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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
SEMCTL(2) Linux Programmer's Manual SEMCTL(2)
semctl - System V semaphore control operations
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semctl(int semid, int semnum, int cmd, ...);
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)
Return 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)
Return 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)
Return 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 Return 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 Return the value of sempid for the 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.
GETVAL Return the value of semval for the semnum-th semaphore of
the set. The calling process must have read permission on
the semaphore set.
GETZCNT Return 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.
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.
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 in the user namespace that governs its IPC
namespace.
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.
POSIX.1-2001, POSIX.1-2008, SVr4.
POSIX.1 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.
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 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.
The sempid value
POSIX.1 defines sempid as the "process ID of [the] last operation" on
a semaphore, and explicitly notes that this value is set by a
successful semop(2) call, with the implication that no other
interface affects the sempid value.
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 also update
sempid for the other operations that update the value of a semaphore:
the SETVAL and SETALL operations, as well as the semaphore
adjustments performed on process termination as a consequence of the
use of the SEM_UNDO flag (see semop(2)).
Linux also updates sempid for SETVAL operations and semaphore
adjustments. However, somewhat inconsistently, up to and including
4.5, Linux did not update sempid for SETALL operations. This was
rectified in Linux 4.6.
ipc(2), semget(2), semop(2), capabilities(7), sem_overview(7),
svipc(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-10-08 SEMCTL(2)
Pages that refer to this page: ipcrm(1), ipc(2), semget(2), semop(2), syscalls(2), svipc(7)