Dynamic Loading of Modules

Dynamic Loading of Modules — portable method for dynamically loading 'plug-ins'

Synopsis


#include <gmodule.h>


                    GModule;
gboolean            g_module_supported                  (void);
gchar*              g_module_build_path                 (const gchar *directory,
                                                         const gchar *module_name);
GModule*            g_module_open                       (const gchar *file_name,
                                                         GModuleFlags flags);
enum                GModuleFlags;
gboolean            g_module_symbol                     (GModule *module,
                                                         const gchar *symbol_name,
                                                         gpointer *symbol);
const gchar*        g_module_name                       (GModule *module);
void                g_module_make_resident              (GModule *module);
gboolean            g_module_close                      (GModule *module);
const gchar*        g_module_error                      (void);

const gchar*        (*GModuleCheckInit)                 (GModule *module);
void                (*GModuleUnload)                    (GModule *module);
#define             G_MODULE_SUFFIX
#define             G_MODULE_EXPORT
#define             G_MODULE_IMPORT

Description

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.

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

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.

If any of the above functions fail, the error status can be found with g_module_error().

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).

If your module introduces static data to common subsystems in the running program, e.g. through calling g_quark_from_static_string ("my-module-stuff"), it must ensure that it is never unloaded, by calling g_module_make_resident().

Example 11. Calling a function defined in a GModule

/* 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,
		   "%s", g_module_error ());
      return FALSE;
    }

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

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

  if (!g_module_close (module))
    g_warning ("%s: %s", filename, g_module_error ());

  return TRUE;
}


Details

GModule

typedef struct _GModule GModule;

The GModule struct is an opaque data structure to represent a Dynamically-Loaded Module. It should only be accessed via the following functions.

g_module_supported ()

gboolean            g_module_supported                  (void);

Checks if modules are supported on the current platform.

Returns :

 TRUE if modules are supported.

g_module_build_path ()

gchar*              g_module_build_path                 (const gchar *directory,
                                                         const gchar *module_name);

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.

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.

For example, calling g_module_build_path() on a Linux system with a directory of /lib and a module_name of "mylibrary" will return /lib/libmylibrary.so. On a Windows system, using \Windows as the directory it will return \Windows\mylibrary.dll.

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.

g_module_open ()

GModule*            g_module_open                       (const gchar *file_name,
                                                         GModuleFlags flags);

Opens a module. If the module has already been opened, its reference count is incremented.

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.

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.

Returns :

 a GModule on success, or NULL on failure.

enum GModuleFlags

typedef enum
{
  G_MODULE_BIND_LAZY	= 1 << 0,
  G_MODULE_BIND_LOCAL	= 1 << 1,
  G_MODULE_BIND_MASK	= 0x03
} GModuleFlags;

Flags passed to g_module_open(). Note that these flags are not supported on all platforms.

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.

g_module_symbol ()

gboolean            g_module_symbol                     (GModule *module,
                                                         const gchar *symbol_name,
                                                         gpointer *symbol);

Gets a symbol pointer from a module, such as one exported by G_MODULE_EXPORT.

Note that a valid symbol can be NULL.

module :

 a GModule.

symbol_name :

 the name of the symbol to find.

symbol :

 returns the pointer to the symbol value.

Returns :

 TRUE on success.

g_module_name ()

const gchar*        g_module_name                       (GModule *module);

Gets the filename from a GModule.

module :

 a GModule.

Returns :

 the filename of the module, or "main" if the module is the main program itself.

g_module_make_resident ()

void                g_module_make_resident              (GModule *module);

Ensures that a module will never be unloaded. Any future g_module_close() calls on the module will be ignored.

module :

 a GModule to make permanently resident.

g_module_close ()

gboolean            g_module_close                      (GModule *module);

Closes a module.

module :

 a GModule to close.

Returns :

 TRUE on success.

g_module_error ()

const gchar*        g_module_error                      (void);

Gets a string describing the last module error.

Returns :

 a string describing the last module error.

GModuleCheckInit ()

const gchar*        (*GModuleCheckInit)                 (GModule *module);

Specifies the type of the module initialization function. 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.

module :

 the GModule corresponding to the module which has just been loaded.

Returns :

 NULL on success, or a string describing the initialization error.

GModuleUnload ()

void                (*GModuleUnload)                    (GModule *module);

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.

module :

 the GModule about to be unloaded.

G_MODULE_SUFFIX

#define G_MODULE_SUFFIX "so"

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".

G_MODULE_EXPORT

#define             G_MODULE_EXPORT

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.

G_MODULE_IMPORT

#define	G_MODULE_IMPORT		extern

Used to declare functions imported from modules.