test if a file is locked?

[Fortepianissimo]

I can lock a file using the following snippet:

---
import fcntl

f=open('flock.txt','w')
fcntl.lockf(f.fileno(),fcntl.LOCK_EX)
---

If some other script tries to open 'flock.txt', it'll wait until the
file is unlocked/closed.

My question is, I'd like to tell a script to quit *immediately* if it
cannot open the file, or, if it knows the file is locked.

The reason I need this is I want to serialize parallel processes using
a lock file. Any suggestion is welcome!

 

[Carl Banks]

I can lock a file using the following snippet:

---
import fcntl

f=open('flock.txt','w')
fcntl.lockf(f.fileno(),fcntl.LOCK_EX)
---

If some other script tries to open 'flock.txt', it'll wait until the
file is unlocked/closed.

My question is, I'd like to tell a script to quit *immediately* if it
cannot open the file, or, if it knows the file is locked.

The reason I need this is I want to serialize parallel processes using
a lock file. Any suggestion is welcome!

Try this:

fcntl.lockf(f.fileno(),fcntl.LOCK_EX|fcntl.LOCK_NB)

 

[Benjamin Han]

Try this:

fcntl.lockf(f.fileno(),fcntl.LOCK_EX|fcntl.LOCK_NB)

Ok looks like I'm still missing something: so how do you get 'f' without
using open() or file()? Either of these will put the script on hold...

Thanks!

 

[Benjamin Han]

Ok looks like I'm still missing something: so how do you get 'f' without
using open() or file()? Either of these will put the script on hold...

I got it, thanks!

 

[Carl Banks]

Ok looks like I'm still missing something: so how do you get 'f' without
using open() or file()? Either of these will put the script on hold...

Thanks!

Have you tried it? (Hint: it's not "open" that waits until the file
is unlocked.)

 

[Guest]

I can lock a file using the following snippet:

---
import fcntl

f=open('flock.txt','w')
fcntl.lockf(f.fileno(),fcntl.LOCK_EX)

This is not true on my system (Linux-2.4.23/glibc-2.3.2), after locking the
file Ican still open it with another script or program (like "cat").

The other script blocks only when I try to lock the file again.

My question is, I'd like to tell a script to quit *immediately* if it
cannot open the file, or, if it knows the file is locked.

try this:
fcntl.lockf(f.fileno(),fcntl.LOCK_EX|fcntl.LOCK_NB)
it will throw an exception if the file is locked:
IOError: [Errno 11] Resource temporarily unavailable