Basics Of Function Pointers In C

Good examples and great 'blow by blow' breakdown. This is a really tough concept to learn, and the C syntax gets a little complicated. This was well laid out, readable, and had a good progression of concepts. Well done!

Now it just needs the matching tutorial on initializing pointers in the data section :-)

The article is missing at least one useful thing: how to declare a typedef for a function pointer. This can be used both to avoid error-prone duplication of declarations and to simplify excessively complex declarations.

Here's a simple example:

http://en.wikipedia.org/wiki/Typedef#Using_typedef_with_func...

Here's a more complex example:

http://www.devx.com/tips/Tip/13829

Here's a tutorial on how to interpret the complex declarations like the one at the top of the previous example:

http://www.codeproject.com/Articles/7042/How-to-interpret-co...

Here's a discussion about using typedefs for function pointers on StackOverflow:

http://stackoverflow.com/questions/1591361/understanding-typ...

See also: The Linux command "cdecl":

http://linux.die.net/man/1/cdecl

Plus, there's an on-line version:

http://www.lemoda.net/c/cdecl

Let's look at the signature of qsort() from libc:

   void qsort(void *base, size_t nmemb, size_t size,
           int (*compar)(const void *, const void *));

qsort is a function inside of libc.so. It's already been compiled. It doesn't know about the "compar" pointer you're going to pass it. That function might not even exist yet.

qsort doesn't know if you're sorting integers, sorting strings, sorting struct foo which is still being ironed out. It knows how to sort arrays. You give it the size of each element, the number of elements, and a function pointer that it will call to compare elements. It will call you back and you can stick your data-structure-specific knowledge into that function.

(This turns out to be a lousy way to sort arrays, by the way. If you used a C++ template instead, the comparison function could be inlined directly in the sort algorithm, which gives you better object code. But you lose the possibility that the caller and callee exist in different modules, for example.)