Class to handle file locking. More...

#include <FileLocker.h>

Public Types
enum	LockType { Read, Write }
	Define the possible lock types. More...
Public Member Functions
	FileLocker ()
	Default constructor creates an invalid fd.
	FileLocker (int fd, uInt start=0, uInt length=0)
	Construct the FileLocker object for the given file descriptor.
	~FileLocker ()
Bool	acquire (LockType=Write, uInt nattempts=0)
	Acquire a write or read lock.
Bool	release ()
	Release a lock.
Bool	canLock (LockType=Write)
	Test if the file can be locked for read or write.
Bool	canLock (uInt &pid, LockType=Write)
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.
Private Attributes
int	itsFD
int	itsError
int	itsStart
int	itsLength
Bool	itsMsgShown
Bool	itsReadLocked
	temporary for SUSE 6.1
Bool	itsWriteLocked

Detailed Description

Class to handle file locking.

Intended use:

Public interface

Review Status

Reviewed By:: UNKNOWN
Date Reviewed:: before2004/08/25
Test programs:: tLockFile

Prerequisite

man page of fcntl

Synopsis

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.

Definition at line 91 of file FileLocker.h.

Member Enumeration Documentation

enum casa::FileLocker::LockType

Define the possible lock types.

Enumerator:

Read	Acquire a read lock.
Write	Acquire a write lock.

Definition at line 95 of file FileLocker.h.

Constructor & Destructor Documentation

casa::FileLocker::FileLocker ( )

Default constructor creates an invalid fd.

casa::FileLocker::FileLocker	(	int	fd,
		uInt	start = `0`,
		uInt	length = `0`
	)			`[explicit]`

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.

casa::FileLocker::~FileLocker ( )

Member Function Documentation

Bool casa::FileLocker::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.