Name

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

Library

glib.lib

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

Detailed 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 first determine whether dynamic loading is supported on the platform by calling g_module_supported(). If it is supported,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 the 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 12. 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;
    }

  /* 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.

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.

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 does not 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 does not 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.

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.

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);

g_module_close ()

gboolean    g_module_close                  (GModule *module);

Closes a module.

g_module_error ()

const gchar* g_module_error                 (void);

Gets 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.

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.

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.

G_MODULE_IMPORT

#define	G_MODULE_IMPORT		extern

Used to declare functions imported from modules.