Function Pointers in C are Underrated

http://vickychijwani.github.com/2012/03/22/function-pointers-in-c-are-underrated/

by vickychijwani • 14 years ago

19 32 14 years ago

vickychijwani.github.com

duaneb• 14 years ago

I don't quite understand what is meant by this. Function pointers are not "underrated"; that implies that it is lower or higher than something. Being the only way to perform an indirect branch in C, it's not really something you rate. It just is.

matt_yoho• 14 years ago

I interpret the statement as being closer to "function pointers are under-discussed".

xymostech• 14 years ago

Indeed. While someone might talk about lambda functions very early into a discussion of Ruby, Python, etc., pointers-to-functions in C are almost never discussed in basic Computer Science.

vickychijwani4• 14 years ago

Through the title, I'm just trying to point out that function pointers are not as well-known in C as in other languages. Programmers should recognize when and how to use them effectively.

Arelius4• 14 years ago

Not as well known to C programmers, or not as well known to programmers who don't program C? I don't know a single C programmer that doesn't regularly use function pointers.

kbd• 14 years ago

TLDR: You can write a comparator function and use it like with C's 'qsort'.

zerostar07• 14 years ago

C function pointers are prime citizens for 42 years. Nobody underrates them. 'cdecl' is a fun way to learn all about them.

C++ 's inline templates beat them in performance though.

mangoman• 14 years ago

This should probably be called "Function pointers in C are AWESOME". because they are. Function Pointers are basically C's implementation for callbacks too! There are some awesome things people can do with fp's, and i especially like C++'s use of function pointers and function objects with respect to the STL algorithms and the use of tr1::bind and std::not1 in the functional header. Cool Stuff!

sophacles• 14 years ago

The linux kernel has a cool linked list implementation for purposes like this, which IMHO is much prettier. Essentially you put a struct list (or hlist) inside of a struct you use in a list. This struct can be a simple pointer to data, or a full struct itself. Whatever. Then the list macros and functions just work on your data structure.

Implementation details here: http://lxr.linux.no/linux+v3.3/include/linux/rculist.h (also look up list.h).

My favorite part is wrapping your head round the container_of macro central to this. ( http://lxr.linux.no/linux+v3.3/tools/perf/util/include/linux... )

Not that function pointers aren't neat or useful, but sometimes there are other cool solutions too :)

mhd2• 14 years ago

Equating function pointers with closures in the second paragraph isn't a good start…

vickychijwani4• 14 years ago

At the most basic level, though, the comparison remains valid, doesn't it? That is all I'm trying to highlight.

sp332• 14 years ago

No. Maybe you're thinking of anonymous functions? A closure binds a function to its scope. https://en.wikipedia.org/wiki/Closure_%28computer_science%29

Arelius4• 14 years ago

No, not really. Closures... close on variables, function pointers, are just pointers to functions.

What you are saying is sort of like saying that Classes and Objects are equivalent, creating a closure creates an instance, wheras a function pointer just references the canonical code.

Also, don't undervalue anonymous local functions, which is another thing that full closures have over C99 functions.

msbarnett• 14 years ago

> At the most basic level, though, the comparison remains valid, doesn't it?

Given that function pointers can't close over variables in the environment, it's a fairly tenuous comparison.

The ability to pass function around is different from the ability of functions to close over their environment. When people talk about closures and their power, they're generally talking about the latter, not the former, at least in my experience.

mhd2• 14 years ago

The fact that it can be used where other languages would use closures doesn't make them equal, and the language here is pretty definite, you're not using "similar", "like" or "can stand in for"… I can do a tree with an array, that doesn't make them equal.

I don't want to sound too harsh, I'm just noting that any blog post that starts out like that is a bit likely to be instantly discarded by some people. And will confuse others later on when they want to learn about closures…

sixbrx3• 14 years ago

The author claims you might know it as a lambda or anonymous function in other languages, but I think function pointers come up short in comparison.

Function pointers can't capture variables from surrounding scopes and so can't depend on runtime values in any easy or safe way.

Just try emulating the following with function pointers:

  def incrementor(n: Int) = (x:Int) => x + n

Sure you could add another input parameter for the C function pointer, but then you couldn't use it anywhere an arity-1 function is required.

jasonlotito3• 14 years ago

sigh

FTA: The point was not to go into the nuances of closures and lambdas, but only to highlight the basic similarity between function pointers / lambdas / anonymous functions.

sixbrx3• 14 years ago

I don't think my comment is sigh worthy really, because the article says you "might know it by the name 'lambda' or ... 'anonymous function'. Now "know it by the name" implies equality/equivalence at least to me (two names for the same referent). I guess it was just a poor choice of words, but that was my point to begin with.

angersock3• 14 years ago

So, with a clever use of a macro (and assuming you use reference-counted smart pointers or have another way of persisting variables at that time on the stack), you can totally make a fire-and-forget closure in C++. Our solution involved declaring inline classes with some minor stringification magic to guarantee uniqueness.

sixbrx3• 14 years ago

Yep, there are some neat ways of emulating the capture, I didn't mean to imply it was impossible - just beyond what workaday programmers like me are willing to do :)

wglb• 14 years ago

If you know Python / Ruby / Lisp, you might know it by the name ‘lambda’, or if you come from a JavaScript background, you might’ve used it under the name ‘anonymous function’.

This is false. Lambdas capture context; pointed-to functions i C do not.

jasonlotito3• 14 years ago

FTA: The point was not to go into the nuances of closures and lambdas, but only to highlight the basic similarity between function pointers / lambdas / anonymous functions.

jacquesm2• 14 years ago

I believe it is closures that capture context, lambdas are just anonymous functions, see:

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

http://en.wikipedia.org/wiki/Closure_%28computer_science%29

A lambda function with all its variables bound is a closure iirc.

jacquesm2• 14 years ago

Function pointers in C are like salt. Use them sparingly and you can greatly improve the readability and size of your code, use too many of them and you'll end up spoiling the broth and it will become a terrible mess.

vickychijwani4• 14 years ago

A lot of people here have pointed out that closures are not the same as function pointers in C, and for that I thank them. I have added that note to the blog post (it should be up on GitHub shortly), but I would also like to say that the reason for introducing function pointers in that way was to establish a sense of familiarity for those coming from Ruby / Python / JavaScript, who are not well-acquainted with their use in C.

smashing• 14 years ago

Clear and easily readable C code is underrated.

Function Pointers in C are Underrated

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