int
semctl(
int semid
, int semnum
, int cmd
, ...
)
)
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
SETVAL
arg.val
,
where
arg
is the fourth argument to
semctl(
).
GETPID
GETNCNT
GETZCNT
GETALL
semid
.
SETALL
semid
to the corresponding values in
arg.array
,
where
arg
is the fourth argument to
semctl(
).
IPC_STAT
arg.buf
,
where
arg
is the fourth argument to
semctl(
).
IPC_SET
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
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.
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.
)
will fail if:
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.
EACCES
]
EINVAL
]
semid
is not a valid message semaphore identifier.
cmd is not a valid command.
EFAULT
]
arg.buf
or
arg.array
specifies an invalid address.
ERANGE
]
cmd
is equal to
SETVAL
or
SETALL
and the value to be set is greater than the system semaphore maximum value.
AT&T
System V UNIX
.