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For a discussion of callback with computer modems, see callback (telecommunications). In computer programming, a callback is a reference to executable code, or a piece of executable code, that is passed as an argument to other code. This allows a lower-level software layer to call a subroutine (or function) defined in a higher-level layer. A callback is often back on the level of the original caller. Contents 1 Use 2 Example 3 Implementation 4 See also 5 External links 6 References // Use Callbacks have a wide variety of uses. For example, imagine a function that reads a configuration file and associates values with options. If the options are identified by a hash, then writing the function so that it takes a callback makes it more flexible: its user can choose whatever hashing algorithm he wishes and the function will continue to work, since it uses the callback to turn option names into hashes; thus, callbacks allow the user of a function to fine-tune it at runtime. Another use is in error signaling. A Unix program, for example, might not want to terminate immediately when it receives SIGTERM; to make sure things get taken care of, it would register the cleanup function as a callback. Callbacks may also be used to control whether a function acts or not: Xlib allows custom predicates to be specified to determine whether a program wishes to handle an event. Example The following code in C demonstrates the use of callbacks for the specific case of dealing with a POSIX-style signal (in this case SIGUSR1). It should be noted, however, that the use of printf(3) is not safe inside a signal handler. #include <stdio.h> #include <signal.h> #include <unistd.h> void sig(int signum) { printf("Received signal number %d!\n", signum); } int main(int argc, char *argv[]) { signal(SIGUSR1, sig); pause(); return 0; } The pause() call will keep this example from doing anything interesting, but it will give you plenty of time to send a signal to this process. (If you're on a unix-like system, try a "kill -USR1 <pid>" to the process ID associated with this sample program. No matter how or when you send it, the callback should respond.) Note that using a loop instead of the pause() would cause the process CPU usage to climb to 100%. Implementation The form of a callback varies among programming languages. C, C++ and Pascal allow function pointers as arguments to other functions. Other languages, such as JavaScript, Python, Perl[1][2] and PHP, simply allow the name of a function to be passed through. The .NET Framework, used in languages such as C# and VB.NET, provides a type-safe encapsulating reference, a 'delegate', to define well-typed function pointers. These can be used as callbacks. Events and event handlers, as used in .NET languages, provide generalized syntax for callbacks. Functional languages generally support first-class functions, which can be passed as callbacks to other functions, stored as data or returned from functions. Some languages, such as Algol 68, Perl, newer versions of C# and VB.NET as well as most functional languages, allow unnamed blocks of code (lambda expressions) to be supplied instead of references to functions defined elsewhere. In some languages, e.g. Scheme, ML, JavaScript, Perl, and many others, such functions can be closures, i.e. they can access and modify variables locally defined in the context in which the function was defined. In object-oriented programming languages without function-valued arguments, such as Java, callbacks can be simulated by passing an abstract class or interface, of which the receiver will call one or more methods, while the calling end provides a concrete implementation. Such objects are effectively a bundle of callbacks, plus the data they need to manipulate. They are useful in implementing various design patterns such as Visitor, Observer, and Strategy. C++ allows objects to provide their own implementation of the function call operation. The Standard Template Library accepts these objects (called functors), as well as function pointers, as parameters to various polymorphic algorithms See also Continuation-passing style Signals and slots Event-driven programming libsigc++, a callback library for C++ Implicit invocation User exit Inversion of control External links Style Case Study #2: Generic Callbacks C++ Callback Solution Basic Instincts: Implementing Callback Notifications Using Delegates Implement Script Callback Framework in ASP.NET Implement callback routines in Java Interfacing C++ member functions with C libraries References ^ "Perl Cookbook - 11.4. Taking References to Functions". http://www.unix.org.ua/orelly/perl/cookbook/ch11_05.htm. Retrieved 2008-03-03.  ^ "Advanced Perl Programming - 4.2 Using Subroutine References". http://www.unix.org.ua/orelly/perl/advprog/ch04_02.htm. Retrieved 2008-03-03.