 . .P The .I ipc_perm structure is defined as follows (the highlighted fields are settable using .BR IPC_SET ): .P .in +4n .EX struct ipc_perm { key_t __key; /* Key supplied to semget(2) */ uid_t \fBuid\fP; /* Effective UID of owner */ gid_t \fBgid\fP; /* Effective GID of owner */ uid_t cuid; /* Effective UID of creator */ gid_t cgid; /* Effective GID of creator */ unsigned short \fBmode\fP; /* Permissions */ unsigned short __seq; /* Sequence number */ }; .EE .in .P The least significant 9 bits of the .I mode field of the .I ipc_perm structure define the access permissions for the shared memory segment. The permission bits are as follows: .TS l l. 0400 Read by user 0200 Write by user 0040 Read by group 0020 Write by group 0004 Read by others 0002 Write by others .TE .P In effect, "write" means "alter" for a semaphore set. Bits 0100, 0010, and 0001 (the execute bits) are unused by the system. .P Valid values for .I op are: .TP .B IPC_STAT Copy information from the kernel data structure associated with .I semid into the .I semid_ds structure pointed to by .IR arg.buf . The argument .I semnum is ignored. The calling process must have read permission on the semaphore set. .TP .B IPC_SET Write the values of some members of the .I semid_ds structure pointed to by .I arg.buf to the kernel data structure associated with this semaphore set, updating also its .I sem_ctime member. .IP The following members of the structure are updated: .IR sem_perm.uid , .IR sem_perm.gid , and (the least significant 9 bits of) .IR sem_perm.mode . .IP The effective UID of the calling process must match the owner .RI ( sem_perm.uid ) or creator .RI ( sem_perm.cuid ) of the semaphore set, or the caller must be privileged. The argument .I semnum is ignored. .TP .B IPC_RMID Immediately remove the semaphore set, awakening all processes blocked in .BR semop (2) calls on the set (with an error return and .I errno set to .BR EIDRM ). The effective user ID of the calling process must match the creator or owner of the semaphore set, or the caller must be privileged. The argument .I semnum is ignored. .TP .BR IPC_INFO " (Linux\-specific)" Return information about system-wide semaphore limits and parameters in the structure pointed to by .IR arg.__buf . This structure is of type .IR seminfo , defined in .I if the .B _GNU_SOURCE feature test macro is defined: .IP .in +4n .EX struct seminfo { int semmap; /* Number of entries in semaphore map; unused within kernel */ int semmni; /* Maximum number of semaphore sets */ int semmns; /* Maximum number of semaphores in all semaphore sets */ int semmnu; /* System\-wide maximum number of undo structures; unused within kernel */ int semmsl; /* Maximum number of semaphores in a set */ int semopm; /* Maximum number of operations for semop(2) */ int semume; /* Maximum number of undo entries per process; unused within kernel */ int semusz; /* Size of struct sem_undo */ int semvmx; /* Maximum semaphore value */ int semaem; /* Max. value that can be recorded for semaphore adjustment (SEM_UNDO) */ }; .EE .in .IP The .IR semmsl , .IR semmns , .IR semopm , and .I semmni settings can be changed via .IR /proc/sys/kernel/sem ; see .BR proc (5) for details. .TP .BR SEM_INFO " (Linux-specific)" Return a .I seminfo structure containing the same information as for .BR IPC_INFO , except that the following fields are returned with information about system resources consumed by semaphores: the .I semusz field returns the number of semaphore sets that currently exist on the system; and the .I semaem field returns the total number of semaphores in all semaphore sets on the system. .TP .BR SEM_STAT " (Linux-specific)" Return a .I semid_ds structure as for .BR IPC_STAT . However, the .I semid argument is not a semaphore identifier, but instead an index into the kernel's internal array that maintains information about all semaphore sets on the system. .TP .BR SEM_STAT_ANY " (Linux-specific, since Linux 4.17)" Return a .I semid_ds structure as for .BR SEM_STAT . However, .I sem_perm.mode is not checked for read access for .I semid meaning that any user can employ this operation (just as any user may read .I /proc/sysvipc/sem to obtain the same information). .TP .B GETALL Return .B semval (i.e., the current value) for all semaphores of the set into .IR arg.array . The argument .I semnum is ignored. The calling process must have read permission on the semaphore set. .TP .B GETNCNT Return the .B semncnt value for the .IR semnum \-th semaphore of the set (i.e., the number of processes waiting for the semaphore's value to increase). The calling process must have read permission on the semaphore set. .TP .B GETPID Return the .B sempid value for the .IR semnum \-th semaphore of the set. This is the PID of the process that last performed an operation on that semaphore (but see NOTES). The calling process must have read permission on the semaphore set. .TP .B GETVAL Return .B semval (i.e., the semaphore value) for the .IR semnum \-th semaphore of the set. The calling process must have read permission on the semaphore set. .TP .B GETZCNT Return the .B semzcnt value for the .IR semnum \-th semaphore of the set (i.e., the number of processes waiting for the semaphore value to become 0). The calling process must have read permission on the semaphore set. .TP .B SETALL Set the .B semval values for all semaphores of the set using .IR arg.array , updating also the .I sem_ctime member of the .I semid_ds structure associated with the set. Undo entries (see .BR semop (2)) are cleared for altered semaphores in all processes. If the changes to semaphore values would permit blocked .BR semop (2) calls in other processes to proceed, then those processes are woken up. The argument .I semnum is ignored. The calling process must have alter (write) permission on the semaphore set. .TP .B SETVAL Set the semaphore value .RB ( semval ) to .I arg.val for the .IR semnum \-th semaphore of the set, updating also the .I sem_ctime member of the .I semid_ds structure associated with the set. Undo entries are cleared for altered semaphores in all processes. If the changes to semaphore values would permit blocked .BR semop (2) calls in other processes to proceed, then those processes are woken up. The calling process must have alter permission on the semaphore set. .SH RETURN VALUE On success, .BR semctl () returns a nonnegative value depending on .I op as follows: .TP .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 . .TP .B IPC_INFO the index of the highest used entry in the kernel's internal array recording information about all semaphore sets. (This information can be used with repeated .B SEM_STAT or .B SEM_STAT_ANY operations to obtain information about all semaphore sets on the system.) .TP .B SEM_INFO as for .BR IPC_INFO . .TP .B SEM_STAT the identifier of the semaphore set whose index was given in .IR semid . .TP .B SEM_STAT_ANY as for .BR SEM_STAT . .P All other .I op values return 0 on success. .P On failure, .BR semctl () returns \-1 and sets .I errno to indicate the error. .SH ERRORS .TP .B EACCES The argument .I op has one of the values .BR GETALL , .BR GETPID , .BR GETVAL , .BR GETNCNT , .BR GETZCNT , .BR IPC_STAT , .BR SEM_STAT , .BR SEM_STAT_ANY , .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 in the user namespace that governs its IPC namespace. .TP .B EFAULT The address pointed to by .I arg.buf or .I arg.array isn't accessible. .TP .B EIDRM The semaphore set was removed. .TP .B EINVAL Invalid value for .I op or .IR semid . Or: for a .B SEM_STAT operation, the index value specified in .I semid referred to an array slot that is currently unused. .TP .B EPERM The argument .I op has the value .B IPC_SET or .B IPC_RMID 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 op has the value .B SETALL or .B SETVAL and the value to which .B semval is to be set (for some semaphore of the set) is less than 0 or greater than the implementation limit .BR SEMVMX . .SH VERSIONS POSIX.1 specifies the .\" POSIX.1-2001, POSIX.1-2008 .I sem_nsems field of the .I semid_ds structure as having the type .IR "unsigned\ short" , and the field is so defined on most other systems. It was also so defined on Linux 2.2 and earlier, but, since Linux 2.4, the field has the type .IR "unsigned\ long" . .\" .SS The sempid value POSIX.1 defines .I sempid as the "process ID of [the] last operation" on a semaphore, and explicitly notes that this value is set by a successful .BR semop (2) call, with the implication that no other interface affects the .I sempid value. .P While some implementations conform to the behavior specified in POSIX.1, others do not. (The fault here probably lies with POSIX.1 inasmuch as it likely failed to capture the full range of existing implementation behaviors.) Various other implementations .\" At least OpenSolaris (and, one supposes, older Solaris) and Darwin also update .I sempid for the other operations that update the value of a semaphore: the .B SETVAL and .B SETALL operations, as well as the semaphore adjustments performed on process termination as a consequence of the use of the .B SEM_UNDO flag (see .BR semop (2)). .P Linux also updates .I sempid for .B SETVAL operations and semaphore adjustments. However, somewhat inconsistently, up to and including Linux 4.5, the kernel did not update .I sempid for .B SETALL operations. This was rectified .\" commit a5f4db877177d2a3d7ae62a7bac3a5a27e083d7f in Linux 4.6. .SH STANDARDS POSIX.1-2008. .SH HISTORY POSIX.1-2001, SVr4. .\" SVr4 documents more error conditions EINVAL and EOVERFLOW. .P Various fields in a \fIstruct semid_ds\fP were typed as .I short under Linux 2.2 and have become .I long 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 an .B IPC_64 flag in .IR op .) .P In some earlier versions of glibc, the .I semun union was defined in \fI\fP, but POSIX.1 requires .\" POSIX.1-2001, POSIX.1-2008 that the caller define this union. On versions of glibc where this union is \fInot\fP defined, the macro .B _SEM_SEMUN_UNDEFINED is defined in \fI\fP. .SH NOTES The .BR IPC_INFO , .BR SEM_STAT , and .B SEM_INFO operations are used by the .BR ipcs (1) program to provide information on allocated resources. In the future these may modified or moved to a .I /proc filesystem interface. .P The following system limit on semaphore sets affects a .BR semctl () call: .TP .B SEMVMX Maximum value for .BR semval : implementation dependent (32767). .P For greater portability, it is best to always call .BR semctl () with four arguments. .SH EXAMPLES See .BR shmop (2). .SH SEE ALSO .BR ipc (2), .BR semget (2), .BR semop (2), .BR capabilities (7), .BR sem_overview (7), .BR sysvipc (7)