---
name: "sem_open"
section: "2"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["macos"]
url: "https://new.carta.tech/sem_open"
---

# sem\_open(2)

initialize and open a named semaphore

## NAME

**sem\_open** – initialize and open a named semaphore

## SYNOPSIS

**#include \<semaphore.h\>**

*sem\_t* *\**\

**sem\_open**(*const char \*name*, *int oflag*, *...*);

The parameters "mode\_t mode" and "unsigned int value" are optional.

## DESCRIPTION

The named semaphore named *name* is initialized and opened as specified by the argument *oflag* and a semaphore descriptor is returned to the calling process.

The value of *oflag* is formed by *or*'ing the following values:

```
O_CREAT		create the semaphore if it does not exist
O_EXCL		error if create and semaphore exists
```

If O\_CREAT is specified, **sem\_open**() requires an additional two arguments. *mode* specifies the permissions for the semaphore as described in [chmod(2)](/chmod/2.md) and modified by the process' umask value (see [umask(2)](/umask/2.md)). The semaphore is created with an initial *value*, which must be less than or equal to SEM\_VALUE\_MAX.

If O\_EXCL is specified and the semaphore exists, **sem\_open**() fails.  The check for the existence of the semaphore and the creation of the semaphore are atomic with respect to all processes calling **sem\_open**() with O\_CREAT and O\_EXCL set.

When a new semaphore is created, it is given the user ID and group ID which correspond to the effective user and group IDs of the calling process. There is no visible entry in the file system for the created object in this implementation.

There are additional restrictions for non-macOS platforms only: if the calling process is signed with a Team ID entitlement, then *name* is ‘namespaced’, meaning calls to **sem\_open**() or **sem\_unlink**() will not observe any semaphores created by processes signed with different Team IDs. If a process lacks a Team ID entitlement, then **sem\_open**() and **sem\_unlink**() operate on a global namespace, which is always searched as a fallback.

The returned semaphore descriptor is available to the calling process until it is closed with **sem\_close**(), or until the caller exits or execs.

If a process makes repeated calls to **sem\_open**(), with the same *name* argument, the same descriptor is returned for each successful call, unless **sem\_unlink**() has been called on the semaphore in the interim.

If **sem\_open**() fails for any reason, it will return a value of SEM\_FAILED and sets *errno*. On success, it returns a semaphore descriptor.

## ERRORS

The named semaphore is opened unless:

- \[EACCES\]

  The required permissions (for reading and/or writing) are denied for the given flags; or O\_CREAT is specified, the object does not exist, and permission to create the semaphore is denied.

- \[EEXIST\]

  O\_CREAT and O\_EXCL were specified and the semaphore exists.

- \[EINTR\]

  The **sem\_open**() operation was interrupted by a signal.

- \[EINVAL\]

  The **shm\_open**() operation is not supported; or O\_CREAT is specified and *value* exceeds SEM\_VALUE\_MAX.

- \[EMFILE\]

  The process has already reached its limit for semaphores or file descriptors in use.

- \[ENAMETOOLONG\]

  *name* exceeded PSEMNAMLEN characters, or Team ID of calling binary exceeded PSEMTEAMIDLEN characters.

- \[ENFILE\]

  Too many semaphores or file descriptors are open on the system.

- \[ENOENT\]

  O\_CREAT is not set and the named semaphore does not exist.

- \[ENOSPC\]

  O\_CREAT is specified, the file does not exist, and there is insufficient space available to create the semaphore.

## SEE ALSO

[sem\_close(2)](/sem_close/2.md), [sem\_post(2)](/sem_post/2.md), sem\_trywait(2), [sem\_unlink(2)](/sem_unlink/2.md), [sem\_wait(2)](/sem_wait/2.md), [semctl(2)](/semctl/2.md), [semget(2)](/semget/2.md), [semop(2)](/semop/2.md), [umask(2)](/umask/2.md)

## HISTORY

**sem\_open**() is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).