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NetBSD 6.1.5 - man page for ttylock (netbsd section 3)

PIDLOCK(3)			   BSD Library Functions Manual 		       PIDLOCK(3)

     pidlock, ttylock, ttyunlock -- locks based on files containing PIDs

     System Utilities Library (libutil, -lutil)

     #include <util.h>

     pidlock(const char *lockfile, int flags, pid_t *locker, const char *info);

     ttylock(const char *tty, int flags, pid_t *locker);

     ttyunlock(const char *tty);

     The pidlock() ttylock(), and ttyunlock() functions attempt to create a lockfile for an arbi-
     trary resource that only one program may hold at a time.  (In the case of ttylock(), this is
     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.  The ttyunlock() function will remove the lockfile cre-
     ated by ttylock().

     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:

	   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  tempo-
	   rary 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.

     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.  If a comment is given, but
     PIDLOCK_NONBLOCK is not, a blank line will be written as the second line of the file.

     The pidlock() function will attempt to create the file lockfile and put the current
     process's pid in it.  The ttylock() 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 test that the
     tty exists in /dev and is a character device, and then create the file in the
     /var/spool/lock directory and prefix the filename with LCK...  Use the ttyunlock() function
     to remove this lock.

     The following flags may be passed in flags:

			 The function should return immediately when a lock is held by another
			 active process.  Otherwise the function will wait (forever, if neces-
			 sary) for the lock to be freed.

			 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.  (The function will
			 never create a lock if PIDLOCK_USEHOSTNAME is specified and no hostname
			 is present.)

     If locker is non-null, it will contain the PID of the locking process, if there is one, on

     If info is non-null and the lock succeeds, the string it points to will be written as the
     third line of the lock file.

     Zero is returned if the operation was successful; on an error a -1 is returned and a stan-
     dard error code is left in the global location errno.

     In addition to the errors that are returned from stat(2), open(2), read(2), write(2), and
     link(2), pidlock() or ttylock() can set errno to the following values on failure:

     [EWOULDBLOCK]	Another running process has a lock and the PIDLOCK_NONBLOCK flag was

     [EFTYPE]		The tty specified in ttylock() is not a character special device.

     The pidlock() and ttylock() functions appeared in NetBSD 1.3.

     Curt Sampson <cjs@NetBSD.org>.

     The lockfile format breaks if a pid is longer than ten digits when printed in decimal form.

     The PID returned will be the pid of the locker on the remote machine if PIDLOCK_USEHOSTNAME
     is specified, but there is no indication that this is not on the local machine.

BSD					  March 19, 2006				      BSD

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