Originální popis anglicky:
semctl - semaphore control operations
Návod, kniha: Linux Programmer's Manual
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int semctl(int semid, int semnum, int cmd, ...);
The function
semctl performs the control operation specified by
cmd on the semaphore set identified by
semid, or on the
semnum-th semaphore of that set. (Semaphores are numbered starting at
0.)
This function has three or four arguments. When there are four, the call is
semctl(semid,semnum,cmd,arg);
where the fourth argument
arg has a type
union semun defined as
follows:
#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 *array; /* array for GETALL, SETALL */
/* Linux specific part: */
struct seminfo *__buf; /* buffer for IPC_INFO */
};
#endif
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 effective user-ID of the calling process must be that of the super-user,
or match the creator or owner of the semaphore set. The argument
semnum is ignored.
- IPC_RMID
- Immediately remove the semaphore set and its data
structures awakening all waiting processes (with an error return and
errno set to EIDRM). The effective user-ID of the calling
process must be that of the super-user, or match the 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 entries are cleared for altered semaphores 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 failure
semctl 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.
All other
cmd values return 0 on success.
On failure,
errno will be set to one of the following:
- EACCES
- The argument cmd has one of the values
GETALL, GETPID, GETVAL, GETNCNT,
GETZCNT, IPC_STAT, SEM_STAT, SETALL, or
SETVAL and the calling process does not have the required
permissions on the semaphore set and does not have the
CAP_IPC_OWNER capability.
- 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 but the effective user ID of the calling process
is not the creator (as found in sem_perm.cuid) or the owner (as
found in sem_perm.uid) of the semaphore set, and the process does
not have the CAP_SYS_ADMIN capability.
- 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).
For greater portability it is best to always call
semctl with four
arguments.
Under Linux, the function
semctl is not a system call, but is implemented
via the system call
ipc(2).
SVr4, SVID. SVr4 documents more error conditions EINVAL and EOVERFLOW.
ipc(2),
semget(2),
semop(2),
ipc(5),
capabilities(7)
