file lock

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flock

flock(2)							System Calls Manual							  flock(2)

NAME

       flock - Applies or removes an advisory lock on an open file

SYNOPSIS

       #include <sys/fcntl.h>

       int flock(      int filedes,	 int operation );

PARAMETERS

       Specifies a file descriptor returned by a successful open() or fcntl() function, identifying the file to which the lock is to be applied or
       removed.  Specifies one of the following constants for flock(), defined in the fcntl.h file: Apply a shared lock.  Apply an exclusive lock.
       Do not block when locking. This value can be logically ORed with either LOCK_SH or LOCK_EX.  Remove a lock.

DESCRIPTION

       The  flock()  function  applies	or removes an advisory lock on the file associated with the filedes file descriptor.  Advisory locks allow
       cooperating processes to perform consistent operations on files, but do not guarantee consistency (that	is,  processes	may  still  access
       files without using advisory locks, possibly resulting in inconsistencies).

       You can use the flock() function to coordinate a file's lock status on local, CFS, and NFS file systems.

       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
       previous 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 errno will be set to [EWOULDBLOCK].

NOTES

       Locks are on files, not file descriptors.  That is, file descriptors duplicated using the dup() or fork() functions do not result in multi-
       ple  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.

       Processes blocked awaiting a lock may be awakened by signals.

       The flock() interface is not part of any UNIX standard.	Therefore, if you are designing and writing applications  to  be  portable  across
       platforms, you should use the fcntl() file locking interface instead of flock().

RETURN VALUES

       Upon successful completion, 0 (zero) is returned.  Otherwise, -1 is returned and errno is set to indicate the error.

ERRORS

       If  the	flock()  function fails, errno may be set to one of the following values: The file is locked and the LOCK_NB option was specified.
       The filedes argument is not a valid open file descriptor.  A signal interrupted the flock call.	The operator is not valid.  The lock table
       is  full.   Too	many  regions  are already locked.  The lock is blocked by some lock from another process.  Putting the calling process to
       sleep while waiting for that lock to become free would cause a deadlock.

RELATED INFORMATION

       Functions: close(2), exec(2), fcntl(2), fork(2), open(2), lockf(3) delim off

																	  flock(2)

UNIX for Advanced & Expert Users