Computer Science
SEMCTL(2) Linux Programmer's Manual SEMCTL(2)
NAME
semctl - semaphore control operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#if defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED)
/* union semun is defined by including <sys/sem.h> */
#else
/* according to X/OPEN we have to define it ourselves */
union semun {
int val; /* value for SETVAL */
struct semid_ds *buf; /* buffer for IPC_STAT, IPC_SET */
unsigned short int *array; /* array for GETALL, SETALL */
struct seminfo *__buf; /* buffer for IPC_INFO */
};
#endif
int semctl (int semid, int semnum, int cmd, union semun
arg)
DESCRIPTION
The function performs the control operation specified by
cmd on the semaphore set (or on the semnum-th semaphore of
the set) identified by semid. The first semaphore of the
set is indicated by a value 0 for semnum.
Legal values for cmd are
IPC_STAT Copy info from the semaphore set data struc-
ture into the structure pointed to by arg.buf.
The argument semnum is ignored. The calling
process must have read access privileges on
the semaphore set.
IPC_SET Write the values of some members of the
semid_ds structure pointed to by arg.buf to
the semaphore set data structure, updating
also its sem_ctime member. Considered members
from the user supplied struct semid_ds pointed
to by arg.buf are
sem_perm.uid
sem_perm.gid
sem_perm.mode /* only lowest 9-bits */
The calling process effective user-ID must be
one among super-user, creator or owner of the
semaphore set. The argument semnum is
ignored.
IPC_RMID Remove immediately the semaphore set and its
data structures awakening all waiting pro-
cesses (with an error return and errno set to
EIDRM). The calling process effective user-ID
must be one among super-user, creator or owner
of the semaphore set. The argument semnum is
ignored.
GETALL Return semval for all semaphores of the set
into arg.array. The argument semnum is
ignored. The calling process must have read
access privileges on the semaphore set.
GETNCNT The system call returns the value of semncnt
for the semnum-th semaphore of the set (i.e.
the number of processes waiting for an
increase of semval for the semnum-th semaphore
of the set). The calling process must have
read access privileges on the semaphore set.
GETPID The system call returns the value of sempid
for the semnum-th semaphore of the set (i.e.
the pid of the process that executed the last
semop call for the semnum-th semaphore of the
set). The calling process must have read
access privileges on the semaphore set.
GETVAL The system call returns the value of semval
for the semnum-th semaphore of the set. The
calling process must have read access privi-
leges on the semaphore set.
GETZCNT The system call returns the value of semzcnt
for the semnum-th semaphore of the set (i.e.
the number of processes waiting for semval of
the semnum-th semaphore of the set to become
0). The calling process must have read access
privileges on the semaphore set.
SETALL Set semval for all semaphores of the set using
arg.array, updating also the sem_ctime member
of the semid_ds structure associated to the
set. Undo entries are cleared for altered
semaphores in all processes. Processes sleep-
ing on the wait queue are awakened if some
semval becomes 0 or increases. The argument
semnum is ignored. The calling process must
have alter access privileges on the semaphore
set.
SETVAL Set the value of semval to arg.val for the
semnum-th semaphore of the set, updating also
the sem_ctime member of the semid_ds structure
associated to the set. Undo entry is cleared
for altered semaphore in all processes. Pro-
cesses sleeping on the wait queue are awakened
if semval becomes 0 or increases. The calling
process must have alter access privileges on
the semaphore set.
RETURN VALUE
On fail the system call returns -1 with errno indicating
the error. Otherwise the system call returns a nonnega-
tive value depending on cmd as follows:
GETNCNT the value of semncnt.
GETPID the value of sempid.
GETVAL the value of semval.
GETZCNT the value of semzcnt.
ERRORS
For a failing return, errno will be set to one among the
following values:
EACCES The calling process has no access permissions
needed to execute cmd.
EFAULT The address pointed to by arg.buf or arg.array
isn't accessible.
EIDRM The semaphore set was removed.
EINVAL Invalid value for cmd or semid.
EPERM The argument cmd has value IPC_SET or IPC_RMID
but the calling process effective user-ID has
insufficient privileges to execute the command.
ERANGE The argument cmd has value SETALL or SETVAL and
the value to which semval has to be set (for
some semaphore of the set) is less than 0 or
greater than the implementation value SEMVMX.
NOTES
The IPC_INFO, SEM_STAT and SEM_INFO control calls are used
by the ipcs(8) program to provide information on allocated
resources. In the future these can be modified as needed
or moved to a proc file system interface.
The following system limit on semaphore sets affects a
semctl call:
SEMVMX Maximum value for semval: implementation depen-
dent (32767).
CONFORMING TO
SVr4, SVID. SVr4 documents more error conditions EINVAL
and EOVERFLOW.
SEE ALSO
ipc(5), shmget(2), shmat(2), shmdt(2).
Linux 0.99.13 November 1, 1993 1
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