SEMCTL(2) System Calls SEMCTL(2)


NAME


semctl - semaphore control operations

SYNOPSIS


#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

int semctl(int semid, int semnum, int cmd...);


DESCRIPTION


The semctl() function provides a variety of semaphore control operations
as specified by cmd. The fourth argument is optional, depending upon the
operation requested. If required, it is of type union semun, which
must be explicitly declared by the application program.

union semun {
int val;
struct semid_ds *buf;
ushort_t *array;
} arg ;


The permission required for a semaphore operation is given as {token},
where token is the type of permission needed. The types of permission
are interpreted as follows:

00400 READ by user
00200 ALTER by user
00040 READ by group
00020 ALTER by group
00004 READ by others
00002 ALTER by others


See the Semaphore Operation Permissions subsection of the DEFINITIONS
section of Intro(2) for more information. The following semaphore
operations as specified by cmd 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 operations 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 operations 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
Intro(2). {READ}


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 /* access permission bits only */

This command can be executed only by a process that has
either the {PRIV_IPC_OWNER} privilege or an effective user ID
equal to the value of msg_perm.cuid or msg_perm.uid in the
data structure associated with msqid. Only a process with the
{PRIV_SYS_IPC_CONFIG} privilege can raise the value of
msg_qbytes.


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 command can be executed only by a
process that has the {PRIV_IPC_OWNER} privilege or an
effective user ID equal to the value of sem_perm.cuid or
sem_perm.uid in the data structure associated with semid.


RETURN VALUES


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 other successful completions return 0; otherwise, -1 is returned and
errno is set to indicate the error.

ERRORS


The semctl() function will fail if:

EACCES
Operation permission is denied to the calling process (see
Intro(2)).


EFAULT
The source or target is not a valid address in the user
process.


EINVAL
The semid argument is not a valid semaphore identifier; the
semnum argument is less than 0 or greater than sem_nsems -1;
or the cmd argument is not a valid command or is IPC_SET and
sem_perm.uid or sem_perm.gid is not valid.


EPERM
The cmd argument is equal to IPC_RMID or IPC_SET, the
effective user ID of the calling process is not equal to the
value of sem_perm.cuid or sem_perm.uid in the data structure
associated with semid, and {PRIV_IPC_OWNER} is not asserted
in the effective set of the calling process.


EOVERFLOW
The cmd argument is IPC_STAT and uid or gid is too large to
be stored in the structure pointed to by arg.buf.


ERANGE
The cmd argument is SETVAL or SETALL and the value to which
semval is to be set is greater than the system imposed
maximum.


ATTRIBUTES


See attributes(5) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+

SEE ALSO


ipcs(1), Intro(2), semget(2), semop(2), attributes(5), privileges(5),
standards(5)


February 1, 2003 SEMCTL(2)