#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h> int
semctl (int semid int semnum int cmd ...);
DESCRIPTION
The
semctl ();
system call
performs the operation indicated by
Fa cmd
on the semaphore set indicated by
Fa semid .
A fourth argument, a
Fa union semun arg ,
is required for certain values of
Fa cmd .
For the commands that use the
Fa arg
argument,
Fa union semun
is defined as follows:
union semun {
int val; /* value for SETVAL */
struct semid_ds *buf; /* buffer for IPC_STAT & IPC_SET */
u_short *array; /* array for GETALL & SETALL */
};
Commands are performed as follows:
IPC_STAT
Fetch the semaphore set's
Fa struct semid_ds ,
storing it in the memory pointed to by
Fa arg.buf .
IPC_SET
Changes the
Fa sem_perm.uid ,
Fa sem_perm.gid ,
and
Fa sem_perm.mode
members of the semaphore set's
Fa struct semid_ds
to match those of the struct pointed to by
Fa arg.buf .
The calling process's effective uid must
match either
Fa sem_perm.uid
or
Fa sem_perm.cuid ,
or it must have superuser privileges.
IPC_RMID
Immediately removes the semaphore set from the system.
The calling
process's effective uid must equal the semaphore set's
Fa sem_perm.uid
or
Fa sem_perm.cuid ,
or the process must have superuser privileges.
GETVAL
Return the value of semaphore number
Fa semnum .
SETVAL
Set the value of semaphore number
Fa semnum
to
Fa arg.val .
Outstanding adjust on exit values for this semaphore in any process
are cleared.
GETPID
Return the pid of the last process to perform an operation on
semaphore number
Fa semnum .
GETNCNT
Return the number of processes waiting for semaphore number
Fa semnum Ns 's
value to become greater than its current value.
GETZCNT
Return the number of processes waiting for semaphore number
Fa semnum Ns 's
value to become 0.
GETALL
Fetch the value of all of the semaphores in the set into the
array pointed to by
Fa arg.array .
SETALL
Set the values of all of the semaphores in the set to the values
in the array pointed to by
Fa arg.array .
Outstanding adjust on exit values for all semaphores in this set,
in any process are cleared.
The
Vt struct semid_ds
is defined as follows:
struct semid_ds {
struct ipc_perm sem_perm; /* operation permission struct */
struct sem *sem_base; /* pointer to first semaphore in set */
u_short sem_nsems; /* number of sems in set */
time_t sem_otime; /* last operation time */
long sem_pad1; /* SVABI/386 says I need this here */
time_t sem_ctime; /* last change time */
/* Times measured in secs since */
/* 00:00:00 GMT, Jan. 1, 1970 */
long sem_pad2; /* SVABI/386 says I need this here */
long sem_pad3[4]; /* SVABI/386 says I need this here */
};
RETURN VALUES
On success, when
Fa cmd
is one of
GETVAL , GETPID , GETNCNT
or
GETZCNT
semctl ();
returns the corresponding value; otherwise, 0 is returned.
On failure, -1 is returned, and
errno
is set to indicate the error.
ERRORS
The
semctl ();
system call
will fail if:
Bq Er EINVAL
No semaphore set corresponds to
Fa semid .
Bq Er EINVAL
The
Fa semnum
argument
is not in the range of valid semaphores for given semaphore set.
Bq Er EPERM
The calling process's effective uid does not match the uid of
the semaphore set's owner or creator.
Bq Er EACCES
Permission denied due to mismatch between operation and mode of
semaphore set.
Bq Er ERANGE
SETVAL
or
SETALL
attempted to set a semaphore outside the allowable range
Bq 0 .. Dv SEMVMX .
