semctl() - Unix, Linux System Call
Advertisements
NAME
semctl - 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 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 short sem_nsems; /* No. of semaphores in set */
};
|
The
ipc_perm structure is defined in <sys/ipc.h> as follows
(the highlighted fields are settable using
IPC_SET):
struct ipc_perm {
key_t key; /* Key supplied to semget() */
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:
Tag | Description |
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() 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; /* # of entries in semaphore map;
unused */
int semmni; /* Max. # of semaphore sets */
int semmns; /* Max. # of semaphores in all
semaphore sets */
int semmnu; /* System-wide max. # of undo
structures; unused */
int semmsl; /* Max. # of semaphores in a set */
int semopm; /* Max. # of operations for semop() */
int semume; /* Max. # of undo entries per
process; unused */
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 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() 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 (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 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() 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() 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:
Tag | Description |
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:
Tag | Description |
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 isnt 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. |
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 shorts under Linux 2.2
and have become longs 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:
Tag | Description |
SEMVMX |
Maximum value for
semval: implementation dependent (32767).
|
For greater portability it is best to always call
semctl() with four arguments.
Under Linux,
semctl() is not a system call, but is implemented via the system call
ipc(2).
CONFORMING TO
SVr4, POSIX.1-2001.
SEE ALSO
Advertisements
|