glib/docs/reference/glib/tmpl/modules.sgml

<!-- ##### SECTION Title ##### -->
Dynamic Loading of Modules

<!-- ##### SECTION Short_Description ##### -->
portable method for dynamically loading 'plug-ins'

<!-- ##### SECTION Long_Description ##### -->
<para>
These functions provide a portable way to dynamically load object files
(commonly known as 'plug-ins').
The current implementation supports all systems that provide
an implementation of dlopen() (e.g. Linux/Sun), as well as HP-UX via its
shl_load() mechanism, and Windows platforms via DLLs.
</para>

<para>
A program which wants to use these functions must be linked to the
libraries output by the command <command>pkg-config --libs gmodule-2.0</command>.
</para>

<para>
To use them you must first determine whether dynamic loading
is supported on the platform by calling g_module_supported().
If it is, you can open a module with g_module_open(),
find the module's symbols (e.g. function names) with g_module_symbol(),
and later close the module with g_module_close().
g_module_name() will return the file name of a currently opened module.
</para>
<para>
If any of the above functions fail, the error status can be found with
g_module_error().
</para>
<para>
The #GModule implementation features reference counting for opened modules,
and supports hook functions within a module which are called when the
module is loaded and unloaded (see #GModuleCheckInit and #GModuleUnload).
</para>
<para>
If your module introduces static data to common subsystems in the running
program, e.g. through calling <literal>g_quark_from_static_string ("my-module-stuff")</literal>,
it must ensure that it is never unloaded, by calling g_module_make_resident().
</para>

<para>
<example>
<title>Calling a function defined in a <structname>GModule</structname></title>
<programlisting>
/* the function signature for 'say_hello' */
typedef void (* SayHelloFunc) (const char *message);

gboolean
just_say_hello (const char *filename, GError **error)
{
  SayHelloFunc  say_hello;
  GModule      *module;

  module = g_module_open (filename, G_MODULE_BIND_LAZY);
  if (!module)
    {
      g_set_error (error, FOO_ERROR, FOO_ERROR_BLAH,
		   "&percnt;s", g_module_error (<!-- -->));
      return FALSE;
    }

  if (!g_module_symbol (module, "say_hello", (gpointer *)&amp;say_hello))
    {
      g_set_error (error, SAY_ERROR, SAY_ERROR_OPEN,
		   "&percnt;s: &percnt;s", filename, g_module_error (<!-- -->));
      if (!g_module_close (module))
	g_warning ("&percnt;s: &percnt;s", filename, g_module_error (<!-- -->));
      return FALSE;
    }

  if (say_hello == NULL)
    {
      g_set_error (error, SAY_ERROR, SAY_ERROR_OPEN, "symbol say_hello is NULL");
      if (!g_module_close (module))
	g_warning ("&percnt;s: &percnt;s", filename, g_module_error (<!-- -->));
      return FALSE;
    }

  /* call our function in the module */
  say_hello ("Hello world!");

  if (!g_module_close (module))
    g_warning ("&percnt;s: &percnt;s", filename, g_module_error (<!-- -->));

  return TRUE;
}
</programlisting>
</example>
</para>

<!-- ##### SECTION See_Also ##### -->
<para>

</para>

<!-- ##### SECTION Stability_Level ##### -->


<!-- ##### SECTION Image ##### -->


<!-- ##### STRUCT GModule ##### -->
<para>
The #GModule struct is an opaque data structure to represent a
<link linkend="glib-Dynamic-Loading-of-Modules">Dynamically-Loaded Module</link>.
It should only be accessed via the following functions.
</para>


<!-- ##### FUNCTION g_module_supported ##### -->
<para>
Checks if modules are supported on the current platform.
</para>

@void: 
@Returns: %TRUE if modules are supported.


<!-- ##### FUNCTION g_module_build_path ##### -->
<para>
A portable way to build the filename of a module. The platform-specific
prefix and suffix are added to the filename, if needed, and the result is
added to the directory, using the correct separator character.
</para>
<para>
The directory should specify the directory where the module can be found.
It can be %NULL or an empty string to indicate that the module is in a standard
platform-specific directory, though this is not recommended since the
wrong module may be found.
</para>
<para>
For example, calling g_module_build_path() on a Linux system with a @directory
of <filename>/lib</filename> and a @module_name of "mylibrary" will return 
<filename>/lib/libmylibrary.so</filename>. On a Windows system, using 
<filename>\Windows</filename> as the directory it will return
<filename>\Windows\mylibrary.dll</filename>.
</para>

@directory: the directory where the module is. This can be %NULL or the empty
string to indicate that the standard platform-specific directories will be 
used, though that is not recommended.
@module_name: the name of the module.
@Returns: the complete path of the module, including the standard library
prefix and suffix. This should be freed when no longer needed.


<!-- ##### MACRO g_module_open ##### -->
<para>
Opens a module. If the module has already been opened, its reference
count is incremented. 
</para>

<para>
First of all g_module_open() tries to open @file_name as a module. If
that fails and @file_name has the ".la"-suffix (and is a libtool archive) 
it tries to open the corresponding module. If that fails and it doesn't 
have the proper module suffix for the platform (#G_MODULE_SUFFIX), this 
suffix will be appended and the corresponding module will be opended. If 
that fails and @file_name doesn't have the ".la"-suffix, this suffix is 
appended and g_module_open() tries to open the corresponding module. If 
eventually that fails as well, %NULL is returned.
</para>

@Returns: a #GModule on success, or %NULL on failure.
<!-- # Unused Parameters # -->
@file_name: the name of the file containing the module, or %NULL to obtain
  a #GModule representing the main program itself.
@flags: the flags used for opening the module. This can be the logical
OR of any of the #GModuleFlags.


<!-- ##### ENUM GModuleFlags ##### -->
<para>
Flags passed to g_module_open(). Note that these flags are
not supported on all platforms.
</para>

@G_MODULE_BIND_LAZY: specifies that symbols are only resolved when needed.
  The default action is to bind all symbols when the module is loaded.
@G_MODULE_BIND_LOCAL: specifies that symbols in the module should
  not be added to the global name space.  The default action on most
  platforms is to place symbols in the module in the global name space,
  which may cause conflicts with existing symbols.
@G_MODULE_BIND_MASK: mask for all flags.

<!-- ##### FUNCTION g_module_symbol ##### -->
<para>
Gets a symbol pointer from a module, such as one exported by #G_MODULE_EXPORT.
</para>
<para>
Note that a valid symbol can be %NULL.
</para>

@module: a #GModule.
@symbol_name: the name of the symbol to find.
@symbol: returns the pointer to the symbol value.
@Returns: %TRUE on success.


<!-- ##### MACRO g_module_name ##### -->
<para>
Gets the filename from a #GModule.
</para>

@Returns: the filename of the module, or "main" if the module is the main
program itself.
<!-- # Unused Parameters # -->
@module: a #GModule.


<!-- ##### FUNCTION g_module_make_resident ##### -->
<para>
Ensures that a module will never be unloaded.
Any future g_module_close() calls on the module will be ignored.
</para>

@module: a #GModule to make permanently resident.


<!-- ##### FUNCTION g_module_close ##### -->
<para>
Closes a module.
</para>

@module: a #GModule to close.
@Returns: %TRUE on success.


<!-- ##### FUNCTION g_module_error ##### -->
<para>
Gets a string describing the last module error.
</para>

@void: 
@Returns: a string describing the last module error.


<!-- ##### USER_FUNCTION GModuleCheckInit ##### -->
<para>
Specifies the type of the module initialization function.
<indexterm zone="g-module-check-init"><primary>g_module_check_init</primary></indexterm>
If a module contains a function named g_module_check_init() it is called
automatically when the module is loaded. It is passed the #GModule structure
and should return %NULL on success or a string describing the initialization
error.
</para>

@module: the #GModule corresponding to the module which has just been loaded.
@Returns: %NULL on success, or a string describing the initialization error.


<!-- ##### USER_FUNCTION GModuleUnload ##### -->
<para>
<indexterm zone="g-module-unload"><primary>g_module_unload</primary></indexterm>
Specifies the type of the module function called when it is unloaded.
If a module contains a function named g_module_unload() it is called
automatically when the module is unloaded.
It is passed the #GModule structure.
</para>

@module: the #GModule about to be unloaded.


<!-- ##### MACRO G_MODULE_SUFFIX ##### -->
<para>
Expands to the proper shared library suffix for the current platform
without the leading dot. For the most Unices and Linux this is "so",
for some HP-UX versions this is "sl" and for Windows this is "dll".
</para>



<!-- ##### MACRO G_MODULE_EXPORT ##### -->
<para>
Used to declare functions exported by modules. This is a no-op on Linux and
Unices, but when compiling for Windows, it marks a symbol to be exported from
the library or executable being built.
</para>



<!-- ##### MACRO G_MODULE_IMPORT ##### -->
<para>
Used to declare functions imported from modules.
</para>