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
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