FileLocker.h

This class handles file locking by means of the fcntl SETLK function. Locking of files on NFS-mounted file systems works correctly as long as the NFS lockd and statd deamons are configured correctly. Otherwise lock requests may be granted incorrectly.

Acquiring a lock can be done for a read or a write lock. Multiple locks on a file can exist as long as they are all read locks. When a write lock is involved, no other lock can exist. It is possible to acquire a lock in 2 ways:

Wait until the lock request is granted; i.e. until the processes holding a lock on the file release their lock.
Do several attempts; between each attempt it sleeps 1 second. Note that nattempts=1 means it returns immediately when the lock request could not be granted.

Example

    int fd = open ("file.name");
    FileLocker lock(fd);
    if (lock.acquire()) {
        ... do something with the file ...
        lock.release();
    }else{
        cout << lock.lastMessage() << endl;
    }

Motivation

Make it possible to lock files in a standard way.

Member Description

enum LockType

Define the possible lock types.

FileLocker()

Default constructor creates an invalid fd.

explicit FileLocker (int fd, uInt start=0, uInt length=0)

Construct the FileLocker object for the given file descriptor. This can be used to lock a segment of the given file. The segment is given by start and length. Length=0 means till the end of the file.

~FileLocker()

Bool acquire (LockType = Write, uInt nattempts = 0)

Acquire a write or read lock. nattempts defines how often it tries to acquire the lock. A zero value indicates an infinite number of times (i.e. wait until the lock is acquired). A positive value means it waits 1 second between each attempt.

Bool release()

Release a lock. The return status indicates if an error occurred.

Bool canLock (LockType = Write)
Bool canLock (uInt& pid, LockType = Write)

Test if the file can be locked for read or write. Optionally the PID of the process holding the lock is returned.

Bool hasLock (LockType = Write) const

Test if the process has a lock for read or write on the file.

int fd() const

Get the fd in use.

int lastError() const

Get the last error.

String lastMessage() const

Get the message belonging to the last error.