flopen(3) [debian man page]

FLOPEN(3)						   BSD Library Functions Manual 						 FLOPEN(3)

NAME

     flopen -- reliably open and lock a file

LIBRARY

     Utility functions from BSD systems (libbsd, -lbsd)

SYNOPSIS

     #include <sys/fcntl.h>
     #include <bsd/libutil.h>

     int
     flopen(const char *path, int flags);

     int
     flopen(const char *path, int flags, mode_t mode);

DESCRIPTION

     The flopen() function opens or creates a file and acquires an exclusive lock on it.  It is essentially equivalent with calling open() with
     the same parameters followed by flock() with an operation argument of LOCK_EX, except that flopen() will attempt to detect and handle races
     that may occur between opening / creating the file and locking it.  Thus, it is well suited for opening lock files, PID files, spool files,
     mailboxes and other kinds of files which are used for synchronization between processes.

     If flags includes O_NONBLOCK and the file is already locked, flopen() will fail and set errno to EWOULDBLOCK.

     As with open(), the additional mode argument is required if flags includes O_CREAT.

RETURN VALUES

     If successful, flopen() returns a valid file descriptor.  Otherwise, it returns -1, and sets errno as described in flock(2) and open(2).

SEE ALSO

     errno(2), flock(2), open(2)

AUTHORS

     The flopen function and this manual page were written by Dag-Erling Smorgrav <des@FreeBSD.org>.

BSD
								   June 6, 2009 							       BSD

FLOCK(2)						      BSD System Calls Manual							  FLOCK(2)

NAME

     flock -- apply or remove an advisory lock on an open file

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/file.h>
     #define   LOCK_SH	      0x01	/* shared file lock */
     #define   LOCK_EX	      0x02	/* exclusive file lock */
     #define   LOCK_NB	      0x04	/* do not block when locking */
     #define   LOCK_UN	      0x08	/* unlock file */

     int
     flock(int fd, int operation);

DESCRIPTION

     The flock() system call applies or removes an advisory lock on the file associated with the file descriptor fd.  A lock is applied by speci-
     fying an operation argument that is one of LOCK_SH or LOCK_EX with the optional addition of LOCK_NB.  To unlock an existing lock operation
     should be LOCK_UN.

     Advisory locks allow cooperating processes to perform consistent operations on files, but do not guarantee consistency (i.e., processes may
     still access files without using advisory locks possibly resulting in inconsistencies).

     The locking mechanism allows two types of locks: shared locks and exclusive locks.  At any time multiple shared locks may be applied to a
     file, but at no time are multiple exclusive, or both shared and exclusive, locks allowed simultaneously on a file.

     A shared lock may be upgraded to an exclusive lock, and vice versa, simply by specifying the appropriate lock type; this results in the pre-
     vious lock being released and the new lock applied (possibly after other processes have gained and released the lock).

     Requesting a lock on an object that is already locked normally causes the caller to be blocked until the lock may be acquired.  If LOCK_NB is
     included in operation, then this will not happen; instead the call will fail and the error EWOULDBLOCK will be returned.

NOTES

     Locks are on files, not file descriptors.	That is, file descriptors duplicated through dup(2) or fork(2) do not result in multiple instances
     of a lock, but rather multiple references to a single lock.  If a process holding a lock on a file forks and the child explicitly unlocks the
     file, the parent will lose its lock.

     The flock(), fcntl(2), and lockf(3) locks are compatible.	Processes using different locking interfaces can cooperate over the same file
     safely.  However, only one of such interfaces should be used within the same process.  If a file is locked by a process through flock(), any
     record within the file will be seen as locked from the viewpoint of another process using fcntl(2) or lockf(3), and vice versa.

     Processes blocked awaiting a lock may be awakened by signals.

RETURN VALUES

     The flock() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate
     the error.

ERRORS

     The flock() system call fails if:

     [EWOULDBLOCK]	The file is locked and the LOCK_NB option was specified.

     [EBADF]		The argument fd is an invalid descriptor.

     [EINVAL]		The argument fd refers to an object other than a file.

     [EOPNOTSUPP]	The argument fd refers to an object that does not support file locking.

     [ENOLCK]		A lock was requested, but no locks are available.

SEE ALSO

     close(2), dup(2), execve(2), fcntl(2), fork(2), open(2), flopen(3), lockf(3)

HISTORY

     The flock() system call appeared in 4.2BSD.

BSD
								 November 9, 2011							       BSD