maillock, mailunlock, touchlock - manage mailbox lockfiles
cc [ flag ... ] file ... -llockfile [ library ]
int maillock( const char *user, int retrycnt );
void mailunlock( void );
void touchlock( void );
The maillock function tries to create a lockfile for the users mailbox
in an NFS-safe (or resistant) way. The algorithm is documented in
The mailbox is typically located in /var/mail. The name of the
lockfile then becomes /var/mail/USERNAME.lock. If the environment
variable $MAIL is set, and it ends with the same username as the
username passed to maillock(), then that file is taken as the mailbox
to lock instead.
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
touchlock function is provided: while holding the lock, it needs to be
refreshed regulary (every minute or so) by calling touchlock () .
Finally the mailunlock function removes the lockfile.
maillock 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 */
These functions are not thread safe. If you need thread safe functions,
or you need to lock other mailbox (like) files that are not in the
standard location, use lockfile_create(3) instead.
These functions call lockfile_create(3) to do the work. That function
might spawn a set group-id executable to do the actual locking if the
current process doesn’t have enough priviliges.
There are some issues with flushing the kernels attribute cache if you
are using NFS - see the lockfile_create(3) manpage.
Miquel van Smoorenburg <email@example.com>
lockfile_create(3), lockfile_touch (3), lockfile_remove(3)