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 structure
into the structure pointed to by arg.buf. The
argument semnum is ignored. The calling process
must have read access privileges on the sema-
phore 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 sup-
plied 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 processes
(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 privileges 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 sleeping 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.
Processes 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 nonnegative
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).