Not having heard any objections to Simon Gerraty's idea of adding
the hostname to the lock file, I've added that, a flag to determine
whether or not to use it, and an optional informational comment to
the lockfile.  Here's the latest version of the manpage.

Do we still have any major objections to adding this to libutil,
and converting the various programs in the tree (including getty)
to use it?

cjs

Curt Sampson    cjs@portal.ca		Info at http://www.portal.ca/
Internet Portal Services, Inc.	
Vancouver, BC   (604) 257-9400		De gustibus, aut bene aut nihil.


.\" $Id: pidlock.3,v 1.2 1996/11/18 04:51:15 cjs Exp cjs $
.Dd November 10, 1996
.Os
.Dt PIDLOCK 3
.Sh NAME
.Nm pidlock ,
.Nm uucplock
.Nd locks based on files containing PIDs
.Sh SYNOPSIS
.Fd #include <util.h>
.Ft int
.Fn pidlock "const char *lockfile" "int flags" "pid_t *locker" "const char *info"
.Ft int
.Fn uucplock "const char *tty" "int flags" "pid_t *locker"
.Sh DESCRIPTION
The
.Fn pidlock
and
.Fn uucplock
functions attempt to create a lockfile for an arbitrary resource that
only one program may hold at a time.  (In the case of
.Fn uucplock ,
this is by convention access to a tty device.) If the
function succeeds in creating the lockfile, it will succeed for
no other program calling it with the same lockfile until the original
calling program has removed the lockfile or exited.
.Pp
These functions use the method of creating a lockfile traditionally
used by UUCP software.  This is described as follows in the
documentation for Taylor UUCP:
.Bd -filled -offset indent
The lock file normally contains the process ID of the locking
process.  This makes it easy to determine whether a lock is still
valid.  The algorithm is to create a temporary file and then link
it to the name that must be locked.  If the link fails because a
file with that name already exists, the existing file is read to
get the process ID.  If the process still exists, the lock attempt
fails.  Otherwise the lock file is deleted and the locking algorithm
is retried.
.Ed
.Pp
.Pp
The PID is stored in ASCII format, with leading spaces to pad it
out to ten characters, and a terminating newline.  This
implementation has been extended to put the hostname
on the second line of the file, terminated with a newline, and
optionally an arbitrary comment on the third line of the file, also
terminated with a newline.
.Pp
The
.Fn pidlock
function will attempt to create the file
.Fa lockfile
and put the current process's pid in it. The
.Fn uucplock
function will do the same, but should be passed only the base name
(with no leading directory prefix) of the tty to be locked; it will
create the file in the
.Pa /var/spool/lock
directory and prefix the filename with
.Pa LCK.. .
.Pp
The following flags may be passed in
.Pa flags :
.Bl -tag -width Dv -offset indent
.It Dv PIDLOCK_NONBLOCK
The function should return immediately when a lock is held by another
active process.  Otherwise the function will wait (forever, if necessary)
for the lock to be freed.
.It Dv PIDLOCK_USEHOSTNAME
The hostname should be compared against the hostname in the second
line of the file (if present), and if they differ, no attempt at
checking for a living process holding the lock will be made, and
the lockfile will never be deleted.  (The process is assumed to be
alive.)  This is used for locking on NFS or other remote filesystems.
[What do we do if no hostname is present?]
.El
.Pp
If
.Pa locker
is non-null, it will contain the PID of the locking process on
return.  (This will be the current process's PID if no process
already holds the lock.)
.Pp
If
.Pa info
is non-null and the lock succeeds, the string it points to will be
written as the third line of the lock file.
.Pp
The
.Fn uucplock
function will
.Fn chown
the lockfile to the
.Em uucp
user if the process is currently running as root.
.Sh RETURN VALUES
Zero is returned if the operation was successful; on an error a -1
is returned and a standard error code is left in the global location errno.
.Sh ERRORS
These are among the values left in
.Va errno
if
.Fn pidlock
or
.Fn uucplock
returns a failure:
.Bl -tag -width Er
.It Bq Er EPERM
The current process does not have some of the privileges necessary
to perform the lock. These include read and write access to the lock
directory, and read access to the current lockfile, if it exists.
.It Bq Er ENOENT
A component of a specified pathname did not exist, or the pathname
was an empty string.
.It Bq Er EWOULBLOCK
Another runnning process has a lock and the
.Dv PIDLOCK_NONBLOCK
flag was specified.
.It Bq Er ENAMETOOLONG
A component of the path name exceeded 255 (MAXNAMELEN) characters,
or an entire path name exceeded 1023 (MAXPATHLEN-1) characters.
.El
.\" .Sh SEE ALSO
.Sh HISTORY
The
.Fn pidlock
and
.Fn uucplock
functions appeared in
.Nx 1.3 .
.\" .Sh AUTHORS
.\" .Sh BUGS
.Pp