sem_open(3pthread)
sem_open --
initialize and open a named semaphore
Synopsis
cc [options] -Kthread file
#include <semaphore.h>
sem_t *sem_open(const char *name, int oflag, ...);
Description
The
sem_open
function establishes a connection between a named semaphore and a process.
Following a call to
sem_open
with semaphore name,
the process may reference 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 it 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 int.
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 name
has been created by
sem_open
with the O_CREAT flag, other processes can connect
to it 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.
Flags other than O_CREAT and O_EXCL
specified in the oflag parameter are ignored.
name
points to a string naming a semaphore object.
name
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
will refer to the same semaphore object,
as long as name has not been removed.
If
name
does not begin with the slash character,
the semaphore is opened in the current
working directory of the process.
If a process makes multiple successful calls to
sem_open
with the same value for
name,
the same semaphore address is returned for each such successful call,
provided that there have been no calls to
sem_unlink
for this semaphore.
Return values
Upon successful completion,
the function returns the address of the semaphore.
Otherwise, it returns a value of SEM_FAILED and sets
errno
to indicate the error.
The symbol SEM_FAILED is defined in the header
semaphore.h.
Diagnostics
If any of the following conditions occur, the
sem_open
function will return SEM_FAILED and set
errno
to the corresponding value:
EACCES-
name exists and the permissions specified by
oflag are denied, or
name
does not exist and permission to create
the named semaphore
is denied.
EEXIST-
O_CREAT and O_EXCL are set and name
already exists.
EINTR-
sem_open was interrupted by a signal.
EINVAL-
sem_open
is not supported for 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
string exceeds PATH_MAX,
or a pathname component is longer than
NAME_MAX while _POSIX_NO_TRUNC is in effect.
ENFILE-
Too many semaphores are currently open in the system.
ENOENT-
O_CREAT is not set and name
does not exist.
ENOSPC-
There is insufficient space for the creation of name.
Standards compliance
The Single UNIX Specification, Version 2; The Open Group.
References
sem_close(3pthread),
sem_post(3pthread),
sem_trywait(3pthread),
sem_unlink(3pthread),
sem_wait(3pthread),
semaphore(4)
© 2004 The SCO Group, Inc. All rights reserved.
UnixWare 7 Release 7.1.4 - 25 April 2004