SEM_OPEN(3) Library Functions Manual SEM_OPEN(3) 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_get- value(3), sem_wait(3), sem_trywait(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 possi- ble 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 ini- tial 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 al- ready 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 al- located 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 indi- cate 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. 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 ex- ists. [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 ex- ist. [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), 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 FreeBSD 5.0. BUGS This implementation places strict requirements on the value of name: it must begin with a slash (`/'), contain no other slash characters, and be less than 14 characters in length not including the terminating null char- acter. FreeBSD 7.0 January 15, 2003 SEM_OPEN(3)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | STANDARDS | HISTORY | BUGS
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