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(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
(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(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
(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(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 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
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.
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 <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.
This page is part of release 3.14 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/.
