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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. NetBSD 8.99.34 May 4, 2018 NetBSD 8.99.34