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.

May 4, 2018 | NetBSD 9.4