SEM_OPEN(3) BSD Library Functions Manual SEM_OPEN(3)NAME
sem_open, sem_close, sem_unlink — named semaphore operations
Standard C Library (libc, -lc)
sem_open(const char *name, int oflag, ...);
sem_unlink(const char *name);
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_wait(3), sem_trywait(3), sem_post(3), and
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 pos‐
sible to grant only “read” permission on a semaphore. The mode
is modified according to the process's file creation mask; see
The fourth argument must be an unsigned int and specifies the
initial value for the semaphore, and must be no greater than
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.
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 success‐
ful; otherwise the value -1 is returned and the global variable errno is
set to indicate the error.
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
[EACCES] The semaphore does not exist, but permission to create
it is denied.
[EEXIST] O_CREAT and O_EXCL are set but the semaphore already
[EINTR] The call was interrupted by a signal.
[EINVAL] The sem_open() operation is not supported for the
[EINVAL] The value argument is greater than SEM_VALUE_MAX.
[ENAMETOOLONG] The name argument is too long.
[ENFILE] The system limit on semaphores has been reached.
[ENOENT] O_CREAT is not set but the named semaphore does not
[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 too long.
[ENOENT] The named semaphore does not exist.
SEE ALSOclose(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”).
Support for named semaphores first appeared in FreeBSD 5.0.
This implementation places strict requirements on the value of name: it
must begin with a slash (‘/’), contain no other slash characters, and be
less than 14 characters in length not including the terminating null
BSD January 15, 2003 BSD