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NAME
semctl - System V semaphore control operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
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 program must define this union as follows:
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) */
};
The semid_ds data structure is defined in <sys/sem.h> as follows:
struct semid_ds {
struct ipc_perm sem_perm; /* Ownership and permissions */
time_t sem_otime; /* Last semop time */
time_t sem_ctime; /* Last change time */
unsigned long sem_nsems; /* No. of semaphores in set */
};
The ipc_perm structure is defined as follows (the highlighted fields are settable using IPC_SET):
struct ipc_perm {
key_t __key; /* Key supplied to semget(2) */
uid_t uid; /* Effective UID of owner */
gid_t gid; /* Effective GID of owner */
uid_t cuid; /* Effective UID of creator */
gid_t cgid; /* Effective GID of creator */
unsigned short mode; /* Permissions */
unsigned short __seq; /* Sequence number */
};
Valid values for cmd are:
IPC_STAT Copy information from the kernel data structure associated with semid into the semid_ds
structure pointed to by arg.buf. The argument semnum is ignored. The calling process must
have read permission on the semaphore set.
IPC_SET Write the values of some members of the semid_ds structure pointed to by arg.buf to the kernel
data structure associated with this semaphore set, updating also its sem_ctime member. The
following members of the structure are updated: sem_perm.uid, sem_perm.gid, and (the least
significant 9 bits of) sem_perm.mode. The effective UID of the calling process must match the
owner (sem_perm.uid) or creator (sem_perm.cuid) of the semaphore set, or the caller must be
privileged. The argument semnum is ignored.
IPC_RMID 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 calling
process must match the creator or owner of the semaphore set, or the caller must be privileged.
The argument semnum is ignored.
IPC_INFO (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 <sys/sem.h> if the _GNU_SOURCE
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 sem_undo */
int semvmx; /* Maximum semaphore value */
int semaem; /* Max. value that can be recorded for
semaphore adjustment (SEM_UNDO) */
};
The semmsl, semmns, semopm, and semmni settings can be changed via /proc/sys/kernel/sem; see
proc(5) for details.
SEM_INFO (Linux-specific)
Returns a seminfo structure containing the same information as for IPC_INFO, 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.
SEM_STAT (Linux-specific)
Returns a semid_ds structure as for IPC_STAT. However, the 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.
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 semnum-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 executed 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 semnum-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 sem_ctime member of
the semid_ds 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 semaphore of the set, updating also the
sem_ctime member of the 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 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.
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 SEM_STAT operations to
obtain information about all semaphore sets on the system.)
SEM_INFO As for IPC_INFO.
SEM_STAT 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, IPC_STAT,
SEM_STAT, SETALL, or SETVAL and the calling process does not have the required permissions on the
semaphore set and does not have the CAP_IPC_OWNER 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 SEM_STAT operation, the index value specified in semid
referred to an array slot that is currently unused.
EPERM The argument cmd has the value IPC_SET or IPC_RMID but the effective user ID of the calling
process is not the creator (as found in sem_perm.cuid) or the owner (as found in sem_perm.uid) of
the semaphore set, and the process does not have the CAP_SYS_ADMIN 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 sem_nsems field of the semid_ds 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 <sys/types.h> and <sys/ipc.h> 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 IPC_INFO, SEM_STAT and SEM_INFO 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 filesystem interface.
Various fields in a struct semid_ds 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 IPC_64 flag in cmd.)
In some earlier versions of glibc, the semun union was defined in <sys/sem.h>, but POSIX.1-2001 requires
that the caller define this union. On versions of glibc where this union is not defined, the macro
_SEM_SEMUN_UNDEFINED is defined in <sys/sem.h>.
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), sem_overview(7), svipc(7)
COLOPHON
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