C++ – Optimal lock file method

cfilelockingunix

Windows has an option to open a file with exclusive access rights. Unix doesn't.

In order to ensure exclusive access to some file or device, it is common practice in Unix to use a lock file usually stored in the /var/lock directory.

The C instruction open( "/var/lock/myLock.lock", O_RDWR | O_CREAT | O_EXCL, 0666 ) returns -1 if the lock file already exist, otherwise it creates it. The function is atomic and ensures there is not race condition.

When the resource is released, the lock file is deleted by the following instruction
remove( "/var/lock/myLock.lock" ).

There are two problems with this method.

The program may terminate without removing the lock. For instance because it is killed, crashes or whatever. The lock file remains in place, and will prevent any access to the resource even though it is not used anymore.
The lock file is created with group and world write privilege but it is a common practice to configure accounts to use a permission mask that will clear the group and world write permission. Thus, if we had a reliable method to determine that the lock is orphan (not use), a user not owner of the file will not be allowed to remove it.

For the record, I use the lock file to ensure exclusive access to the device connected to the serial port (/dev/ttyUSBx in fact). Advisory method, requiring cooperation, is OK. But exclusive access should be ensured between different users.

Is there a better synchronization method than the lock file? How to determine if the process who created the lock file is still running?
How to make it possible for another user to remove the lock file if not in use?

One solution I came up with was to use the file as Unix socket file. If the file exist, try to connect using the file. If it fails, we may assume the owner process of the file is dead. This requires to have a thread looping on socket accept() in the owner process. Unfortunately, the system wouldn't be atomic anymore.

Best Answer

Take a look at the enlightening presentation File Locking Tricks and Traps:

This short talk presents several common pitfalls of file locking and a few useful tricks for using file locking more effectively.

Edit: To address your questions more precisely:

Is there a better synchronization method than the lock file?

As @Hasturkun already mentioned and as the presentation above told, the system call you need to use is flock(2). If the resource you'd like to share across many users is already file-based (in your case it is /dev/ttyUSBx), then you can flock the device file itself.

How to determine if the process who created the lock file is still running?

You don't have to determine this, as the flock-ed lock will be automatically released upon closing the file descriptor associated with your file, even if the process was terminated.

How making it possible for another user to remove the lock file if not in use?

If you would lock the device file itself, then there will be no need to remove the file. Even if you would decide to lock an ordinary file in /var/lock, with flock you will not need to remove the file in order to release the lock.

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Python – How to check whether a file exists without exceptions

If the reason you're checking is so you can do something like if file_exists: open_it(), it's safer to use a try around the attempt to open it. Checking and then opening risks the file being deleted or moved or something between when you check and when you try to open it.

If you're not planning to open the file immediately, you can use os.path.isfile

Return True if path is an existing regular file. This follows symbolic links, so both islink() and isfile() can be true for the same path.

import os.path
os.path.isfile(fname)

if you need to be sure it's a file.

Starting with Python 3.4, the pathlib module offers an object-oriented approach (backported to pathlib2 in Python 2.7):

from pathlib import Path

my_file = Path("/path/to/file")
if my_file.is_file():
    # file exists

To check a directory, do:

if my_file.is_dir():
    # directory exists

To check whether a Path object exists independently of whether is it a file or directory, use exists():

if my_file.exists():
    # path exists

You can also use resolve(strict=True) in a try block:

try:
    my_abs_path = my_file.resolve(strict=True)
except FileNotFoundError:
    # doesn't exist
else:
    # exists

Python – How to copy a file in Python

shutil has many methods you can use. One of which is:

from shutil import copyfile
copyfile(src, dst)

# 2nd option
copy(src, dst)  # dst can be a folder; use copy2() to preserve timestamp

Copy the contents of the file named src to a file named dst. Both src and dst need to be the entire filename of the files, including path.
The destination location must be writable; otherwise, an IOError exception will be raised.
If dst already exists, it will be replaced.
Special files such as character or block devices and pipes cannot be copied with this function.
With copy, src and dst are path names given as strs.

Another shutil method to look at is shutil.copy2(). It's similar but preserves more metadata (e.g. time stamps).

If you use os.path operations, use copy rather than copyfile. copyfile will only accept strings.

Best Answer

Related Solutions

Python – How to check whether a file exists without exceptions

Python – How to copy a file in Python

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