1OVERVIEW
2========
3
4Plugins are a way to extend vlock's functionality.  They can define
5hooks that are called at certain points in a vlock session.
6
7There are two separate types of plugins:  modules and scripts.  Modules
8are shared objects that are loaded into vlock's address space.  They run
9with the same privileges as vlock and thus are very powerful but also
10dangerous.  Scripts may be any kind of executables located in vlock's
11script directory.  They are run in separate processes with lowered
12privileges, i.e. the same as the user who started vlock.
13
14For simple tasks scripts should be preferred over modules.  They are
15easier to develop and test and have a lower impact on security and
16stability.
17
18NB:  The following interface is not yet declared stable.  It is not guaranteed
19that plugins (modules or scripts) that work with vlock 2.2 will work with
20future versions.
21
22DEPENDENCIES
23============
24
25Plugins may depend on each other in several ways.  There are six
26different types of dependencies. Each dependency type is represented by
27a list of plugin names.  The way of declaring them is different for
28modules and scripts but their names and meaning are the same.
29
30Resolving the dependencies is done after all initially requested plugins
31are loaded and may fail if dependencies cannot be met.
32
33The names and meaning of the dependencies are as follows:
34
35requires:
36  The plugins listed here must be loaded for the declaring plugin to
37  work.  If any of the plugins is not loaded yet it will be loaded
38  automatically.  Dependency resolving fails if a plugin cannot be
39  loaded.
40
41needs:
42  The plugins listed here must be loaded for the declaring plugin to
43  work.  Dependency resolving fails if any of the plugins listed here is
44  not loaded.
45
46depends:
47  The plugins listed here must be loaded for the declaring plugin to
48  work.  If any of the plugins listed here is not loaded the declaring
49  plugin is automatically unloaded.  Dependency resolving fails if the
50  declaring plugin is already required by some other plugin.
51
52conflicts:
53  The plugins listed here must not be loaded at the same time as the
54  declaring plugin.  Dependency resolving fails if any of the plugins
55  listed here is loaded.
56
57The other two dependencies are used to specify the order of the plugins:
58
59preceeds:
60  The plugins listed here must come after the declaring plugin.
61
62succeeds:
63  The plugins listed here must come before the declaring plugin.
64
65Sorting the plugins may fail if the "preceeds" and "succeeds"
66dependencies introduce circles.
67
68HOOKS
69=====
70
71There are four different hooks that plugins may declare:
72
73vlock_start:
74  This hook is called once immediately after vlock is initialized and
75  before any authentication prompt.  If a plugin signals an error in
76  this hook vlock aborts and calls the vlock_end hooks of all previously
77  called modules.
78
79vlock_end:
80  This hook is called once after successful authentication or if vlock
81  is killed by SIGTERM.  Errors in this hook are ignored.
82
83vlock_save:
84  This hook is called after the vlock message is displayed every time
85  the timeout expires or the escape key is pressed.  If a plugin signals
86  an error in this hook its vlock_save_abort hook is called and both
87  hooks are not called again afterwards.
88
89vlock_save_abort:
90  This hook is called after vlock_save was called and any key was
91  pressed.  If a plugin signals an error in this hook both this hook and
92  the vlock_save hook are not called again.
93
94Note: Hooks should not block.  Screensavers should be executed in a
95background process or thread.  The only exception would be hooks that
96suspend the machine (though these technically do not block in the common
97sense).
98
99MODULES
100=======
101
102Modules are shared objects that are loaded into vlock's address space.
103They export hook functions and dependencies as global functions.  To
104ensure definitions modules should include vlock_plugin.h from the module
105subdirectory of the vlock source distribution.
106
107dependencies
108------------
109
110Dependencies are declared as NULL terminated arrays of const char
111pointers.  Empty lists can be just left out.  Example::
112
113  /* From nosysrq.c */
114  const char *preceeds[] = { "new", "all", NULL };
115  const char *depends[] = { "all", NULL };
116
117hooks
118-----
119
120Hooks are boolean functions that take a void pointer pointer.  Their
121return status indicates success or failure.  The argument points to a
122void pointer that may be set freely.  It may be used to maintain state
123between the different hooks.  It is initialized to NULL.  Hook functions
124must not block and not terminate the program.  On error they may print
125the cause of the error to stderr in addition to returning false.
126
127example
128-------
129
130Please see modules/example_module.c in the vlock source distribution.
131
132SCRIPTS
133=======
134
135Scripts are executables that are started as child processes of vlock.
136They run with the same privileges as the user starting vlock instead of
137the privileges of the vlock process.  They communicate with vlock
138through command line arguments and pipes.
139
140dependencies
141------------
142
143To get the dependencies of a script it is run once for each dependency
144item with the dependency name as the single command line argument.  Its
145standard output is redirected to a pipe that is read by vlock.  The
146plugin should print the dependency items, if any, separated by arbitrary
147white space (carriage return, space or newline) and then exit.  No
148errors are detected in this process.
149
150hooks
151-----
152
153After the dependencies are read the script is run one last time this
154time with the string "hooks" as the single command line argument.  Its
155standard input is redirected from a pipe that is written to by vlock.
156Whenever a hook should be executed its name followed by a new line
157character are written to the pipe.  The script's standard output and
158standard error are redirected to /dev/null.  The script should only exit
159if end-of-file is detected on standard in even in cases where no
160subsequent hooks need to be executed.  Error detection is limited to
161detecting if the script exits prematurely.  There is currently no way
162for a script what kind of error happened.
163
164example
165-------
166
167Please see scripts/example_script.sh in the vlock source distribution.
168