lockdev
LOCKDEV(3) Linux Programmer’s Manual LOCKDEV(3)
NAME
lockdev, liblockdev, dev_testlock, dev_lock, dev_relock, dev_unlock -
manage device lockfiles
SYNOPSIS
#include <lockdev.h>
pid_t dev_testlock( const char * devname);
pid_t dev_lock( const char * devname);
pid_t dev_relock( const char * devname, pid_t pid);
pid_t dev_unlock( const char * devname, pid_t pid);
cc [ flag ... ] file ... -llockdev [ library ]
DESCRIPTION
The lockdev functions act on device locks normally located in
/var/lock . The lock is acquired creating a pair of files hardlinked
between them and named after the device name (as mandated by FSSTND)
and the device’s major and minor numbers (as in SVr4 locks). This per-
mits to circumvent a problem using only the FSSTND lock method when
the same device exists under different names (for convenience or when
a device must be accessable by more than one group of users).
The lock file names are typically in the form LCK..ttyS1 and
LCK.004.065 , but is provided a way to easily modify them to use the
library on different architectures. The content of those files is the
pid of the process who owns the lock.
The dev_testlock() function simply checks if the device is in some way
locked and if the owner of the lock is still active (otherwise it
removes the lock). It recognise a valid lock even if only one of the
two lock files exists (and is owned by an existing process), thus per-
mitting a safe use of this library together with programs using only
FSSTND or SVr4 lock style.
The dev_lock() function first checks if the device is already locked
and then tries to acquire the lock building the two lock files. First
it creates the file which name contains the major and minor numbers
(in SVr4 style), then it creates the file with the device name in its
name. This order reduces the clashes with other processes trying to
lock the same device (even with a different name) and using this
library. It has no problem with processes that uses only the FSSTND
algorithm.
The dev_relock() function changes the owner of an existing lock; if
the pid of the old owner is provided, then it checks if the lock was
correctly assigned (otherwise there is the possibility of a process
acquiring a lock which was owned by another unrelated process). If the
device was not locked, locks it.
The dev_unlock() function removes the existing locks on the device. If
the pid of the owner of the lock is provided, then it checks if the
lock is assigned to that process, avoiding to remove locks assigned to
other existing processes.
RETURN VALUES
All the functions in lockdev library return ZERO on successfull com-
pletion of the function (dev_testlock returns zero if there is no lock
on the device), otherwise, if the device is currently locked by an
existing process, they return the pid of the process owner of the
lock. They return a negative number when some kind of error happens.
Actually they all return only (-1).
DEBUGGING
The API has symbols used only for debugging purposis
int liblockdev_debug
void liblockdev_incr_debug( void );
void liblockdev_reset_debug( void );
which can be used when the liblockdev library is compiled with -DDEBUG
flag as when using make install-dbg , which compiles a debug shared
library and installs it under /usr/local/lib/debug (or
/usr/lib/debug).
The value of the global integer is set to 1 by the DEBUG define, and
can be set to a different value passing a flag like -DDEBUG=3 during
compilation of the library, or setting the environment variable
LIBLOCKDEV_DEBUG to the wanted value before executing your program.
During execution of your program, the flag’s value can be changed from
your program or from another terminal, respectively using the function
liblockdev_incr_debug() , or sending SIGUSR1 to the running process,
to increment the value of the integer by one, or using the function
liblockdev_reset_debug() , or sending SIGUSR2 to the running process,
to set to zero the value of the global integer.
Direct manipulation of the global integer is strongly deprecated,
because the data structure of the symbol (actually an integer) could
be changed later in some way, or even become a macro.
The library prints on stdout some informations like error conditions
(level of 1), normal termination conditions (2) or function calling
(3).
To use the debug shared library, simply define in your environment the
variable LD_LIBRARY_PATH=/usr/lib/debug (or /usr/local/lib/debug if
built using make install-dbg) and call gdb or directly your program
without any need to recompile it. As you can check with ldd, your pro-
gram will load the debug library instead of the normal one.
Beware that if your program is setuid or setgid, you must become root
to let this work, because ld.so ignores the LD_LIBRARY_PATH variable
for security reasons.
On Debian GNU/Linux systems exists a debug binary package named
liblockdev1-dbg which installs a shared library built with all debug-
ging options (and the -DDEBUG flag) into /usr/lib/debug .
FILES
/var/lock/LCK..<device>
/var/lock/LCK.<major>.<minor>
/usr/lib/liblockdev.so.1
/usr/lib/debug/liblockdev.so.1
HISTORY
(c) 1997 by Fabrizio Polacco <fab@prosa.it>.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as pub-
lished by the Free Software Foundation; version 2 dated June, 1991.
Linux Manpage 26 Dec 1997 LOCKDEV(3)
