Updated: 2022/Sep/29

Please read Privacy Policy. It's for your privacy.

SEM_OPEN(3)                Library Functions Manual                SEM_OPEN(3)

NAME
     sem_open, sem_close, sem_unlink - named semaphore operations

LIBRARY
     POSIX Real-time Library (librt, -lrt)

SYNOPSIS
     #include <semaphore.h>

     sem_t *
     sem_open(const char *name, int oflag, ...);

     int
     sem_close(sem_t *sem);

     int
     sem_unlink(const char *name);

DESCRIPTION
     The sem_open() function creates or opens the named semaphore specified by
     name.  The returned semaphore may be used in subsequent calls to
     sem_getvalue(3), sem_timedwait(3), sem_trywait(3), sem_wait(3),
     sem_post(3), and sem_close().

     The following bits may be set in the oflag argument:

     O_CREAT      Create the semaphore if it does not already exist.

                  The third argument to the call to sem_open() must be of type
                  mode_t and specifies the mode for the semaphore.  Only the
                  S_IWUSR, S_IWGRP, and S_IWOTH bits are examined; it is not
                  possible to grant only "read" permission on a semaphore.
                  The mode is modified according to the process's file
                  creation mask; see umask(2).

                  The fourth argument must be an unsigned int and specifies
                  the initial value for the semaphore, and must be no greater
                  than SEM_VALUE_MAX.

     O_EXCL       Create the semaphore if it does not exist.  If the semaphore
                  already exists, sem_open() will fail.  This flag is ignored
                  unless O_CREAT is also specified.

     The sem_close() function closes a named semaphore that was opened by a
     call to sem_open().

     The sem_unlink() function removes the semaphore named name.  Resources
     allocated to the semaphore are only deallocated when all processes that
     have the semaphore open close it.

RETURN VALUES
     If successful, the sem_open() function returns the address of the opened
     semaphore.  If the same name argument is given to multiple calls to
     sem_open() by the same process without an intervening call to
     sem_close(), the same address is returned each time.  If the semaphore
     cannot be opened, sem_open() returns SEM_FAILED and the global variable
     errno is set to indicate the error.

     The sem_close() and sem_unlink() functions return the value 0 if
     successful; otherwise the value -1 is returned and the global variable
     errno is set to indicate the error.

ERRORS
     The sem_open() function will fail if:

     [EACCES]           The semaphore exists and the permissions specified by
                        oflag at the time it was created deny access to this
                        process; or the semaphore does not exist, but
                        permission to create it is denied.

     [EEXIST]           O_CREAT and O_EXCL are set but the semaphore already
                        exists.

     [EINTR]            The call was interrupted by a signal.

     [EINVAL]           The name argument does not begin with a `/' or
                        contains more slashes.  This is implementation-
                        specific behavior and allowed by ISO/IEC 9945-1:1996
                        ("POSIX.1").  Or, the value argument is greater than
                        SEM_VALUE_MAX.

     [ENAMETOOLONG]     The specified name is longer than NAME_MAX, or longer
                        than the implementing file system will allow.

     [ENFILE]           The system limit on semaphores has been reached.

     [ENOENT]           O_CREAT is not set and the named semaphore does not
                        exist.

     [ENOSPC]           There is not enough space to create the semaphore.

     The sem_close() function will fail if:

     [EINVAL]           The sem argument is not a valid semaphore.

     The sem_unlink() function will fail if:

     [EACCES]           Permission is denied to unlink the semaphore.

     [ENAMETOOLONG]     The specified name is longer than NAME_MAX, or longer
                        than the implementing file system will allow.

     [ENOENT]           The named semaphore does not exist.

SEE ALSO
     close(2), open(2), umask(2), unlink(2), sem_getvalue(3), sem_post(3),
     sem_trywait(3), sem_wait(3), sem(4)

STANDARDS
     The sem_open(), sem_close(), and sem_unlink() functions conform to
     ISO/IEC 9945-1:1996 ("POSIX.1").

HISTORY
     Support for named semaphores first appeared in NetBSD 2.0.

NetBSD 10.99                      May 4, 2018                     NetBSD 10.99