Sunday, March 12, 2006

What is a "thunk"?

A "thunk" appears to be an overloaded term in computer science. Use of the word "thunk" goes back to Algol 60 implementation. In general, as I understand it, thunk is a function which is used to replace something. More often than not, it is auto-generated. This "something" could be an expression (in a programming language) or an address.

In programming language world, a known usecase is call-by-name. The mechanism of dynamic linking (.DLL/.so files) uses local thunks to invoke dynamic linker at run-time and replace the thunk with the actual function for all the later invocations of the function. Please see how this technique can be exploited to modify program behavior at run-time.

In the C++ world, transparent replacement of addresses is very useful. An example is, adjusting pointers when virtual functions are invoked using one of the many possible base classes of the derived most class when multiple inheritance is involved. Depending upon which base class is used, the "this" pointer is adjusted using thunk methods to point at the right position in the object.

Helpful source: