Callback (computer programming)

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Callback" computer programming – news · newspapers · books · scholar · JSTOR (September 2015) (Learn how and when to remove this message)

In computer programming, a callback is a function that is stored as data (a reference) and designed to be called by another function – often back to the original abstraction layer.

A function that accepts a callback parameter may be designed to call back before returning to its caller which is known as synchronous or blocking. The function that accepts a callback may be designed to store the callback so that it can be called back after returning which is known as asynchronous, non-blocking or deferred.

Programming languages support callbacks in different ways such as function pointers, lambda expressions and blocks.

A callback can be likened to leaving instructions with a tailor for what to do when a suit is ready, such as calling a specific phone number or delivering it to a given address. These instructions represent a callback: a function provided in advance to be executed later, often by a different part of the system and not necessarily by the one that received it.

The term callback can be misleading, as it does not necessarily imply a return to the original caller, unlike a telephone callback.