22.7.3 Nested Functions

A nested function is a function defined inside another function. (The ability to do this is indispensable for automatic translation of certain programming languages into C.) The nested function’s name is local to the block where it is defined. For example, here we define a nested function named square, then call it twice:

foo (double a, double b)
{
  double square (double z) { return z * z; }

  return square (a) + square (b);
}

The nested function definition can access all the variables of the containing function that are visible at the point of its definition. This is called lexical scoping. For example, here we show a nested function that uses an inherited variable named offset:

bar (int *array, int offset, int size)
{
  int access (int *array, int index)
    { return array[index + offset]; }
  int i;
  …
  for (i = 0; i < size; i++)
    … access (array, i) …
}

Nested function definitions can appear wherever automatic variable declarations are allowed; that is, in any block, interspersed with the other declarations and statements in the block.

The nested function’s name is visible only within the parent block; the name’s scope starts from its definition and continues to the end of the containing block. If the nested function’s name is the same as the parent function’s name, there will be no way to refer to the parent function inside the scope of the name of the nested function.

Using extern or static on a nested function definition is an error.

It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function. You can do this safely, but you must be careful:

hack (int *array, int size, int addition)
{
  void store (int index, int value)
    { array[index] = value + addition; }

  intermediate (store, size);
}

Here, the function intermediate receives the address of store as an argument. If intermediate calls store, the arguments given to store are used to store into array. store also accesses hack’s local variable addition.

It is safe for intermediate to call store because hack’s stack frame, with its arguments and local variables, continues to exist during the call to intermediate.

Calling the nested function through its address after the containing function has exited is asking for trouble. If it is called after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, it will refer to memory containing junk or other data. It’s not wise to take the risk.

The GNU C Compiler implements taking the address of a nested function using a technique called trampolines. This technique was described in Lexical Closures for C++ (Thomas M. Breuel, USENIX C++ Conference Proceedings, October 17–21, 1988).

A nested function can jump to a label inherited from a containing function, provided the label was explicitly declared in the containing function (see Local Labels). Such a jump returns instantly to the containing function, exiting the nested function that did the goto and any intermediate function invocations as well. Here is an example:

bar (int *array, int offset, int size)
{
  /* Explicitly declare the label failure. */
  __label__ failure;
  int access (int *array, int index)
    {
      if (index > size)
        /* Exit this function,
           and return to bar. */
        goto failure;
      return array[index + offset];
    }

  int i;
  …
  for (i = 0; i < size; i++)
    … access (array, i) …
  …
  return 0;

 /* Control comes here from access
    if it does the goto.  */
 failure:
  return -1;
}

To declare the nested function before its definition, use auto (which is otherwise meaningless for function declarations; see auto and register). For example,

bar (int *array, int offset, int size)
{
  auto int access (int *, int);
  …
  … access (array, i) …
  …
  int access (int *array, int index)
    {
      …
    }
  …
}