THUNK - Late Binding
A thunk is a computer programming technique for resolving late binding of items.
Description
The essence of a thunk is to use a place holder to stand as a proxy until the last possible moment before using an item with the executable code.
Late Binding
One of the best ways to implement Late Binding in any area of life, is to cross that bridge when we come to it. A thunk is a technique to cross that bridge only when we come to it.
Examples of Use
The Call by Name calling convention passes the name of the actual parameter to a function call. Where Call by Reference resolves the actual paramter's contents by a pointer or reference object. Call by Name resolves to the actual object to the name of the item. The name of any actual paremeter can only be known at run-time. Just as the pointer value of any actual parameter is unknown at run-time with Call by Reference.
To use a thunk, the compiler marks a spot for a reference to the named thing at runtime to achieve late binding. Call by Name uses the the lexographical significance of parameters in the call stack. Using a thunk allows late binding of a function invokation with the stack history at runtime.
When the function comes to an unknown parameter during execution, the thunk is fired to resolve the this-call-Name-of the actual parameter. While this sounds odd, in practice it normally only means adding to the most recent stack-frame.
Earlier use of thunk techniques were the working Algol 60 implementations. See also: B5000
The essense of a Virtual Memory management system is that there is more memory allowed to be in-use by a program than actually exists physically. Typically Virtual Memory systems use a thunk to allocate empty pages from a free-page pool. Late Binding in this way means that no free-pages need to be unnecessarily pre-committed. Memory is allocated only when the first used, not before.
OS/2 & Windows 16-bit Address hack
A piece of code is executed to provide an address. The most common usage is in the Win16 / Win32 API, where thunking is used to convert a 16 bit address into a 32 bit equivalent or vice versa. The ubiquitous early example was "wsock32.dll", a thunking layer added to allow Win32 Internet applications to use the Win16 "winsock.dll" Sockets library originally written for Windows 3.11. Similar thunking was required in many cases in OS/2 2.x—while most of the operating system was 32-bit, many parts of the kernel and device drivers were 16-bit for compatibility reasons.
Description
In Lisp terminology, a thunk is a function that takes no arguments, especially if it is passed as an argument to another function.
Example
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