NAME
semctl - System V semaphore control operations SYNOPSIS
#include
#include
#include
int semctl(int semid, int semnum, int cmd, ...); DESCRIPTION semctl() performs the control operation specified by cmd on the System V semaphore set identified by semid, or on the semnum-th semaphore of that set. (The semaphores in a set are numbered starting at 0.) This function has three or four arguments, depending on cmd. When there are four, the fourth has the type union semun. The calling pro‐ gram must define this union as follows: union semun { int val; /* Value for SETVAL */ struct semidds *buf; /* Buffer for IPCSTAT, IPCSET */ unsigned short *array; /* Array for GETALL, SETALL */ struct seminfo *buf; /* Buffer for IPCINFO
(Linux-specific) */ }; The semidds data structure is defined in
as follows: struct semidds { struct ipcperm semperm; /* Ownership and permissions */ timet semotime; /* Last semop time */ timet semctime; /* Last change time */ unsigned long semnsems; /* No. of semaphores in set */ }; The ipcperm structure is defined as follows (the highlighted fields are settable using IPCSET): struct ipcperm { keyt key; /* Key supplied to semget(2) */ uidt uid; /* Effective UID of owner */ gidt gid; /* Effective GID of owner */ uidt cuid; /* Effective UID of creator */ gidt cgid; /* Effective GID of creator */ unsigned short mode; /* Permissions */ unsigned short seq; /* Sequence number */ }; Valid values for cmd are: IPCSTAT Copy information from the kernel data structure associated with semid into the semidds structure pointed to by arg.buf. The argument semnum is ignored. The calling process must have read permission on the semaphore set. IPCSET Write the values of some members of the semidds structure pointed to by arg.buf to the kernel data structure associated with this semaphore set, updating also its semctime member. The following members of the structure are updated: semperm.uid, semperm.gid, and (the least significant 9 bits of) semperm.mode. The effective UID of the calling process must match the owner (semperm.uid) or creator (semperm.cuid) of the semaphore set, or the caller must be privileged. The argument semnum is ignored. IPCRMID Immediately remove the semaphore set, awakening all processes blocked in semop(2) calls on the set (with an error return and errno set to EIDRM). The effective user ID of the call‐ ing process must match the creator or owner of the semaphore set, or the caller must be privileged. The argument semnum is ignored. IPCINFO (Linux-specific)
Returns information about system-wide semaphore limits and parameters in the structure pointed to by arg.buf. This structure is of type seminfo, defined in
if the GNUSOURCE feature test macro is defined: 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 semundo */ int semvmx; /* Maximum semaphore value */ int semaem; /* Max. value that can be recorded for semaphore adjustment (SEMUNDO) */ }; The semmsl, semmns, semopm, and semmni settings can be changed via /proc/sys/kernel/sem; see proc(5) for details.
SEMINFO (Linux-specific) Returns a seminfo structure containing the same information as for IPCINFO, except that the following fields are returned with information about system resources consumed by semaphores: the semusz field returns the number of semaphore sets that currently exist on the system; and the semaem field returns the total number of semaphores in all semaphore sets on the system.
SEMSTAT (Linux-specific) Returns a semidds structure as for IPCSTAT. However, the semid argument is not a semaphore identifier, but instead an index into the kernel's internal array that maintains infor‐ mation about all semaphore sets on the system. GETALL Return semval (i.e., the current value) for all semaphores of the set into arg.array. The argument semnum is ignored. The calling process must have read permission on the semaphore set. GETNCNT The system call returns the value of semncnt for the sem‐
num-th semaphore of the set (i.e., the number of processes
waiting for an increase of semval for the semnum-th semaphore of the set). The calling process must have read permission on the semaphore set.
GETPID The system call returns the value of sempid for the semnum-th semaphore of the set (i.e., the PID of the process that exe‐
cuted the last semop(2) call for the semnum-th semaphore of the set). The calling process must have read permission on the semaphore set.
GETVAL The system call returns the value of semval for the semnum-th semaphore of the set. The calling process must have read permission on the semaphore set. GETZCNT The system call returns the value of semzcnt for the sem‐
num-th semaphore of the set (i.e., the number of processes
waiting for semval of the semnum-th semaphore of the set to become 0). The calling process must have read permission on the semaphore set. SETALL Set semval for all semaphores of the set using arg.array, updating also the semctime member of the semidds structure associated with the set. Undo entries (see semop(2)) are cleared for altered semaphores in all processes. If the changes to semaphore values would permit blocked semop(2) calls in other processes to proceed, then those processes are woken up. The argument semnum is ignored. The calling process must have alter (write) permission on the semaphore set.
SETVAL Set the value of semval to arg.val for the semnum-th sema‐ phore of the set, updating also the semctime member of the semidds structure associated with the set. Undo entries are cleared for altered semaphores in all processes. If the changes to semaphore values would permit blocked 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. RETURN VALUE
On failure semctl() returns -1 with errno indicating the error. Otherwise the system call returns a nonnegative value depending on cmd as follows: GETNCNT the value of semncnt. GETPID the value of sempid. GETVAL the value of semval. GETZCNT the value of semzcnt. IPCINFO the index of the highest used entry in the kernel's inter‐ nal array recording information about all semaphore sets. (This information can be used with repeated SEMSTAT opera‐ tions to obtain information about all semaphore sets on the system.) SEMINFO As for IPCINFO. SEMSTAT the identifier of the semaphore set whose index was given in semid. All other cmd values return 0 on success. ERRORS On failure, errno will be set to one of the following: EACCES The argument cmd has one of the values GETALL, GETPID, GETVAL, GETNCNT, GETZCNT, IPCSTAT, SEMSTAT, SETALL, or SETVAL and the calling process does not have the required permissions on the semaphore set and does not have the CAPIPCOWNER capability. EFAULT The address pointed to by arg.buf or arg.array isn't accessible. EIDRM The semaphore set was removed. EINVAL Invalid value for cmd or semid. Or: for a SEMSTAT operation, the index value specified in semid referred to an array slot that is currently unused. EPERM The argument cmd has the value IPCSET or IPCRMID but the effective user ID of the calling process is not the creator (as found in semperm.cuid) or the owner (as found in semperm.uid) of the semaphore set, and the process does not have the CAPSYSADMIN capability. ERANGE The argument cmd has the value SETALL or SETVAL and the value to which semval is to be set (for some semaphore of the set) is less than 0 or greater than the implementation limit SEMVMX. CONFORMING TO
SVr4, POSIX.1-2001.
POSIX.1-2001 specifies the semnsems field of the semidds structure as having the type 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 unsigned long. NOTES The inclusion of
and isn't required on Linux or by any version of POSIX. However, some old implementations required the inclusion of these header files, and the SVID also documented their inclusion. Applications intended to be portable to such old systems may need to include these header files. The IPCINFO, SEMSTAT and SEMINFO operations are used by the ipcs(1) program to provide information on allocated resources. In the future these may modified or moved to a /proc file system interface. Various fields in a struct semidds were typed as short under Linux 2.2 and have become 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 IPC64 flag in cmd.) In some earlier versions of glibc, the semun union was defined in
, but POSIX.1-2001 requires that the caller define this union. On versions of glibc where this union is not defined, the macro SEMSEMUNUNDEFINED is defined in . The following system limit on semaphore sets affects a semctl() call: SEMVMX Maximum value for semval: implementation dependent (32767). For greater portability it is best to always call semctl() with four arguments. SEE ALSO ipc(2), semget(2), semop(2), capabilities(7), semoverview(7), svipc(7) COLOPHON This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2013-06-03 SEMCTL(2)