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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 arbitrary 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 created 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 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.

     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 necessary) 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 return.

     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 standard 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 specified.

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

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

     Curt Sampson <>.

     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