DragonFly On-Line Manual Pages
SEM_OPEN(3) DragonFly Library Functions Manual SEM_OPEN(3)
sem_open, sem_close, sem_unlink -- named semaphore operations
POSIX Threads Library (libpthread, -lpthread)
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. 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.
The current implementation uses shared memory mappings of files. The
semaphore files are created at the path pointed to by name. If name is
an absolute path, /var/run/sem is prepended to name. The environnment
variable LIBTHREAD_SEM_PREFIX can be set to change this value.
It is not possible to grant only ``read'' permission on a semaphore.
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 and 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.
[EMFILE] Too many filedescriptors are in use by this process.
[ENAMETOOLONG] The name argument is too long or a pathname component
is too long.
[ENFILE] The system limit on semaphores or open files has been
[ENOENT] O_CREAT is not set but the named semaphore does not
[ENOSPC] There is not enough space to create the semaphore.
[ENOMEM] There is insufficient memory for the creation of the
new named 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 or a pathname component
is too long.
[ENOENT] The named semaphore does not exist.
close(2), open(2), umask(2), unlink(2), sem_getvalue(3), sem_post(3),
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 DragonFly 3.9.
DragonFly 4.1 June 4, 2014 DragonFly 4.1