Originální popis anglicky:
semctl - XSI semaphore control operations
Návod, kniha: POSIX Programmer's Manual
#include <sys/sem.h>
int semctl(int
semid, int
semnum , int cmd,
...);
The
semctl() function operates on XSI semaphores (see the Base
Definitions volume of IEEE Std 1003.1-2001, Section 4.15,
Semaphore). It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in
Realtime .
The
semctl() function provides a variety of semaphore control operations
as specified by
cmd. The fourth argument is optional and depends upon
the operation requested. If required, it is of type
union semun, which
the application shall explicitly declare:
union semun {
int val;
struct semid_ds *buf;
unsigned short *array;
} arg;
The following semaphore control operations as specified by
cmd are
executed with respect to the semaphore specified by
semid and
semnum. The level of permission required for each operation is shown
with each command; see
XSI Interprocess Communication . The
symbolic names for the values of
cmd are defined in the
<sys/sem.h> header:
- GETVAL
- Return the value of semval; see
<sys/sem.h>. Requires read permission.
- SETVAL
- Set the value of semval to arg.val, where
arg is the value of the fourth argument to semctl(). When
this command is successfully executed, the semadj value
corresponding to the specified semaphore in all processes is cleared.
Requires alter permission; see XSI Interprocess Communication
.
- GETPID
- Return the value of sempid. Requires read
permission.
- GETNCNT
- Return the value of semncnt. Requires read
permission.
- GETZCNT
- Return the value of semzcnt. Requires read
permission.
The following values of
cmd operate on each
semval in the set of
semaphores:
- GETALL
- Return the value of semval for each semaphore in the
semaphore set and place into the array pointed to by arg.array,
where arg is the fourth argument to semctl(). Requires read
permission.
- SETALL
- Set the value of semval for each semaphore in the
semaphore set according to the array pointed to by arg.array, where
arg is the fourth argument to semctl(). When this command is
successfully executed, the semadj values corresponding to each
specified semaphore in all processes are cleared. Requires alter
permission.
The following values of
cmd are also available:
- IPC_STAT
- Place the current value of each member of the
semid_ds data structure associated with semid into the
structure pointed to by arg.buf, where arg is the fourth
argument to semctl(). The contents of this structure are defined in
<sys/sem.h>. Requires read permission.
- IPC_SET
- Set the value of the following members of the
semid_ds data structure associated with semid to the
corresponding value found in the structure pointed to by arg.buf,
where arg is the fourth argument to semctl():
sem_perm.uid
sem_perm.gid
sem_perm.mode
The mode bits specified in
IPC General Description are copied into the
corresponding bits of the
sem_perm.mode associated with
semid.
The stored values of any other bits are unspecified.
This command can only be executed by a process that has an effective user ID
equal to either that of a process with appropriate privileges or to the value
of
sem_perm.cuid or
sem_perm.uid in the
semid_ds data
structure associated with
semid.
- IPC_RMID
- Remove the semaphore identifier specified by semid
from the system and destroy the set of semaphores and semid_ds data
structure associated with it. This command can only be executed by a
process that has an effective user ID equal to either that of a process
with appropriate privileges or to the value of sem_perm.cuid or
sem_perm.uid in the semid_ds data structure associated with
semid.
If successful, the value returned by
semctl() 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
- 0.
Otherwise,
semctl() shall return -1 and set
errno to indicate the
error.
The
semctl() function shall fail if:
- EACCES
- Operation permission is denied to the calling process; see
XSI Interprocess Communication .
- EINVAL
- The value of semid is not a valid semaphore
identifier, or the value of semnum is less than 0 or greater than
or equal to sem_nsems, or the value of cmd is not a valid
command.
- EPERM
- The argument cmd is equal to IPC_RMID or IPC_SET and
the effective user ID of the calling process is not equal to that of a
process with appropriate privileges and it is not equal to the value of
sem_perm.cuid or sem_perm.uid in the data structure
associated with semid.
- ERANGE
- The argument cmd is equal to SETVAL or SETALL and
the value to which semval is to be set is greater than the
system-imposed maximum.
The following sections are informative.
None.
The fourth parameter in the SYNOPSIS section is now specified as
"..." in order to avoid a clash with the ISO C
standard when referring to the union
semun (as defined in Issue 3) and
for backwards-compatibility.
The POSIX Realtime Extension defines alternative interfaces for interprocess
communication. Application developers who need to use IPC should design their
applications so that modules using the IPC routines described in
XSI
Interprocess Communication can be easily modified to use the alternative
interfaces.
None.
None.
XSI Interprocess Communication ,
Realtime ,
semget() ,
semop() ,
sem_close() ,
sem_destroy() ,
sem_getvalue() ,
sem_init() ,
sem_open() ,
sem_post() ,
sem_unlink() ,
sem_wait() , the Base
Definitions volume of IEEE Std 1003.1-2001,
<sys/sem.h>
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
