NAME
semctl —
semaphore control
operations
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/sem.h>
int
semctl(
int
semid,
int semnum,
int cmd,
...);
DESCRIPTION
The
semctl() system call provides a number of control
operations on the semaphore specified by
semnum and
semid. The operation to be performed is specified in
cmd (see below). The fourth argument is optional and
depends upon the operation requested. If required, it is a union of the
following fields:
int val; /* value for SETVAL */
struct semid_ds *buf; /* buffer for IPC_{STAT,SET} */
u_short *array; /* array for GETALL & SETALL */
The
semid_ds structure used in the
IPC_SET
and
IPC_STAT
commands
is defined in
<sys/sem.h> and
contains the following members:
struct ipc_perm sem_perm; /* operation permissions */
unsigned short sem_nsems; /* number of sems in set */
time_t sem_otime; /* last operation time */
time_t sem_ctime; /* last change time */
The
ipc_perm structure used inside the
semid_ds structure is defined in
<sys/ipc.h> and contains the
following members:
uid_t cuid; /* creator user id */
gid_t cgid; /* creator group id */
uid_t uid; /* user id */
gid_t gid; /* group id */
mode_t mode; /* permission (lower 9 bits) */
semctl() provides the following operations:
-
-
GETVAL
- Return the value of the semaphore.
-
-
SETVAL
- Set the value of the semaphore to
arg.val, where arg is the
fourth argument to semctl().
-
-
GETPID
- Return the pid of the last process that did an operation on
this semaphore.
-
-
GETNCNT
- Return the number of processes waiting to acquire the
semaphore.
-
-
GETZCNT
- Return the number of processes waiting for the value of the
semaphore to reach 0.
-
-
GETALL
- Return the values of all the semaphores associated with
semid.
-
-
SETALL
- Set the values of all the semaphores that are associated
with the semaphore identifier semid to the
corresponding values in arg.array, where
arg is the fourth argument to
semctl().
-
-
IPC_STAT
- Gather information about a semaphore and place the
information in the structure pointed to by arg.buf,
where arg is the fourth argument to
semctl().
-
-
IPC_SET
- Set the value of the sem_perm.uid,
sem_perm.gid and sem_perm.mode
fields in the structure associated with the semaphore. The values are
taken from the corresponding fields in the structure pointed to by
arg.buf, there arg is the
fourth argument to semctl(). This operation can only be
executed by the super-user, or a process that has an effective user id
equal to either sem_perm.cuid or
sem_perm.uid in the data structure associated with
the semaphore.
-
-
IPC_RMID
- Remove the semaphores associated with
semid from the system and destroy the data
structures associated with it. Only the super-user or a process with an
effective uid equal to the sem_perm.cuid or
sem_perm.uid values in the data structure associated
with the semaphore can do this.
The permission to read or change a semaphore (see
semop(2)) is determined by the
sem_perm.mode field in the same way as is done with
files (see
chmod(2)), but the
effective uid can match either the
sem_perm.cuid field
or the
sem_perm.uid field, and the effective gid can
match either
sem_perm.cgid or
sem_perm.gid.
RETURN VALUES
For the
GETVAL
,
GETPID
,
GETNCNT
, and
GETZCNT
operations,
semctl() returns one of the values described
above if successful. All other operations will make
semctl()
return 0 if no errors occur. Otherwise -1 is returned and
errno set to reflect the error.
ERRORS
semctl() will fail if:
-
-
- [
EACCES
]
- The caller has no operation permission for this
semaphore.
-
-
- [
EFAULT
]
- arg.buf or
arg.array specifies an invalid address.
-
-
- [
EINVAL
]
- semid is not a valid message
semaphore identifier.
cmd is not a valid command.
-
-
- [
EPERM
]
- cmd is equal to
IPC_SET
or IPC_RMID
and
the caller is not the super-user, nor does the effective uid match either
the sem_perm.uid or
sem_perm.cuid fields of the data structure
associated with the semaphore.
-
-
- [
ERANGE
]
- cmd is equal to
SETVAL
or SETALL
and the
value to be set is greater than the system semaphore maximum value.
SEE ALSO
semget(2),
semop(2)
STANDARDS
The
semctl system call conforms to
X/Open
System Interfaces and Headers Issue 5 (“XSH5”).
HISTORY
Semaphores appeared in the first release of
AT&T
System V UNIX.