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NAME
sem_open - initialize and open a named semaphore (REALTIME)
SYNOPSIS
#include <semaphore.h> sem_t *sem_open(const char *name, int oflag, ...);
DESCRIPTION
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.
RETURN VALUE
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.
ERRORS
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.
EXAMPLES
None.
APPLICATION USAGE
The sem_open() function is part of the Semaphores option and need not be available on all
implementations.
RATIONALE
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.
FUTURE DIRECTIONS
None.
SEE ALSO
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>
COPYRIGHT
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 .