NAME¶
lockfile_create, lockfile_remove, lockfile_touch, lockfile_check - manage
lockfiles
SYNOPSIS¶
#include <lockfile.h>
cc [ flag ... ] file ... -llockfile [
library ]
int lockfile_create( const char *lockfile, int
retrycnt, int flags );
int lockfile_remove( const char *lockfile );
int lockfile_touch( const char *lockfile );
int lockfile_check( const char *lockfile, int
flags );
DESCRIPTION¶
The
lockfile_create function creates a lockfile in an NFS safe way.
If flags is set to
L_PID then lockfile_create will not only check for an
existing lockfile, but it will read the contents as well to see if it contains
a process id in ASCII. If so, the lockfile is only valid if that process still
exists.
If the lockfile is on a shared filesystem, it might have been created by a
process on a remote host. Thus the process-id checking is useless and the
L_PID flag should not be set. In this case, there is no good way to see if a
lockfile is stale. Therefore if the lockfile is older then 5 minutes, it will
be removed. That is why the
lockfile_touch function is provided: while
holding the lock, it needs to be refreshed regulary (every minute or so) by
calling
lockfile_touch () .
The
lockfile_check function checks if a valid lockfile is already present
without trying to create a new lockfile.
Finally the
lockfile_remove function removes the lockfile.
RETURN VALUES¶
lockfile_create returns one of the following status codes:
#define L_SUCCESS 0 /* Lockfile created */
#define L_NAMELEN 1 /* Recipient name too long (> 13 chars) */
#define L_TMPLOCK 2 /* Error creating tmp lockfile */
#define L_TMPWRITE 3 /* Can't write pid int tmp lockfile */
#define L_MAXTRYS 4 /* Failed after max. number of attempts */
#define L_ERROR 5 /* Unknown error; check errno */
lockfile_check returns 0 if a valid lockfile is present. If no lockfile
or no valid lockfile is present, -1 is returned.
lockfile_touch and
lockfile_remove return 0 on success. On failure
-1 is returned and
errno is set appropriately. It is not an error to
lockfile_remove() a non-existing lockfile.
ALGORITHM¶
The algorithm that is used to create a lockfile in an atomic way, even over NFS,
is as follows:
- 1
- A unique file is created. In printf format, the name of the file is
.lk%05d%x%s. The first argument (%05d) is the current process id. The
second argument (%x) consists of the 4 minor bits of the value returned by
time(2). The last argument is the system hostname.
- 2
- Then the lockfile is created using link(2). The return value of
link is ignored.
- 3
- Now the lockfile is stat()ed. If the stat fails, we go to step
6.
- 4
- The stat value of the lockfile is compared with that of the
temporary file. If they are the same, we have the lock. The temporary file
is deleted and a value of 0 (success) is returned to the caller.
- 5
- A check is made to see if the existing lockfile is a valid one. If it
isn't valid, the stale lockfile is deleted.
- 6
- Before retrying, we sleep for n seconds. n is initially 5
seconds, but after every retry 5 extra seconds is added up to a maximum of
60 seconds (an incremental backoff). Then we go to step 2 up to
retries times.
REMOTE FILE SYSTEMS AND THE KERNEL ATTRIBUTE CACHE¶
These functions do not lock a file - they
generate a
lockfile.
However in a lot of cases, such as Unix mailboxes, all concerned programs
accessing the mailboxes agree on the fact that the presence of
<filename>.lock means that <filename> is locked.
If you are using
lockfile_create to create a lock on a file that resides
on a remote server, and you already have that file open, you need to flush the
NFS attribute cache after locking. This is needed to prevent the following
scenario:
- o
- open /var/mail/USERNAME
- o
- attributes, such as size, inode, etc are now cached in the kernel!
- o
- meanwhile, another remote system appends data to /var/mail/USERNAME
- o
- grab lock using lockfile_create()
- o
- seek to end of file
- o
- write data
Now the end of the file really isn't the end of the file - the kernel cached the
attributes on open, and st_size is not the end of the file anymore. So after
locking the file, you need to tell the kernel to flush the NFS file attribute
cache.
The only
portable way to do this is the POSIX
fcntl() file locking
primitives - locking a file using
fcntl() has the fortunate side-effect
of invalidating the NFS file attribute cache of the kernel.
lockfile_create() cannot do this for you for two reasons. One, it just
creates a lockfile- it doesn't know which file you are actually trying to
lock! Two, even if it could deduce the file you're locking from the filename,
by just opening and closing it, it would invalidate any existing POSIX locks
the program might already have on that file (yes, POSIX locking semantics are
insane!).
So basically what you need to do is something like this:
fd = open("/var/mail/USER");
.. program code ..
lockfile_create("/var/mail/USER.lock", x, y);
/* Invalidate NFS attribute cache using POSIX locks */
if (lockf(fd, F_TLOCK, 0) == 0) lockf(fd, F_ULOCK, 0);
You have to be careful with this if you're putting this in an existing program
that might already be using fcntl(), flock() or lockf() locking- you might
invalidate existing locks.
There is also a non-portable way. A lot of NFS operations return the updated
attributes - and the Linux kernel actually uses these to update the attribute
cache. One of these operations is
chmod(2).
So stat()ing a file and then chmod()ing it to st.st_mode will not actually
change the file, nor will it interfere with any locks on the file, but it will
invalidate the attribute cache. The equivalent to use from a shell script
would be
chmod u=u /var/mail/USER
PERMISSIONS¶
If you are on a system that has a mail spool directory that is only writable by
a special group (usually "mail") you cannot create a lockfile
directly in the mailspool directory without special permissions.
Lockfile_create and lockfile_remove check if the lockfile ends in
$USERNAME.lock, and if the directory the lockfile is writable by group
"mail". If so, an external set group-id mail executable (
dotlockfile(1) ) is spawned to do the actual locking / unlocking.
FILES¶
/usr/lib/liblockfile.so.1
AUTHOR¶
Miquel van Smoorenburg <miquels@cistron.nl>
SEE ALSO¶
dotlockfile(1),
maillock(3),
touchlock (3),
mailunlock(3)