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 .