It's not quite clear what you want to do, nevertheless, here are some ideas.

Create a fifo with mkfifo. If you open it for writing, without O_NONBLOCK, the open will block until someone opens the fifo for reading. Likely open it for reading and it will block until someone opens it for writing.

Get strace to ptrace the system calls of a process, and set it up to log open and similar calls.

Use the fuser program to watch what processes have a certain file open.

Use the dnotify feature of linux to watch for created, accessed, or modified files in a directory, see linux/Documentation/dnotify.txt. (Update 2006 jun 14: similar in spirit to dnotify is leases whcih is also linux-specific and is documented in the fcntl manpage. I see that traveler also noted this in his reply.)

Update: If a file is to be appended sequentially, and you just want to wait for a write, you can use select.

In reply to Re: Monitoring when a file opened by ambrus
in thread Monitoring when a file opened by Angel

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