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NAME
semctl - System V 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 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; /* 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 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.
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:
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.
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, 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.
CONFORMING TO
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.
NOTES
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.
The sempid value
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.
EXAMPLES
See shmop(2).
SEE ALSO
ipc(2), semget(2), semop(2), capabilities(7), sem_overview(7), sysvipc(7)
COLOPHON
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