On Wed, 19 Mar 2003, Simon Marlow wrote:

...

I am planning a Haskell project and I need to access files. Since the program will be automatically started whenever a mail comes in I will need to be able to lock the access to files. Is there any support for this in some library?

Yes, the Posix library distributed with GHC has support for file locking. See

http://www.haskell.org/ghc/docs/latest/html/hslibs/sec-posix.html

This library is currently undergoing a rewrite - the functionality will be available under System.Posix in future releases of GHC (and possibly Hugs & NHC).

I didn't find this when I needed to lock files, so my solution (under Unix) was to write a little C code and call it via the FFI. I used a single lock file, since my application was a CGI script which runs fairly rarely -- there's no need for a finer grain of locking. Here's the C code (taken from the web and slightly adapted): #include <stdio.h> #include <fcntl.h> void claimLock() { struct flock fl; int fd; fl.l_type = F_WRLCK; /* F_RDLCK, F_WRLCK, F_UNLCK */ fl.l_whence = SEEK_SET; /* SEEK_SET, SEEK_CUR, SEEK_END */ fl.l_start = 0; /* Offset from l_whence */ fl.l_len = 0; /* length, 0 = to EOF */ fl.l_pid = getpid(); /* our PID */ fd = open("lock", O_WRONLY); fcntl(fd, F_SETLKW, &fl); /* F_GETLK, F_SETLK, F_SETLKW */ } Here's the corresponding Haskell declaration: foreign import claimLock :: IO () John Hughes