NAME

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

SYNOPSIS

#include <sys/file.h>

int flock (fd, operation)
int fd, operation;

DESCRIPTION

flock applies or removes an advisory lock on the file associated with the file descriptor fd. The operation parameter is formed from the values below:

LOCK_SHshared lock
LOCK_EXexclusive lock
LOCK_NBdon’t block when locking
LOCK_UNunlock

A lock is applied by specifying an operation which is the inclusive or of LOCK_SH or LOCK_EX and, possibly, 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 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 which is already locked normally causes the caller to 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. 

Processes blocked awaiting a lock may be awakened by signals. 

Advisory locks are not maintained between machines by the Network File System (NFS). For instance, a lock made on the server will not be noticed on a client.  However, a lock made on a client will be noticed by other processes on the same client. 

RETURN VALUE

Zero is returned if the operation was successful; on an error a −1 is returned and an error code is left in the global location errno. 

ERRORS

The flock 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. 

SEE ALSO

open(2), close(2), dup(2), exec(2), fork(2)

CX/UX Programmer’s Reference Manual