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sem_open(3) [osf1 man page]

sem_open(3)						     Library Functions Manual						       sem_open(3)

sem_open - Opens/creates a named semaphore for use by a process (P1003.1b) LIBRARY
Realtime Library (, librt.a) SYNOPSIS
#include <semaphore.h> sem_t *sem_open ( const char *name, int oflag, mode_t mode, unsigned int value); PARAMETERS
name Points to a string naming the semaphore object. oflag Specifies whether the semaphore is to be created (O_CREAT flag bit set) or only opened (O_CREAT flag bit clear). If O_CREAT is set, the O_EXCL flag bit may additionally be set to specify that the call should fail if a semaphore of the same name already exists. The O_CREAT and O_EXCL flags are defined in the <fcntl.h> header file. mode Specifies the semaphore's permission bits. This argument is used only when the semaphore is being created. value Specifies the initial value to be given to the semaphore. This argument is used only when the semaphore is being created. DESCRIPTION
Use the sem_open function to establish the connection between a named semaphore and a process. Subsequently, the calling process can ref- erence the semaphore by using the address returned from the call. The semaphore is available in subsequent calls to sem_wait, sem_trywait, sem_post, and sem_getvalue functions. The semaphore remains usable by the process until the semaphore is closed by a successful call to the sem_close function. The O_CREAT flag bit in the oflag parameter controls whether the semaphore is created or only opened by the call to sem_open. A created semaphore's user ID is set to the user ID of the calling process and its group ID is set to a system default group or to the group ID of the process. The semaphore's permission bits are set to the value of the mode argument, except for those set in the file mode creation mask of the process. After a semaphore is created, other processes can open the semaphore by calling sem_open with the same value for the name argument. RETURN VALUES
On successful completion, this function opens the semaphore and returns the semaphore's descriptor. Otherwise, the function returns -1 and sets errno to indicate the error. ERRORS
The sem_open function fails under the following conditions: [EACCES] The named semaphore exists and the permissions specified by oflag are denied, or the named semaphore does not exist and the permissions specified by oflag are denied. [EEXIST] O_CREAT and O_EXCL are set and the named semaphore already exists. [EINTR] The sem_open operation was interrupted by a signal. [EINVAL] The sem_open operation is not supported for the given name. Or, O_CREAT was specified in oflag and value was greater than {SEM_VALUE_MAX}. [EMFILE] Too many semaphore descriptors or file descriptors are currently in use by this process. [ENAMETOOLONG] The length of the name string exceeds {PATH_MAX}, or a pathname component is longer than {NAME_MAX} while {_POSIX_NO_TRUNC} is in effect. [ENFILE] Too many semaphores are currently open in the system. [ENOENT] O_CREAT is not set and the named semaphore does not exist. [ENOSPC] Insufficient space exists for the creation of a new named semaphore. RELATED INFORMATION
Functions: sem_close(3), sem_post(3), sem_trywait(3), sem_unlink(3), sem_wait(3) Guide to Realtime Programming delim off sem_open(3)

Check Out this Related Man Page

SEM_OPEN(3)						   BSD Library Functions Manual 					       SEM_OPEN(3)

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_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 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. [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 exists. [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 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 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 NetBSD 2.0. 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 character. BSD
January 21, 2005 BSD

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