From Wikipedia, the free encyclopedia
In computer programming, a
callback is executable code
that is passed as an argument to other code. It 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.
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:
the user of it can use 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>
void sig(int signum)
{
printf("Received signal number %d!\n", signum);
}
int main(int argc, char *argv[])
{
signal(SIGUSR1, sig);
while(1);
return 0;
}
The while loop 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.)
Implementation
The form of a callback varies among programming languages.
- C and C++ allow function pointers as arguments to
other functions.
- Several programming languages (though especially functional programming languages
such as Scheme or ML) allow closures, a generalization
of function pointers, as arguments to other functions.
- In object-oriented programming
languages, a call can accept an object that implements some
abstract interface, without specifying in detail how the object
should do so. The programmer who implements that object may use the
interface's methods exclusively for application-specific code. 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
- .NET
Framework provides a type-safe encapsulating reference, a 'delegate', to
manage function pointers. These can be used
for callback operations.
- Perl supports subroutine
references.[1][2]
- Some systems have built-in programming languages to support
extension and adaptation. These languages provide callbacks without
the need for separate software development tools.
See also
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