semctl(2)semctl(2)Namesemctl - semaphore control operations
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semctl (semid, semnum, cmd, arg)
int semid, cmd;
int semnum;
union semun {
int val;
struct semid_ds *buf;
ushort array[];
} arg;
Description
The system call provides a variety of semaphore control operations as
specified by cmd. The following cmds are executed with respect to the
semaphore specified by semid and semnum:
GETVAL Return the value of semval. For further information,
see
SETVAL Set the value of semval to arg.val. When this command
is successfully executed, the semadj value corresponding
to the specified semaphore in all processes is cleared.
GETPID Return the value of sempid.
GETNCNT Return the value of semncnt.
GETZCNT Return the value of semzcnt.
The following cmds return and set every semval in the set of sema‐
phores:
GETALL Place semvals into array pointed to by arg.array.
SETALL Set semvals according to the array pointed to by
arg.array When this command is successfully executed,
the semadj values corresponding to each specified sema‐
phore in all processes are cleared.
The following cmds are also available:
IPC_STAT Place the current value of each member of the data
structure associated with semid into the structure
pointed to by arg.buf. The contents of this structure
are defined in
IPC_SET Set the value of the following members of the data
structure associated with semid to the corresponding
value found in the structure pointed to by arg.buf:
sem_perm.uid
sem_perm.gid
sem_perm.mode /* only low 9 bits */
This command can only be executed by a process that has an effective
user ID equal to superuser or to the values of sem_perm.uid or
sem_perm.cuid in the data structure associated with semid.
IPC_RMID Remove the semaphore identifier specified by semid from
the system and destroy the set of semaphores and data
structure associated with it. This cmd can only be exe‐
cuted by a process that has an effective user ID equal
to either that of superuser or to the value of
sem_perm.uid in the data structure associated with
semid.
Return Values
Upon successful completion, the value returned depends on cmd, as fol‐
lows:
GETVAL The value of semval.
GETPID The value of sempid.
GETNCNT The value of semncnt.
GETZCNT The value of semzcnt.
All other A value of 0.
Otherwise, a value of -1 is returned and errno is set to indicate the error.
Diagnostics
The system call fails if any of the following is true:
[EINVAL] The semid is not a valid semaphore identifier.
[EINVAL] The semnum is less than zero or greater than sem_nsems.
[EINVAL] The cmd is not a valid command.
[EACCES] Operation permission is denied to the calling process.
For further information, see
[ERANGE] The cmd is SETVAL or SETALL, and the value to which sem‐
val is to be set is greater than the system imposed max‐
imum.
[EPERM] The cmd is equal to IPC_RMID or IPC_SET and the effec‐
tive user ID of the calling process is not equal to that
of superuser nor to the value of sem_perm.uid in the
data structure associated with semid.
[EFAULT] The arg.buf points to an illegal address.
See Alsoerrno(2), intro(2), semget(2), semop(2)semctl(2)
