---
name: "sem_open"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/sem_close/3"
---

# sem\_open(3)

named semaphore operations

Part of [sem\_close(3)](/sem_close/3.md).

## NAME

**sem\_open**, **sem\_close**, **sem\_unlink** – named semaphore operations

## LIBRARY

Standard C Library (libc, -lc)

## 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_getvalue/3.md), [sem\_wait(3)](/sem_wait/3.md), [sem\_trywait(3)](/sem_trywait/3.md), [sem\_post(3)](/sem_post/3.md), and **sem\_close**().

This implementation places strict requirements on the value of *name*: it must begin with a slash (‘/’) and contain no other slash characters.

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)](/umask/2.md).

  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.

- \[EACCES\]

  The semaphore does not exist, but permission to create it is denied.

- \[EEXIST\]

  O\_CREAT and O\_EXCL are set but the semaphore already exists.

- \[EINTR\]

  The call was interrupted by a signal.

- \[EINVAL\]

  The **sem\_open**() operation is not supported for the given *name*.

- \[EINVAL\]

  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 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 too long.

- \[ENOENT\]

  The named semaphore does not exist.

## SEE ALSO

[close(2)](/close/2.md), [open(2)](/open/2.md), [umask(2)](/umask/2.md), [unlink(2)](/unlink/2.md), [sem\_getvalue(3)](/sem_getvalue/3.md), [sem\_post(3)](/sem_post/3.md), [sem\_trywait(3)](/sem_trywait/3.md), [sem\_wait(3)](/sem_wait/3.md)

## 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 FreeBSD 5.0.