Now you made me curios. How would you go about doing it with functions? Keep in mind that the goal is to interleave conditions and the code in switch-like way.

The goal is to implement the desired functionality in a comprehensible way. Given that, the best option would be to use an array, and to initialize it with the easy array intialization syntax that C provides:

    struct select_condition conds[] = {
        { channel_1, SELECT_OUT, NULL, 42 }, 
        { channel_2, SELECT_IN, &i, 0 }
    };

No need to overcomplicate it or stuff a bunch of functionality into macros in order to make C look like some other language.

Seems like the 'classic' C way to go about it would be to use scanf/printf-style format strings:

    int mode = select("<%i | >%i", channel1, 42, channel2, &i);
    switch(mode) {
    case 0: foo(); break;
    case 1: bar(i); break;
    }

The goal is not to look like golang but to maintain locality: Implementation of one thing should be at one place, not at two different places.