pidlock, ttylock, ttyunlock - locks based on files containing PIDs
System Utilities Library (libutil, -lutil)
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
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
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.
These are among the values left in errno if pidlock() or ttylock()
returns a failure:
[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.
[ENOENT] A component of a specified pathname did not exist, or
the pathname was an empty string.
[EWOULBLOCK] Another runnning process has a lock and the
PIDLOCK_NONBLOCK flag was specified.
[ENAMETOOLONG] A component of the path name exceeded 255 (MAXNAMELEN)
characters, or an entire path name exceeded 1023 (MAXPATHLEN-1)
The pidlock() and ttylock() functions appeared in NetBSD 1.3.
Curt Sampson <email@example.com>
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 November 10, 1996 BSD
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