sem_open (3p) - Linux Manuals

sem_open: initialize and open a named semaphore (REALTIME)

PROLOG

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

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.

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 .