学步园

From Wikipedia, the free encyclopedia

Dynamic loading is a mechanism by which a computer program can, at runtime, load a library (or other binary) into memory, retrieve the addresses of functions and variables contained in the library, execute those functions or access those variables, and unload the library from memory. Unlike static linking and delayed loading, this mechanism allows a computer program to startup in the absence of these libraries, to discover available libraries, and to potentially gain additional functionality. ^[1][2]

[edit] Uses

Dynamic loading is most frequently used in implementing computer plugins ^[1]. For example, the Apache Web Server's *.dso "dynamic shared object" plugin files are libraries which are loaded at runtime with dynamic loading ^[3]. Dynamic loading is also used in implementing computer programs where multiple different libraries may supply the requisite functionality and where the user has the option to select which library or libraries to provide.

[edit] In C/C++

Not all systems support dynamic loading. UNIX-like operating systems such as Mac OS X, Linux, and Solaris provide dynamic loading with the C programming language "dl" library. The Windows operating system provides dynamic loading through the Windows API.

[edit] Summary

[edit] Loading the Library

Loading the library is accomplished with LoadLibrary or LoadLibraryEx on Windows and with dlopen on UNIX-like operating systems. Examples follow:

[edit] Linux

void* sdl_library = dlopen("libsdl.so", RTLD_LAZY);
if(sdl_library == NULL) {
   // report error ...
} else {
   // use the result in a call to dlsym
}

[edit] Mac OS X

As a UNIX library:

void* sdl_library = dlopen("libsdl.dylib", RTLD_LAZY);
if(sdl_library == NULL) {
   // report error ...
} else {
   // use the result in a call to dlsym
}

As an OS X Framework:

void* sdl_library = dlopen("/Library/Frameworks/SDL.framework/SDL", RTLD_LAZY);
if(sdl_library == NULL) {
   // report error ...
} else {
   // use the result in a call to dlsym
}

[edit] Windows

HMODULE sdl_library = LoadLibrary("SDL.dll");
if( sdl_library == NULL) {
   // report error ...
} else {
   // use the result in a call to GetProcAddress
}

[edit] Extracting Library Contents

Extracting the contents of a dynamically loaded library is achieved with GetProcAddress on Windows and with dlsym on UNIX-like operating systems.

[edit] UNIX/BSD-like Operating Systems (Linux, Mac OS X, Solaris, etc.)

void* initializer = dlsym(sdl_library,"SDL_Init");
if(initializer == NULL) {
   // report error ...
} else {
   // cast initializer to its proper type and use
}

[edit] Windows

void* initializer = GetProcAddress(sdl_library,"SDL_Init");
if(initializer == NULL) {
   // report error ...
} else {
   // cast initializer to its proper type and use
}

[edit] Converting Extracted Library Contents

The C++ programming language prohibits conversion between type void* and a pointer to a function. The following code snippet demonstrates a workaround which allows for this conversion:

typedef void (*sdl_init_function_type)(void);
sdl_init_function_type init_func = *((sdl_init_function_type*)(&initializer));

The above snippet will give a warning on some compilers with a warning: dereferencing type-punned pointer will break strict-aliasing rules. A better workaround on those systems is:

typedef void (*sdl_init_function_type)(void);
union { sdl_init_function_type func; void * obj; } alias;
alias.obj = initializer;
sdl_init_function_type init_func = alias.func;

This makes use of the fact that reading from a different union member than the one most recently written to (called "type-punning") is common, and explictly allowed even if strict aliasing is in force, provided the memory is accessed through the union type directly^[4].

[edit] Unloading the Library

Loading a library causes memory to be allocated; the library must be deallocated in order to avoid a memory leak. Additionally, failure to unload a library can prevent filesystem operations on the file which contains the library. Unloading the library is accomplished with FreeLibrary on Windows and with dlclose on UNIX-like operating systems.

[edit] UNIX/BSD-like Operating Systems (Linux, Mac OS X, Solaris, etc.)

dlclose(sdl_library);

[edit] Windows

FreeLibrary(sdl_library);

[edit] Special Library

Both Windows and UNIX implementations of dynamic loading allow programmers to extract symbols from the currently executing process. In both of these APIs, the currently executing process can be "loaded" such that the result can be used in the same manner as the result from dynamically loading a library with LoadLibrary or dlopen.

[edit] UNIX/BSD-like Operating Systems (Linux, Mac OS X, Solaris, etc.)

void* this_process = dlopen(0,0);

[edit] Windows

HMODULE this_process;
GetModuleHandleEx(0,0,&this_process);

[edit] In Java

In the Java programming language, classes can be dynamically loaded using the ClassLoader object. For example:

Class type = ClassLoader.getSystemClassLoader().loadClass(name);
Object obj = type.newInstance();

http://en.wikipedia.org/wiki/Dynamic_loading