Programming

If a process already has the entire file locked for read and write using newstruct.l_type = F_WRLCK; what would happen if another process would try to open it in read only mode using open(filename, O_RDONLY); ?

I want to check if the file exists and I want it to work even if another process has it locked. (I'm not sure what open would return if the file is already locked by another process...)

What happens when you run the ls command on a directory you have permissions to see?

You see all of the files. You see files that are locked, too. This is because ls is looking at file metadata, not the data in the file itself. So it does not open the file, you can think of it as opening the directory instead.

stat() works the same way, it works against file metadata, so it does not matter if another process has the file locked, stat() will work anyway. When you stat a file you are checking it's existence.

More to the point:access() is the system call (unistd.h> that you should use to check file existence and your permissions to it.

Is this what you are asking?

Yes, thank you, it is good to know that you can get file attributes even if the file is locked. I was not aware of access().

I hope you are aware that file locking doesn't prevent any process from doing anything - apart from setting a conflicting lock of course - if you didn't explicitly request mandatory locking.

The default is advisory locking, which expects processes to check for and honor locks voluntarily.

Mandatory locking is frowned upon on systems that have it (System V derivatives) and difficult (Linux) or impossible (BSD derivatives) to enable on others.

Also, I would like to know if fopen, fread & fwrite are thread-safe ? Can multiple users fopen, fread & fwrite files at exactly the same time with no problems or is that the whole point why fcntl.h locking was written ?