SEMCTL(2) Linux Programmers Manual SEMCTL(2)
NAME
semctl - 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
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 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 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 short sem_nsems; /* No. of semaphores in set */
};
The ipc_perm structure is defined in as follows (the high
lighted 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 pro
cess 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 associ
ated 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 signif
icant 9 bits of) sem_perm.mode. The effective UID of the
calling process must match the owner (sem_perm.uid) or cre
ator (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 pro
cesses 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 argu
ment 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 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 kernels 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 (i.e., the
number of processes waiting for the value of this semaphore
to increase) 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 pro
cess must have read permission on the semaphore set.
GETPID The system call returns the value of sempid for the sem
num-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 sem
num-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 (i.e., the
number of processes waiting for the value of this semaphore
to become zero) 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 struc
ture 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 pro
cesses. 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 non-negative 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 kernels 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, GET
VAL, GETNCNT, GETZCNT, IPC_STAT, SEM_STAT, SETALL, or SETVAL
and the calling process does not have the required permis
sions on the semaphore set and does not have the
CAP_IPC_OWNER capability.
EFAULT The address pointed to by arg.buf or arg.array isnt acces
sible.
EIDRM The semaphore set was removed.
EINVAL Invalid value for cmd or semid. Or: for a SEM_STAT opera
tion, 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.
NOTES
The IPC_INFO, SEM_STAT and SEM_INFO operations are used by the ipcs(8)
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 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
, 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 .
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
This page is part of release 3.05 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 2008-05-20 SEMCTL(2)
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