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)
