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 lockfile_create(3).
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 */ #define L_RMSTALE 8 /* Failed to remove stale lockfile */
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
lockfile_create(3), lockfile_touch (3), lockfile_remove(3)
dotlockfile(1), lockfile_create(3), lockfile-progs(1).