semctl - semaphore control operations
#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)
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 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 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 sem-
num-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 sem-
num-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 sem-
num-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 sem-
num-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.
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.
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.
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.
Various fields in a struct semid_ds were shorts under Linux 2.2 and
have become longs under Linux 2.4. To take advantage of this, a recompilation
under glibc-2.1.91 or later should suffice. (The kernel distinguishes
old and new calls by a IPC_64 flag in cmd.)
The following system limit on semaphore sets affects a semctl call:
SEMVMX Maximum value for semval: implementation dependent (32767).
SVr4, SVID. SVr4 documents more error conditions EINVAL and EOVERFLOW.
ipc(5), shmget(2), shmat(2), shmdt(2)
Linux 2.4.1 2001-02-18 SEMCTL(2)
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