Editing Closure (computer programming) (section)

=== Callbacks (C) ===
Some [[C (programming language)|C]] libraries support [[Callback (computer programming)|callbacks]]. This is sometimes implemented by providing two values when registering the callback with the library: a function pointer and a separate <code>void*</code> pointer to arbitrary data of the user's choice. When the library executes the callback function, it passes along the data pointer. This enables the callback to maintain state and to refer to information captured at the time it was registered with the library. The idiom is similar to closures in functionality, but not in syntax. The <code>void*</code> pointer is not [[Type safety|type safe]] so this C idiom differs from type-safe closures in C#, Haskell or ML.

Callbacks are used extensively in [[graphical user interface]] (GUI) [[widget toolkit]]s to implement [[event-driven programming]] by associating general functions of graphical widgets (menus, buttons, check boxes, sliders, spinners, etc.) with application-specific functions implementing the specific desired behavior for the application.

====Nested function and function pointer (C)====
With a [[GNU Compiler Collection]] (GCC) extension, a nested function<ref>{{cite web
|url = https://gcc.gnu.org/onlinedocs/gcc/Nested-Functions.html
|title = Nested functions}}</ref> can be used and a function pointer can emulate closures, provided the function does not exit the containing scope. The next example is invalid because <code>adder</code> is a top-level definition (depending on compiler version, it could produce a correct result if compiled with no optimizing, i.e., at <code>-O0</code>):

<syntaxhighlight lang="c">
#include <stdio.h>

typedef int (*fn_int_to_int)(int); // type of function int->int

fn_int_to_int adder(int number) {
  int add (int value) { return value + number; }
  return &add; // & operator is optional here because the name of a function in C is a pointer pointing on itself
}

int main(void) {
  fn_int_to_int add10 = adder(10);
  printf("%d\n", add10(1));
  return 0;
}
</syntaxhighlight>

But moving <code>adder</code> (and, optionally, the <code>typedef</code>) in <code>main</code> makes it valid:

<syntaxhighlight lang="c">
#include <stdio.h>

int main(void) {
  typedef int (*fn_int_to_int)(int); // type of function int->int
  
  fn_int_to_int adder(int number) {
    int add (int value) { return value + number; }
    return add;
  }
  
  fn_int_to_int add10 = adder(10);
  printf("%d\n", add10(1));
  return 0;
}
</syntaxhighlight>

If executed this now prints <code>11</code> as expected.