1.\" $NetBSD: pidlock.3,v 1.5 2002/10/01 19:36:30 wiz Exp $ 2.\" 3.\" Copyright 1996, 1997 by Curt Sampson <cjs@netbsd.org> 4.\" 5.\" Redistribution and use in source and binary forms, with or without 6.\" modification, are permitted provided that the following conditions 7.\" are met: 8.\" 1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\" 11.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 12.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 13.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 14.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 15.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 16.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 17.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 18.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 19.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 20.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 21.\" POSSIBILITY OF SUCH DAMAGE. 22.\" 23.Dd November 10, 1996 24.Os 25.Dt PIDLOCK 3 26.Sh NAME 27.Nm pidlock , 28.Nm ttylock , 29.Nm ttyunlock 30.Nd locks based on files containing PIDs 31.Sh LIBRARY 32.Lb libutil 33.Sh SYNOPSIS 34.Fd #include \*[Lt]util.h\*[Gt] 35.Ft int 36.Fn pidlock "const char *lockfile" "int flags" "pid_t *locker" "const char *info" 37.Ft int 38.Fn ttylock "const char *tty" "int flags" "pid_t *locker" 39.Ft int 40.Fn ttyunlock "const char *tty" 41.Sh DESCRIPTION 42The 43.Fn pidlock 44.Fn ttylock , 45and 46.Fn ttyunlock 47functions attempt to create a lockfile for an arbitrary resource that 48only one program may hold at a time. 49(In the case of 50.Fn ttylock , 51this is access to a tty device.) 52If the 53function succeeds in creating the lockfile, it will succeed for 54no other program calling it with the same lockfile until the original 55calling program has removed the lockfile or exited. 56The 57.Fn ttyunlock 58function will remove the lockfile created by 59.Fn ttylock . 60.Pp 61These functions use the method of creating a lockfile traditionally 62used by UUCP software. 63This is described as follows in the documentation for Taylor UUCP: 64.Bd -filled -offset indent 65The lock file normally contains the process ID of the locking process. 66This makes it easy to determine whether a lock is still valid. 67The algorithm is to create a temporary file and then link 68it to the name that must be locked. 69If the link fails because a file with that name already exists, 70the existing file is read to get the process ID. 71If the process still exists, the lock attempt fails. 72Otherwise the lock file is deleted and the locking algorithm 73is retried. 74.Ed 75.Pp 76The PID is stored in ASCII format, with leading spaces to pad it 77out to ten characters, and a terminating newline. 78This implementation has been extended to put the hostname 79on the second line of the file, terminated with a newline, and 80optionally an arbitrary comment on the third line of the file, also 81terminated with a newline. 82If a comment is given, but 83.Dv PIDLOCK_NONBLOCK 84is not, a blank line will be written as the second line of the file. 85.Pp 86The 87.Fn pidlock 88function will attempt to create the file 89.Fa lockfile 90and put the current process's pid in it. 91The 92.Fn ttylock 93function will do the same, but should be passed only the base name 94(with no leading directory prefix) of the tty to be locked; it will 95test that the tty exists in 96.Pa /dev 97and is a character device, and then create 98the file in the 99.Pa /var/spool/lock 100directory and prefix the filename with 101.Pa LCK.. . 102Use the 103.Fn ttyunlock 104function to remove this lock. 105.Pp 106The following flags may be passed in 107.Pa flags : 108.Bl -tag -width Dv -offset indent 109.It Dv PIDLOCK_NONBLOCK 110The function should return immediately when a lock is held by another 111active process. 112Otherwise the function will wait (forever, if necessary) 113for the lock to be freed. 114.It Dv PIDLOCK_USEHOSTNAME 115The hostname should be compared against the hostname in the second 116line of the file (if present), and if they differ, no attempt at 117checking for a living process holding the lock will be made, and 118the lockfile will never be deleted. 119(The process is assumed to be alive.) 120This is used for locking on NFS or other remote filesystems. 121(The function will never create a lock if 122.Dv PIDLOCK_USEHOSTNAME 123is specified and no hostname is present.) 124.El 125.Pp 126If 127.Pa locker 128is non-null, it will contain the PID of the locking process, if there 129is one, on return. 130.Pp 131If 132.Pa info 133is non-null and the lock succeeds, the string it points to will be 134written as the third line of the lock file. 135.Sh RETURN VALUES 136Zero is returned if the operation was successful; on an error a -1 137is returned and a standard error code is left in the global location errno. 138.Sh ERRORS 139These are among the values left in 140.Va errno 141if 142.Fn pidlock 143or 144.Fn ttylock 145returns a failure: 146.Bl -tag -width Er 147.It Bq Er EPERM 148The current process does not have some of the privileges necessary 149to perform the lock. 150These include read and write access to the lock 151directory, and read access to the current lockfile, if it exists. 152.It Bq Er ENOENT 153A component of a specified pathname did not exist, or the pathname 154was an empty string. 155.It Bq Er EWOULBLOCK 156Another runnning process has a lock and the 157.Dv PIDLOCK_NONBLOCK 158flag was specified. 159.It Bq Er ENAMETOOLONG 160A component of the path name exceeded 255 (MAXNAMELEN) characters, 161or an entire path name exceeded 1023 (MAXPATHLEN-1) characters. 162.El 163.\" .Sh SEE ALSO 164.Sh HISTORY 165The 166.Fn pidlock 167and 168.Fn ttylock 169functions appeared in 170.Nx 1.3 . 171.Sh AUTHORS 172Curt Sampson \*[Lt]cjs@netbsd.org\*[Gt] 173.Sh BUGS 174The lockfile format breaks if a pid is longer than ten digits when 175printed in decimal form. 176.Pp 177The PID returned will be the pid of the locker on the remote machine if 178.Dv PIDLOCK_USEHOSTNAME 179is specified, but there is no indication that this is not on the local 180machine. 181