Dynamic loading: Difference between revisions

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{{Redirect-distinguish2|Dynamically loaded library|[[dynamic-link library|dynamically linked library]]}}

{{Use dmy dates|date=July 2019|cs1-dates=y}}

'''Dynamic loading''' is a mechanism by which a [[computer program]] can, at [[Run time (program lifecycle phase)|run time]], load a [[Library (computing)|library]] (or other [[Executable file|binary]]) into memory, retrieve the addresses of functions and variables contained in the library, execute those [[library function|functions]] or access those variables, and unload the library from memory. It is one of the fourthree mechanisms by which a computer program can use some other software within the program; the others are spawning a [[child process]], [[static linking]], and [[dynamic linking]]. Unlike static linking and dynamic linking, dynamic loading allows a [[computer program]] to start up in the absence of these libraries, to discover available libraries, and to potentially gain additional functionality.<ref name="autobook"/><ref name="Elf_Dynamic_Loading"/>

==History==

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<syntaxhighlight lang="c">

void* sdl_library = dlopen("libSDL.so", RTLD_LAZY);

if (!sdl_library == NULL) {

// report error ...

} else {

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<syntaxhighlight lang="c">

void* sdl_library = dlopen("libSDL.dylib", RTLD_LAZY);

if (!sdl_library == NULL) {

// report error ...

} else {

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<syntaxhighlight lang="c">

void* sdl_library = dlopen("/Library/Frameworks/SDL.framework/SDL", RTLD_LAZY);

if (!sdl_library == NULL) {

// report error ...

} else {

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<syntaxhighlight lang="c">

HMODULE sdl_library = LoadLibrary(TEXT("SDL.dll"));

if (!sdl_library == NULL) {

// report error ...

} else {

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<syntaxhighlight lang="c">

void* initializer = dlsym(sdl_library, "SDL_Init");

if (!initializer == NULL) {

// report error ...

} else {

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====Windows====

<syntaxhighlight lang="c">

FARPROC initializer = GetProcAddress(sdl_library, "SDL_Init");

if (!initializer == NULL) {

// report error ...

} else {

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<syntaxhighlight lang="c">

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;

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====Unix-like operating systems (Solaris, Linux, *BSD, macOS, etc.)====

<syntaxhighlight lang="c">

void* this_process = dlopen(NULL, 0);

</syntaxhighlight>

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HMODULE this_process_again;

GetModuleHandleEx(0, 0, &this_process_again);

</syntaxhighlight>

==In Java==

{{further|Java Classloaderclass loader}}

In the [[Java programming language]], [[Java class|classes]] can be dynamically loaded using the '''{{Javadoc:SE|java/lang|ClassLoader}}''' object. For example: