How to conditionally determine which functions are called at compile time?

Question

I'm working on implementing a very, very basic component system in C, but now I am at a point where I want to 'dynamically' call some functions. The set-up is very easy: the main program is simply an endless while loop, in which some conditions are checked and in which a "process" function is called for each enabled component.

For example, now it works like this:

while (1) {
  input_process();
  network_process();
  graphics_process();
}

But I want to separate it into separate components, and somehow define in a central place which parts are used. This could be done with simple defines, like so:

#define HAS_NETWORK
...
while (1) {
  input_process();
#ifdef HAS_NETWORK
  network_process();
#endif
  graphics_process();
}

As you can see this is alright for 1 or maybe only a few components, but if I want to do this for all of these (input, network and graphics) and additional components in the future, I would have to put separate #ifdefs in there for each of them, and that's quite tedious.

In pseudo code, what I'm trying to accomplish is the following:

components = {'input', 'network', 'graphics'}
...
foreach component in components
  execute component_process()

This way components could easily be added in the future. I don't really mind if the checking is done compile time or run time (although I obviously prefer compile time, but I can imagine run time is easier to implement). I have no idea how to even start.

Answer 1

You need pointers to functions, create an array of pointers to functions and index it dynamically.

Here link about function pointers.

Answer 2

Compile-time solution: a pre-build step and include directive inside that loop, e.g.

while (1) {
#include "components.generated.c"
}

A basic script to generate that file might look like (Python):

components = ('input', 'networking', 'graphics')
# this could also be e.g. a glob on a directory or a config file

with open('components.generated.c', 'w') as fp:
    for component in components:
        print >>fp, '%s_process();' % component

Any decent build system will allow you to do that.