What is a 'thunk'?

Question

I've seen it used in programming (specifically in the C++ domain) and have no idea what it is. Presumably it is a design pattern, but I could be wrong. Can anyone give a good example of a thunk?

Footstone

Just as an FYI, a thunk is also sometimes called a 'trampoline' (in the general case, maybe not in the C++ domain). – Rhpositive

@MichaelBurr, the only context I have seen the term "trampoline" used is Detours and in that context a trampoline is not a thunk. – Noumenon

The term is the type of thing that does not have a specific definition so it varies in definition. – Noumenon

Answer 1

A thunk usually refers to a small piece of code that is called as a function, does some small thing, and then JUMPs to another location (usually a function) instead of returning to its caller. Assuming the JUMP target is a normal function, when it returns, it will return to the thunk's caller.

Thunks can be used to implement lots of useful things efficiently

• protocol translation -- when calling from code that uses one calling convention to code that uses a different calling convention, a thunk can be used to translate the arguments appropriately. This only works if the return conventions are compatible, but that is often the case

• virtual function handling -- when calling a virtual function of a multiply-inherited base class in C++, there needs to be a fix-up of the this pointer to get it to point to the right place. A thunk can do this.

• dynamic closures -- when you build a dynamic closure, the closure function needs to be able to get at the context where it was created. A small thunk can be built (usually on the stack) which sets up the context info in some register(s) and then jumps to a static piece of code that implements the closure's function. The thunk here is effectively supplying one or more hidden extra arguments to the function that are not provided by the call site.

Sanders

This is the best explanation, because it explains what the thunk is instead of what it usually does in typical use cases to implement different things. Other answers focus too much on those particular implementations instead of the general idea. – Greeley

Not sure about other compilers, but Visual Studio in particular seems to be very fond of thunks. To my knowledge, it uses: adjustor thunks (to adjust this), default/copy constructor closures (for better CRT integration of user-provided ones with default parameters, mainly for DLL export or constructing arrays), vcall thunks (to make sure pointer-to-member-functions work properly with virtual functions), vtordisp thunks (for classes that inherit & override virtual functions from virtual bases, and also have user-provided ctors and/or dtors), native wrappers (to call managed C++/CLI – Tiemroth

functions from native ISO C++ code), and something called "UDT returning" (which appears to be a thunk for adjusting user-defined types returned by operators, but I'm not sure how to generate it; I think it's deprecated). There are probably others, too. I guess you can never say Microsoft thunk not; Descartes would be proud. – Tiemroth

Answer 2

The word thunk has at least three related meanings in computer science. A "thunk" may be:

• a piece of code to perform a delayed computation (similar to a closure)
• a feature of some virtual function table implementations (similar to a wrapper function)
• a mapping of machine data from one system-specific form to another, usually for compatibility reasons

I have usually seen it used in the third context.

http://en.wikipedia.org/wiki/Thunk

Flavoring

Interesting; I usually hear the second form, but I guess it depends what kind of work you do more often – Flirt

Specifically, related by being automatically generated very short blocks of machine code - even the first case is normally just giving context to a precompiled implementation function. – Sinistrad

I think #1 refers to Chris Dodds answer , but is not as broad: its doesn't just include delays, but any 'thing'. So in todays age, I would guess most people (web developers) would find #1 to be the common usage. – Horologist

Answer 3

The term thunk originally referred to the mechanism used by the Royal Radar Establishment implementation of pass-by-name in their Algol60 compiler. In general it refers to any way to induce dynamic behavior when referencing an apparently static object. The term was invented by Brian Wichmann, who when asked to explain pass-by-name said "Well you go out to load the value from memory and then suddenly - thunk - there you are evaluating an expression."

Thunks have been put in hardware (cf. KDF9, Burroughs mainframes). There are several ways to implement them in software, all very machine, language and compiler specific.

The term has come to be generalized beyond pass-by-name, to include any situation in which an apparently or nominally static data reference induces dynamic behavior. Related terms include "trampoline" and "future".

Larrigan

Thanks for the etymology. I hate programming terms whose definition seems to be an arbitrary lookup in a table. – Puritanism

Where did the Well you go out to load the value from memory and then suddenly - thunk - there you are evaluating an expression. quote come from? – Ardellardella

Answer 4

Some compilers for object-oriented languages such as C++ generate functions called "thunks" as an optimization of virtual function calls in the presence of multiple or virtual inheritance.

Taken from: http://en.wikipedia.org/wiki/Thunk#Thunks_in_object-oriented_programming

Kyrakyriako

Answer 5

This question has already been asked on SO, see:

What is a 'thunk', as used in Scheme or in general?

From what I can tell, it's akin to a lambda statement, where you may not want to return the value until you need to evaluate it; or it can also be compared to a property getter which by design executes some code in order to return a value while yet having the interface form that comes across more like a variable, but also has polymorphic behavior that can be swapped out whether by inheritance or by swapping out the function pointer that would evaluate and return a value at runtime based on compile-time or environmental characteristics.

Congest

Answer 6

There's considerable variation in use. Almost universally, a thunk is a function that's (at least conceptually) unusually small and simple. It's usually some sort of adapter that gives you the correct interface to something or other (some data, another function, etc.) but is at least seen as doing little else.

It's almost like a form of syntactic sugar, except that (at least as usually used) syntactic sugar is supposed to make things look the way the human reader wants to see them, and a thunk is to make something look the way the compiler wants to see it.

Alleen

Sounds like the opposite of syntactic sugar to me :) – Deroo

Syntactic sugar for compilers then? Almost, but not quite, entirely unlike syntactic sugar. – Microphone

Perhaps a syntactic sourener? – Tiemroth

Answer 7

I was distressed to find no general 'computer science' definition of this term matching its de-facto usage as known historically to me. The first real-life encounter I can recall where it was actually called that was in the OS/2 days and the 16-32 bit transition. It appears "thunking" is like irony in its application today.

My rough general understanding is that the thunk is a stub routine that just does nothing or routes across some fundamental boundary in kind between systems as in the mentioned historical cases.

So the sense is like a synesthesia of being dropped from the one environment to the other making (metaphorically/as a simile) a "thunk" sound.

Wreckfish

Interesting tip. I was wondering about the actual etymology of this word too, and I imagined people playing "bush telegraph", where one of the persons in the stream silently (and in many cases unknowingly) transforms the message on the way. – Greeley

Answer 8

I'm going to look this up, but I thought thunking was the process employed by a 32-bit processor to run legacy 16-bit code.

I used to use it as an analogy for how you have to restrict how fast you talk and what words you use when talking to dumb people.

Yeah, it's in the Wikipedia link (the part about 32-bit, not my nerdalogy).

https://en.wikipedia.org/wiki/Thunk

Much of the literature on interoperability thunks relates to various Wintel platforms, including MS-DOS, OS/2,[8]Windows[9][10] and .NET, and to the transition from 16-bit to 32-bit memory addressing. As customers have migrated from one platform to another, thunks have been essential to support legacy software written for the older platforms.

(emphasis added by me)

Trend

Answer 9

The earliest use of "thunk" I know of is from late '50s in reference to Algol60 pass-by-name argument evaluation in function calls. Algol was originally a specification language, not a programming language, and there was some question about how pass-by-name could be implemented on a computer.

The solution was to pass the entry point of what was essentially a lambda. When the callee evaluated the parameter, control fell through - thunk! - into the caller's context where the lambda was evaluated and it's result became the value of the parameter in the callee.

In tagged hardware, such as the Burroughs machines, the evaluation was implicit: an argument could be passed as a data value as in ordinary pass-by-value, or by thunk for pass-by-name, with different tags in the argument metadata. A load operation hardware checked the tag and either returned the simple value or automatically invoked the lambda thunk.

Larrigan

Answer 10

Per Kyle Simpson's definition, a thunk is a way to abstract the component of time out of asynchronous code.

Southworth

Answer 11

An earlier version of The New Hacker's Dictionary claimed a thunk is a function that takes no arguments, and that it was a simple late-night solution to a particularly tricky problem, with "thunk" being the supposed past tense of "think", because they ought to have thunk of it long time ago.

Hitherward

Answer 12

In OCaml, it’s a function which takes unit “()” as a param (takes no params, often used for side effects)

Mohamed