sem_open(3RT) Realtime Library Functions sem_open(3RT)NAMEsem_open - initialize/open a named semaphore
SYNOPSIS
cc [ flag... ] file... -lrt [ library... ]
#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 sema‐
phore and a process (or LWP or thread). Following a call to sem_open()
with semaphore name name, the process may reference the semaphore asso‐
ciated with name using the address returned from the call. This sema‐
phore may be used in subsequent calls to sem_wait(3RT), sem_try‐
wait(3RT), sem_post(3RT), and sem_close(3RT). The semaphore remains
usable by this process until the semaphore is closed by a successful
call to sem_close(3RT), _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 pro‐
cesses 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 con‐
forms 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(3RT) for this semaphore.
References to copies of the semaphore produce undefined results.
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 per‐
missions 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 sema‐
phore 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.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Standard │
├─────────────────────────────┼─────────────────────────────┤
│MT-Level │MT-Safe │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOexec(2), exit(2), umask(2), sem_close(3RT), sem_post(3RT),
sem_unlink(3RT), sem_wait(3RT), sysconf(3C), attributes(5), stan‐
dards(5)SunOS 5.10 28 Jun 2002 sem_open(3RT)
