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Plugins are a way to extend vlock's functionality. They can define
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hooks that are called at certain points in a vlock session.
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There are two separate types of plugins: modules and scripts. Modules
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are shared objects that are loaded into vlock's address space. They run
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with the same privileges as vlock and thus are very powerful but also
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dangerous. Scripts may be any kind of executables located in vlock's
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script directory. They are run in separate processes with lowered
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privileges, i.e. the same as the user who started vlock.
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For simple tasks scripts should be preferred over modules. They are
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easier to develop and test and have a lower impact on security and
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NB: The following interface is not yet declared stable. It is not guaranteed
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that plugins (modules or scripts) that work with vlock 2.2 will work with
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Plugins may depend on each other in several ways. There are six
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different types of dependencies. Each dependency type is represented by
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a list of plugin names. The way of declaring them is different for
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modules and scripts but their names and meaning are the same.
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Resolving the dependencies is done after all initially requested plugins
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are loaded and may fail if dependencies cannot be met.
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The names and meaning of the dependencies are as follows:
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The plugins listed here must be loaded for the declaring plugin to
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work. If any of the plugins is not loaded yet it will be loaded
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automatically. Dependency resolving fails if a plugin cannot be
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The plugins listed here must be loaded for the declaring plugin to
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work. Dependency resolving fails if any of the plugins listed here is
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The plugins listed here must be loaded for the declaring plugin to
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work. If any of the plugins listed here is not loaded the declaring
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plugin is automatically unloaded. Dependency resolving fails if the
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declaring plugin is already required by some other plugin.
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The plugins listed here must not be loaded at the same time as the
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declaring plugin. Dependency resolving fails if any of the plugins
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listed here is loaded.
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The other two dependencies are used to specify the order of the plugins:
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The plugins listed here must come after the declaring plugin.
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The plugins listed here must come before the declaring plugin.
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Sorting the plugins may fail if the "preceeds" and "succeeds"
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dependencies introduce circles.
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There are four different hooks that plugins may declare:
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This hook is called once immediately after vlock is initialized and
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before any authentication prompt. If a plugin signals an error in
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this hook vlock aborts and calls the vlock_end hooks of all previously
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This hook is called once after successful authentication or if vlock
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is killed by SIGTERM. Errors in this hook are ignored.
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This hook is called after the vlock message is displayed every time
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the timeout expires or the escape key is pressed. If a plugin signals
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an error in this hook its vlock_save_abort hook is called and both
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hooks are not called again afterwards.
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This hook is called after vlock_save was called and any key was
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pressed. If a plugin signals an error in this hook both this hook and
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the vlock_save hook are not called again.
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Note: Hooks should not block. Screensavers should be executed in a
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background process or thread. The only exception would be hooks that
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suspend the machine (though these technically do not block in the common
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Modules are shared objects that are loaded into vlock's address space.
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They export hook functions and dependencies as global functions. To
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ensure definitions modules should include vlock_plugin.h from the module
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subdirectory of the vlock source distribution.
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Dependencies are declared as NULL terminated arrays of const char
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pointers. Empty lists can be just left out. Example::
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const char *preceeds[] = { "new", "all", NULL };
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const char *depends[] = { "all", NULL };
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Hooks are boolean functions that take a void pointer pointer. Their
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return status indicates success or failure. The argument points to a
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void pointer that may be set freely. It may be used to maintain state
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between the different hooks. It is initialized to NULL. Hook functions
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must not block and not terminate the program. On error they may print
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the cause of the error to stderr in addition to returning false.
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Please see modules/example_module.c in the vlock source distribution.
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Scripts are executables that are started as child processes of vlock.
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They run with the same privileges as the user starting vlock instead of
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the privileges of the vlock process. They communicate with vlock
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through command line arguments and pipes.
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To get the dependencies of a script it is run once for each dependency
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item with the dependency name as the single command line argument. Its
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standard output is redirected to a pipe that is read by vlock. The
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plugin should print the dependency items, if any, separated by arbitrary
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white space (carriage return, space or newline) and then exit. No
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errors are detected in this process.
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After the dependencies are read the script is run one last time this
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time with the string "hooks" as the single command line argument. Its
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standard input is redirected from a pipe that is written to by vlock.
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Whenever a hook should be executed its name followed by a new line
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character are written to the pipe. The script's standard output and
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standard error are redirected to /dev/null. The script should only exit
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if end-of-file is detected on standard in even in cases where no
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subsequent hooks need to be executed. Error detection is limited to
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detecting if the script exits prematurely. There is currently no way
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for a script what kind of error happened.
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Please see scripts/example_script.sh in the vlock source distribution.
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PLUGINS
