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semctl(2) System Calls Manual semctl(2)
semctl - System V semaphore control operations
Standard C library (libc, -lc)
#include <sys/sem.h>
int semctl(int semid, int semnum, int op, ...);
semctl() performs the control operation specified by op 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 op. 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; /* Creation time/time of last
modification via semctl() */
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 creation of semaphore set or time of last semctl()
IPCSET, SETVAL, or SETALL operation.
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 op 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 semid_ds structure 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 VERSIONS). 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.