1LOCKDEV(3) Linux Programmer's Manual LOCKDEV(3)
2
6 lockdev, liblockdev, dev_testlock, dev_lock, dev_relock, dev_unlock -
7 manage device lockfiles
8
10 #include <lockdev.h>
11 pid_t dev_testlock( const char * devname);
13 pid_t dev_lock( const char * devname);
14 pid_t dev_relock( const char * devname, pid_t pid);
15 pid_t dev_unlock( const char * devname, pid_t pid);
16 cc [ flag ... ] file ... -llockdev [ library ]
18
20 The lockdev functions act on device locks normally located in /var/lock
21 . The lock is acquired creating a pair of files hardlinked between
22 them and named after the device name (as mandated by FSSTND) and the
23 device's major and minor numbers (as in SVr4 locks). This permits to
24 circumvent a problem using only the FSSTND lock method when the same
25 device exists under different names (for convenience or when a device
26 must be accessable by more than one group of users).
27 The lock file names are typically in the form LCK..ttyS1 and
28 LCK.004.065 , but is provided a way to easily modify them to use the
29 library on different architectures. The content of those files is the
30 pid of the process who owns the lock.
31 The dev_testlock() function simply checks if the device is in some way
33 locked and if the owner of the lock is still active (otherwise it
34 removes the lock). It recognise a valid lock even if only one of the
35 two lock files exists (and is owned by an existing process), thus per‐
36 mitting a safe use of this library together with programs using only
37 FSSTND or SVr4 lock style.
38 The dev_lock() function first checks if the device is already locked
40 and then tries to acquire the lock building the two lock files. First
41 it creates the file which name contains the major and minor numbers (in
42 SVr4 style), then it creates the file with the device name in its name.
43 This order reduces the clashes with other processes trying to lock the
44 same device (even with a different name) and using this library. It has
45 no problem with processes that uses only the FSSTND algorithm.
46 The dev_relock() function changes the owner of an existing lock; if the
48 pid of the old owner is provided, then it checks if the lock was cor‐
49 rectly assigned (otherwise there is the possibility of a process
50 acquiring a lock which was owned by another unrelated process). If the
51 device was not locked, locks it.
52 The dev_unlock() function removes the existing locks on the device. If
54 the pid of the owner of the lock is provided, then it checks if the
55 lock is assigned to that process, avoiding to remove locks assigned to
56 other existing processes.
57
60 All the functions in lockdev library return ZERO on successfull comple‐
61 tion of the function (dev_testlock returns zero if there is no lock on
62 the device), otherwise, if the device is currently locked by an exist‐
63 ing process, they return the pid of the process owner of the lock. They
64 return a negative number when some kind of error happens. Actually they
65 all return only (-1).
66
69 The API has symbols used only for debugging purposis
70 int liblockdev_debug
72 void liblockdev_incr_debug( void );
73 void liblockdev_reset_debug( void );
74 which can be used when the liblockdev library is compiled with -DDEBUG
76 flag as when using make install-dbg , which compiles a debug shared
77 library and installs it under /usr/local/lib/debug (or /usr/lib/debug).
78 The value of the global integer is set to 1 by the DEBUG define, and
79 can be set to a different value passing a flag like -DDEBUG=3 during
80 compilation of the library, or setting the environment variable
81 LIBLOCKDEV_DEBUG to the wanted value before executing your program.
82 During execution of your program, the flag's value can be changed from
83 your program or from another terminal, respectively using the function
84 liblockdev_incr_debug() , or sending SIGUSR1 to the running process, to
85 increment the value of the integer by one, or using the function
86 liblockdev_reset_debug() , or sending SIGUSR2 to the running process,
87 to set to zero the value of the global integer.
88 Direct manipulation of the global integer is strongly deprecated,
89 because the data structure of the symbol (actually an integer) could be
90 changed later in some way, or even become a macro.
91 The library prints on stdout some informations like error conditions
93 (level of 1), normal termination conditions (2) or function calling
94 (3).
95 To use the debug shared library, simply define in your environment the
97 variable LD_LIBRARY_PATH=/usr/lib/debug (or /usr/local/lib/debug if
98 built using make install-dbg) and call gdb or directly your program
99 without any need to recompile it. As you can check with ldd, your pro‐
100 gram will load the debug library instead of the normal one.
101 Beware that if your program is setuid or setgid, you must become root
102 to let this work, because ld.so ignores the LD_LIBRARY_PATH variable
103 for security reasons.
104 On Debian GNU/Linux systems exists a debug binary package named
106 liblockdev1-dbg which installs a shared library built with all debug‐
107 ging options (and the -DDEBUG flag) into /usr/lib/debug .
108
111 /var/lock/LCK..<device>
112 /var/lock/LCK.<major>.<minor>
113 /usr/lib/liblockdev.so.1
114 /usr/lib/debug/liblockdev.so.1
115
118 (c) 1997 by Fabrizio Polacco <fab@prosa.it>.
119 This program is free software; you can redistribute it and/or modify it
120 under the terms of the GNU Lesser General Public License as published
121 by the Free Software Foundation; version 2 dated June, 1991.
122Linux Manpage 26 Dec 1997 LOCKDEV(3)