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 fields of the
semid_ds structure are as follows:
- sem_perm
- This is an ipc_perm structure (see below) that specifies the access
permissions on the semaphore set.
- sem_otime
- Time of last semop(2) system call.
- sem_ctime
- Time of last semctl(2) system call that changed a member of the
above structure or of one semaphore belonging to the set.
- sem_nsems
- Number of semaphores in the set. Each semaphore of the set is referenced
by a nonnegative integer ranging from 0 to sem_nsems-1.
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 */
};
The least significant 9 bits of the
mode field of the
ipc_perm
structure define the access permissions for the shared memory segment. The
permission bits are as follows:
0400 |
Read by user |
0200 |
Write by user |
0040 |
Read by group |
0020 |
Write by group |
0004 |
Read by others |
0002 |
Write by others |
In effect, "write" means "alter" for a semaphore set. Bits
0100, 0010, and 0001 (the execute bits) are unused by the system.
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.
- SEM_STAT_ANY (Linux-specific, since Linux 4.17)
- Return a seminfo structure containing the same information as for
SEM_STAT. However, sem_perm.mode is not checked for read
access for semid meaning that any user can employ this operation
(just as any user may read /proc/sysvipc/sem to obtain the same
information).
- 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 semncnt value for the semnum-th semaphore of the
set (i.e., the number of processes waiting for the semaphore's value to
increase). The calling process must have read permission on the semaphore
set.
- GETPID
- Return the sempid value 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 semval (i.e., the semaphore value) for the semnum-th
semaphore of the set. The calling process must have read permission on the
semaphore set.
- GETZCNT
- Return the semzcnt value for the semnum-th semaphore of the
set (i.e., the number of processes waiting for the semaphore value to
become 0). The calling process must have read permission on the semaphore
set.
- SETALL
- Set the semval values 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 semaphore value (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 or SEM_STAT_ANY 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.
- SEM_STAT_ANY
- as for SEM_STAT.
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, SEM_STAT_ANY, 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.
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 Linux 4.5,
the kernel did not update
sempid for
SETALL operations. This was
rectified in Linux 4.6.
See
shmop(2).
ipc(2),
semget(2),
semop(2),
capabilities(7),
sem_overview(7),
sysvipc(7)