Tribblix: manual page: sem

SEM_OPEN(3C) Standard C Library Functions SEM_OPEN(3C)

NAME

sem_open - initialize/open a named semaphore

SYNOPSIS

#include <semaphore.h>

sem_t *sem_open(const char *name, int oflag,
/* unsigned long mode, unsigned int value */ ...);

DESCRIPTION

The sem_open() function establishes a connection between a named
semaphore and a process (or LWP or thread). 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(3C), sem_trywait(3C), sem_post(3C), and sem_close(3C). The
semaphore remains usable by this process until the semaphore is
closed by a successful call to sem_close(3C), _Exit(2), 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 (see umask(2)).
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. The first
character of name must be a slash (/) character and the remaining
characters of name cannot include any slash characters. For maximum
portability, name should include no more than 14 characters, but
this limit is not enforced.

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(3C) for this semaphore.

References to copies of the semaphore produce undefined results.

The sem_init(3C) function is used with unnamed semaphores.

RETURN VALUES

Upon successful completion, the function returns the address of the
semaphore. Otherwise, it will return a value of SEM_FAILED and set
errno to indicate the error. The symbol SEM_FAILED is defined in the
header <semaphore.h>. No successful return from sem_open() will
return the value SEM_FAILED.

ERRORS

If any of the following conditions occur, the sem_open() function
will return SEM_FAILED and set errno to the corresponding value:

EACCES
The named semaphore exists and the O_RDWR permissions
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() function was interrupted by a signal.

EINVAL
The sem_open() operation is not supported for the
given name, or O_CREAT was set in oflag and value is
greater than SEM_VALUE_MAX.

EMFILE
The number of open semaphore descriptors in this
process exceeds SEM_NSEMS_MAX, or the number of open
file descriptors in this process exceeds OPEN_MAX.

ENAMETOOLONG
The length of 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 the named semaphore does not
exist.

ENOSPC
There is insufficient space for the creation of the
new named semaphore.

ENOSYS
The sem_open() function is not supported by the
system.