A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
can, at run time, 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 loadtime linking, this mechanism allows a computer program
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
to startup in the absence of these libraries, to discover available libraries, and to potentially gain additional functionality.
History
Dynamic loading was a common technique for IBM/360 Operating systems (1960s to, the - still extant - Z/Architecture
Z/Architecture
z/Architecture, initially and briefly called ESA Modal Extensions , refers to IBM's 64-bit computing architecture for IBM mainframe computers. IBM introduced its first z/Architecture-based system, the zSeries Model 900, in late 2000. Later z/Architecture systems include the IBM z800, z990, z890,...
I/O may refer to:* Input/output, a system of communication for information processing systems* Input-output model, an economic model of flow prediction between sectors...
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
COBOL is one of the oldest programming languages. Its name is an acronym for COmmon Business-Oriented Language, defining its primary domain in business, finance, and administrative systems for companies and governments....
In computer programming, a runtime library is a special program library used by a compiler, to implement functions built into a programming language, during the execution of a computer program...
. As far as the application programmer is concerned, the loading is largely transparent, since it is mostly handled by the operating system (or its I/O subsystem). The main advantages are:
A patch is a piece of software designed to fix problems with, or update a computer program or its supporting data. This includes fixing security vulnerabilities and other bugs, and improving the usability or performance...
) to the subsystems fixed all programs at once, without the need to relink them
Libraries could be protected from unauthorized modification
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
In computer science, transaction processing is information processing that is divided into individual, indivisible operations, called transactions. Each transaction must succeed or fail as a complete unit; it cannot remain in an intermediate state...
Customer Information Control System is a transaction server that runs primarily on IBM mainframe systems under z/OS and z/VSE.CICS is a transaction manager designed for rapid, high-volume online processing. This processing is mostly interactive , but background transactions are possible...
(1970s onwards) uses dynamic loading extensively both for its kernel
Kernel (computing)
In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources...
and for normal application program loading. Corrections to application programs could be made offline and new copies of changed programs loaded dynamically without needing to restart CICS (that can, and frequently does, run 24/7
24/7
24/7 is an abbreviation which stands for "24 hours a day, 7 days a week", usually referring to a business or service available at all times without interruption...
).
Uses
Dynamic loading is most frequently used in implementing software plugins. For example, the Apache Web Server's *.dso "dynamic shared object" plugin files are libraries which are loaded at runtime with dynamic loading. 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.
In C/C++
Not all systems support dynamic loading. UNIX-like
Unix-like
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
, and Solaris provide dynamic loading with the C programming language "dl" library. The Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
The Windows API, informally WinAPI, is Microsoft's core set of application programming interfaces available in the Microsoft Windows operating systems. It was formerly called the Win32 API; however, the name "Windows API" more accurately reflects its roots in 16-bit Windows and its support on...
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
The Windows API, informally WinAPI, is Microsoft's core set of application programming interfaces available in the Microsoft Windows operating systems. It was formerly called the Win32 API; however, the name "Windows API" more accurately reflects its roots in 16-bit Windows and its support on...
Header file inclusion
#include <dlfcn.h>
#include <windows.h>
Definitions for header
dl
(libdl.so, libdl.dylib, etc. depending on the OS
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
)
Kernel32.dll
Loading the library
dlopen
LoadLibrary LoadLibraryEx
Extracting contents
dlsym
GetProcAddress
Unloading the library
dlclose
FreeLibrary
Loading the Library
Loading the library is accomplished with LoadLibrary or LoadLibraryEx on Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification....
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s. Examples follow:
Most UNIX-like operating systems (Linux, *BSD, Solaris, etc.)
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
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
}
Windows
HMODULE sdl_library = LoadLibrary("SDL.dll");
if( sdl_library NULL) {
// report error ...
} else {
// use the result in a call to GetProcAddress
}
Extracting Library Contents
Extracting the contents of a dynamically loaded library is achieved with GetProcAddress on Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s.
UNIX-like operating systems (Linux, *BSD, Mac OS X, Solaris, etc.)
NULL) {
// report error ...
} else {
// cast initializer to its proper type and use
}
Windows
FARPROC initializer = GetProcAddress(sdl_library,"SDL_Init");
if(initializer NULL) {
// report error ...
} else {
// cast initializer to its proper type and use
}
Converting Extracted Library Contents
The result of dlsym or GetProcAddress has to be converted to the desired destination before it can be used.
Windows
In the Windows case, the conversion is straightforward, since FARPROC is essentially already a function pointer:
typedef INT_PTR (*FARPROC)(void);
This can be problematic when the address of an object is to be retrieved rather than a function. However, usually one wants to extract functions anyway, so this is normally not a problem.
typedef void (*sdl_init_function_type)(void);
sdl_init_function_type init_func = (sdl_init_function_type) initializer;
UNIX (POSIX)
According to the POSIX specification, the result of dlsym is a void pointer. Therefore, the specification actually contains a defect, since both ISO C
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
and the C++ programming language prohibit conversion between object pointers and function pointers (in fact, a function pointer is not required to even have the same size as an object pointer). Therefore, strictly speaking, a legal conversion between type void* and a pointer to a function cannot exist.
On most systems in use today, function and object pointers are de facto convertible. The following code snippet demonstrates one workaround which allows to perform the conversion anyway on many systems:
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: warning: dereferencing type-punned pointer will break strict-aliasing rules. Another workaround 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;
which disables the warning even if strict aliasing is in effect. This makes use of the fact that reading from a different union member than the one most recently written to (called "type punning
Type punning
In computer science, type punning is a common term for any programming technique that subverts or circumvents the type system of a programming language in order to achieve an effect that would be difficult or impossible to achieve within the bounds of the formal language.In C and C++, constructs...
") is common, and explicitly allowed even if strict aliasing is in force, provided the memory is accessed through the union type directly. However, this is not strictly the case here, since the function pointer is copied to be used outside the union.
Solving the function pointer problem on POSIX systems
The fact remains that any conversion between function and object pointers has to be regarded as an (inherently non-portable) implementation extension, and that no "correct" way for a direct conversion exists, since in this regard the POSIX and ISO standards contradict each other.
Because of this problem, the POSIX documentation on dlsym (issue 6) stated that "a future version may either add a new function to return function pointers, or the current interface may be deprecated in favor of two new functions: one that returns data pointers and the other that returns function pointers". However, the most current version of the standard (issue 7, 2008) simply states that function pointers have to be convertible to void* for POSIX compliance, leaving compiler makers to choose which standard they adhere to.
If the contents of the library can be changed (i.e. in the case of a custom library), in addition to the function itself a pointer to it can be exported. Since a pointer to a function pointer is itself an object pointer, this pointer can always be legally retrieved by call to dlsym and subsequent conversion. However, this approach requires maintaining separate pointers to all functions that are to be used externally, and the benefits are usually small.
Unloading the Library
Loading a library causes memory to be allocated; the library must be deallocated in order to avoid a memory leak
Memory leak
A memory leak, in computer science , occurs when a computer program consumes memory but is unable to release it back to the operating system. In object-oriented programming, a memory leak happens when an object is stored in memory but cannot be accessed by the running code...
. Additionally, failure to unload a library can prevent filesystem operations on the file
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
which contains the library. Unloading the library is accomplished with FreeLibrary on Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s. However, unloading a DLL can lead to program crashes if objects in the main application refer to memory allocated within the DLL. For example, if a DLL introduces a new class and the DLL is closed, further operations on instances of that class from the main application will likely cause a memory access violation. Likewise, if the DLL introduces a factory function for instantiating dynamically-loaded classes, calling or dereferencing that function after the DLL is closed leads to undefined behaviour.
UNIX-like operating systems (Linux, *BSD, Mac OS X, Solaris, etc.)
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
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.
UNIX-like operating systems (Linux, *BSD, Mac OS X, Solaris, etc.)
In the Java programming language, classes can be dynamically loaded using the object. For example:
Class type = ClassLoader.getSystemClassLoader.loadClass(name);
Object obj = type.newInstance;
In computer science, a static library or statically-linked library is a set of routines, external functions and variables which are resolved in a caller at compile-time and copied into a target application by a compiler, linker, or binder, producing an object file and a stand-alone executable...
The Apache HTTP Server, commonly referred to as Apache , is web server software notable for playing a key role in the initial growth of the World Wide Web. In 2009 it became the first web server software to surpass the 100 million website milestone...
