semctl(2) semctl(2)
NAME
semctl - semaphore control operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
union semun {
int val;
struct semidds *buf;
ushort *array;
};
int semctl(int semid, int semnum, int cmd, ... /* union semun arg
*/);
DESCRIPTION
semctl 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 [see intro(2)]. {READ}
SETVAL Set the value of semval to arg.val. {ALTER}. When
this command is successfully executed, the semadj
value corresponding to the specified semaphore in
all processes is cleared.
GETPID Return the value of (int) sempid. {READ}
GETNCNT Return the value of semncnt. {READ}
GETZCNT Return the value of semzcnt. {READ}
The following cmds return and set, respectively, every semval in the
set of semaphores.
GETALL Place semvals into array pointed to by arg.array.
{READ}
SETALL Set semvals according to the array pointed to by
arg.array. {ALTER}. When this cmd is successfully
executed, the semadj values corresponding to each
specified semaphore in all processes are cleared.
The following cmds are also available:
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semctl(2) semctl(2)
IPCSTAT 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 intro(2). {READ}
IPCSET 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:
semperm.uid
semperm.gid
semperm.mode /* only access permission bits */
This command can be executed only by a process that
has an effective user ID equal to either that of
super-user, or to the value of semperm.cuid or
semperm.uid in the data structure associated with
semid.
IPCRMID Remove the semaphore identifier specified by semid
from the system and destroy the set of semaphores
and data structure associated with it. This command
only be executed only by a process that has an
effective user ID equal to either that of super-
user, or to the value of semperm.cuid or
semperm.uid in the data structure associated with
semid.
semctl fails if one or more of the following are true:
EACCES Operation permission is denied to the calling process
[see intro(2)].
EINVAL semid is not a valid semaphore identifier.
EINVAL semnum is less than 0 or greater than semnsems.
EINVAL cmd is not a valid command.
EINVAL cmd is IPCSET and semperm.uid or semperm.gid is not
valid.
EOVERFLOW cmd is IPCSTAT and uid or gid is too large to be
stored in the structure pointed to by arg.buf.
ERANGE cmd is SETVAL or SETALL and the value to which semval
is to be set is greater than the system imposed
maximum.
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semctl(2) semctl(2)
EPERM cmd is equal to IPCRMID or IPCSET and the effective
user ID of the calling process is not equal to that of
super-user, or to the value of semperm.cuid or
semperm.uid in the data structure associated with
semid.
EFAULT arg.buf points to an illegal address.
SEE ALSO
intro(2), semget(2), semop(2).
DIAGNOSTICS
Upon successful completion, the value returned depends on cmd as
follows:
GETVAL the value of semval
GETPID the value of (int) sempid
GETNCNT the value of semncnt
GETZCNT the value of semzcnt
all others
a value of 0
Otherwise, a value of -1 is returned and errno is set to indicate the
error.
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