SEMCTL(S) UNIX System V SEMCTL(S)
Name
semctl - 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 semctl 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 (see intro(S)).
{READ}
SETVAL Set the value of semval to arg.val. {ALTER}
When this cmd is successfully executed, the
semadj value corresponding to the specified
semaphore in all processes is cleared.
GETPID Return the value of 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:
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(S). {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 /* only low 9 bits */
This cmd can only be executed by a process
that has an effective user ID equal to
either that of super-user, or to the value
of sem_perm.cuid or sem_perm.uid 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 executed by a
process that has an effective user ID equal
to either that of super-user, or to the
value of sem_perm.cuid or sem_perm.uid in
the data structure associated with semid.
The semctl system call fails if one or more of the
following is true:
[EINVAL] semid is not a valid semaphore identifier.
[EINVAL] semnum is less than zero or greater than
sem_nsems.
[EINVAL] cmd is not a valid command.
[EACCES] Operation permission is denied to the
calling process (see intro(S)).
[ERANGE] cmd is SETVAL or SETALL and the value to
which semval is to be set is greater than
the system imposed maximum.
[EPERM] cmd is equal to IPC_RMID or IPC_SET and the
effective user ID of the calling process is
not equal to that of super-user or to the
value of sem_perm.cuid or sem_perm.uid in
the data structure associated with semid.
[EFAULT] arg.buf points to an illegal address.
See Also
intro(S), semget(S), semop(S)
Diagnostics
Upon successful completion, the value returned depends on
cmd as follows:
GETVAL The value of semval.
GETPID The value of 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.
Standards Conformance
semctl is conformant with:
AT&T SVID Issue 2, Select Code 307-127;
and The X/Open Portability Guide II of January 1987.
(printed 6/20/89)