Originální popis anglicky:
sem_open - initialize and open a named semaphore (
REALTIME)
Návod, kniha: POSIX Programmer's Manual
#include <semaphore.h>
sem_t *sem_open(const char *
name, int
oflag , ...);
The
sem_open() function shall establish a connection between a named
semaphore and a process. Following a call to
sem_open() with semaphore
name
name, the process may reference the semaphore associated with
name using the address returned from the call. This semaphore may be
used in subsequent calls to
sem_wait(),
sem_trywait(),
sem_post(), and
sem_close(). The semaphore remains usable by
this process until the semaphore is closed by a successful call to
sem_close(),
_exit(), or one of the
exec functions.
The
oflag argument controls whether the semaphore is created or merely
accessed by the call to
sem_open(). The following flag bits may be set
in
oflag:
- O_CREAT
- This flag is used to create a semaphore if it does not
already exist. If O_CREAT is set and the semaphore already exists, then
O_CREAT has no effect, except as noted under O_EXCL. Otherwise,
sem_open() creates a named semaphore. The O_CREAT flag requires a
third and a fourth argument: mode, which is of type mode_t,
and value, which is of type unsigned. The semaphore is
created with an initial value of value. Valid initial values for
semaphores are less than or equal to {SEM_VALUE_MAX}.
The user ID of the semaphore is set to the effective user ID of the process; the
group ID of the semaphore is set to a system default group ID or to the
effective group ID of the process. The permission bits of the semaphore are
set to the value of the
mode argument except those set in the file mode
creation mask of the process. When bits in
mode other than the file
permission bits are specified, the effect is unspecified.
After the semaphore named
name has been created by
sem_open() with
the O_CREAT flag, other processes can connect to the semaphore by calling
sem_open() with the same value of
name.
- O_EXCL
- If O_EXCL and O_CREAT are set, sem_open() fails if
the semaphore name exists. The check for the existence of the
semaphore and the creation of the semaphore if it does not exist are
atomic with respect to other processes executing sem_open() with
O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT is not set, the
effect is undefined.
If flags other than O_CREAT and O_EXCL are specified in the
oflag
parameter, the effect is unspecified.
The
name argument points to a string naming a semaphore object. It is
unspecified whether the name appears in the file system and is visible to
functions that take pathnames as arguments. The
name argument conforms
to the construction rules for a pathname. If
name begins with the slash
character, then processes calling
sem_open() with the same value of
name shall refer to the same semaphore object, as long as that name has
not been removed. If
name does not begin with the slash character, the
effect is implementation-defined. The interpretation of slash characters other
than the leading slash character in
name is implementation-defined.
If a process makes multiple successful calls to
sem_open() with the same
value for
name, the same semaphore address shall be returned for each
such successful call, provided that there have been no calls to
sem_unlink() for this semaphore.
References to copies of the semaphore produce undefined results.
Upon successful completion, the
sem_open() function shall return the
address of the semaphore. Otherwise, it shall return a value of SEM_FAILED and
set
errno to indicate the error. The symbol SEM_FAILED is defined in
the
<semaphore.h> header. No successful return from
sem_open() shall return the value SEM_FAILED.
If any of the following conditions occur, the
sem_open() function shall
return SEM_FAILED and set
errno to the corresponding value:
- EACCES
- The named semaphore exists and the permissions specified by
oflag are denied, or the named semaphore does not exist and
permission to create the named semaphore is denied.
- EEXIST
- O_CREAT and O_EXCL are set and the named semaphore already
exists.
- EINTR
- The sem_open() operation was interrupted by a
signal.
- EINVAL
- The sem_open() operation is not supported for the
given name, or O_CREAT was specified in oflag and value was
greater than {SEM_VALUE_MAX}.
- EMFILE
- Too many semaphore descriptors or file descriptors are
currently in use by this process.
- ENAMETOOLONG
- The length of the name argument exceeds {PATH_MAX}
or a pathname component is longer than {NAME_MAX}.
- ENFILE
- Too many semaphores are currently open in the system.
- ENOENT
- O_CREAT is not set and the named semaphore does not
exist.
- ENOSPC
- There is insufficient space for the creation of the new
named semaphore.
The following sections are informative.
None.
The
sem_open() function is part of the Semaphores option and need not be
available on all implementations.
Early drafts required an error return value of -1 with the type
sem_t
* for the
sem_open() function, which is not guaranteed to be
portable across implementations. The revised text provides the symbolic error
code SEM_FAILED to eliminate the type conflict.
None.
semctl() ,
semget() ,
semop() ,
sem_close() ,
sem_post() ,
sem_timedwait() ,
sem_trywait() ,
sem_unlink() ,
sem_wait() , the Base Definitions volume of
IEEE Std 1003.1-2001,
<semaphore.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
.
