flock(2) [redhat man page]

FLOCK(2)						     Linux Programmer's Manual							  FLOCK(2)

NAME

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

SYNOPSIS

       #include <sys/file.h>

       int flock(int fd, int operation);

DESCRIPTION

       Apply or remove an advisory lock on the open file specified by fd.  The parameter operation is one of the following:

	      LOCK_SH	Place a shared lock.  More than one process may hold a shared lock for a given file at a given time.

	      LOCK_EX	Place an exclusive lock.  Only one process may hold an exclusive lock for a given file at a given time.

	      LOCK_UN	Remove an existing lock held by this process.

       A call to flock() may block if an incompatible lock is held by another process.	To make a non-blocking request, include LOCK_NB (by ORing)
       with any of the above operations.

       A single file may not simultaneously have both shared and exclusive locks.

       Locks created by flock() are associated with a file, or, more precisely, an open file table entry.  This means that duplicate file descrip-
       tors  (created  by,  for  example,  fork(2) or dup(2)) refer to the same lock, and this lock may be modified or released using any of these
       descriptors.  Furthermore, the lock is released either by an explicit LOCK_UN operation on any of these duplicate descriptors, or when  all
       such descriptors have been closed.

       A  process may only hold one type of lock (shared or exclusive) on a file.  Subsequent flock() calls on an already locked file will convert
       an existing lock to the new lock mode.

       Locks created by flock() are preserved across an execve(2).

       A shared or exclusive lock can be placed on a file regardless of the mode in which the file was opened.

RETURN VALUE

       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS

       EWOULDBLOCK
	      The file is locked and the LOCK_NB flag was selected.

       EBADF  fd is not a not an open file descriptor.

       EINTR  While waiting to acquire a lock, the call was interrupted by delivery of a signal caught by a handler.

       EINVAL operation is invalid.

       ENOLCK The kernel ran out of memory for allocating lock records.

CONFORMING TO

       4.4BSD (the flock(2) call first appeared in 4.2BSD).  A version of flock(2), possibly implemented in terms of  fcntl(2),  appears  on  most
       Unices.

NOTES

       flock(2)  does  not lock files over NFS.  Use fcntl(2) instead: that does work over NFS, given a sufficiently recent version of Linux and a
       server which supports locking.

       Since kernel 2.0, flock(2) is implemented as a system call in its own right rather than being emulated in the GNU C library as  a  call	to
       fcntl(2).   This yields true BSD semantics: there is no interaction between the types of lock placed by flock(2) and fcntl(2), and flock(2)
       does not detect deadlock.

       flock(2) places advisory locks only; given suitable permissions on a file, a process is free to ignore the use of flock(2) and perform  I/O
       on the file.

       flock(2) and fcntl(2) locks have different semantics with respect to forked processes and dup(2).

SEE ALSO

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

       There are also locks.txt and mandatory.txt in /usr/src/linux/Documentation.

Linux								    2002-04-24								  FLOCK(2)

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