maillock, mailunlock, touchlock - functions to manage lockfile(s) for
cc [ flag ... ] file ... -lmail [ library ... ]
int maillock(const char *user, int retrycnt);
The maillock() function attempts to create a lockfile for the user's
mailfile. If a lockfile already exists, and it has not been modified in
the last 5 minutes, maillock() will remove the lockfile and set its own
It is crucial that programs locking mail files refresh their locks at
least every three minutes to maintain the lock. Refresh the lockfile by
calling the touchlock() function with no arguments.
The algorithm used to determine the age of the lockfile takes into
account clock drift between machines using a network file system. A
zero is written into the lockfile so that the lock will be respected
by systems running the standard version of System V.
If the lockfile has been modified in the last 5 minutes the process
will sleep until the lock is available. The sleep algorithm is to sleep
for 5 seconds times the attempt number. That is, the first sleep will
be for 5 seconds, the next sleep will be for 10 seconds, etc. until the
number of attempts reaches retrycnt.
When the lockfile is no longer needed, it should be removed by calling
The user argument is the login name of the user for whose mailbox the
lockfile will be created. maillock() assumes that user's mailfiles are
in the ``standard'' place as defined in <maillock.h>.
Upon successful completion, .maillock() returns 0. Otherwise it returns
user mailbox files
user mailbox lockfiles
See attributes(5) for descriptions of the following attributes:
│ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│MT-Level │ Unsafe │
The mailunlock() function will only remove the lockfile created from
the most previous call to maillock(). Calling maillock() for different
users without intervening calls to mailunlock() will cause the ini‐
tially created lockfile(s) to remain, potentially blocking subsequent
message delivery until the current process finally terminates.
Mar 29, 1999 MAILLOCK(3MAIL)