View Source: CastingPointerToFunction - Waikato Linux Users Group

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[C] is probably the only language you can do this in (except maybe [Forth]).  Other languages either have no way of doing this, or don't treat data and code differently.
(Feel free to add examples for other languages here.)

Once you've got a pointer to your machine code (usually an array of bytes or ints), you need to cast it to the appropriate type and call it.  Eg if you have a pointer to a function which takes a char *, you can call it like this:

 ((int (*)(char *)) p)("hello, world");

The secret here is the parenthesis around the star.  To declare a variable to hold the above, use this:

 int (*func_t)(char *);

In C (and [C++]), a function name is really just a pointer to a function, and you can think of () as an operator to make a call to that function. Using a typedef to hold the function type might make that clear:

 typedef int (*func_t)(char *);

 int foo(char *x) {
  printf("%s",x);
 }


 func_t p = foo; /* assign function pointer to p */
 return p("Narf!"); /* call that function, and return the result */

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And you need to do this why?

* Jon was using it to "compile" something then jump to it in C, by casting an array as a function, then calling it.

* I use it frequently to do things like have a lookup table of name to function.  For example in ircu, there is a function called "parse" which takes a line from a user, splits it up into a function and it's arguments, then scans through a table of functions and their names, when it finds one that matches that command, it calls the function with the arguments it parsed earlier.  Nice 'n zippy.

* A library back-end may provide functionality that should behave differently for different front ends. For example, a library that wants to print out diagnostic messages uses a function pointer, and then a console app can pass a pointer to a function that prints out to the terminal (or stdout(3) or stderr(3)) while a graphical app can create a callback function that pops up a [GUI] window or something.

* Some functions take another function as an argument. Probably the most well-known example is qsort(3) the quick sort algorithm -- you need to pass it an array and a function for comparing array elements. Also see the example below.


from atexit(3):
       #include <stdlib.h>
       int atexit(void (*function)(void));

you can specify functions to be executed after main() finishes.
You do this by calling atexit(functionname), as a function name
without () is a pointer to that function. However, if you want to this for a function that returns something other than null, you have to type cast it. Eg:
 (int endwin() is an ncurses function).

      atexit(endwin);
 source.cpp:227: passing `int (*)()' as argument 1 of `atexit(void (*)())'
      atexit(void)endwin);
 source.cpp:227: void value not ignored as it ought to be

The trick is to cast the pointer to a function pointer of the required type:
     atexit( (void(*)())endwin );

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It is worth noticing that in languages where functions are first class objects (such as [LISP] and [Scheme]), passing around pointers to functions is entirely normal (and type safe); even in [Java] and [Python] [Introspection] allows type safe access to such functionality.
2 pages link to CastingPointerToFunction:
Last edited on Monday, May 15, 2006 2:48:05 pm by AristotlePagaltzis
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