Thread: Cannot understand this C preprocessor function

There's this trick for "commenting out" large swathes of code when testing something:

Code:

#if 0
some code...
#endif

This NRFX_CHECK function-style macro does the same thing. If NRFX_SAADC_ENABLED is 0, then NRFX_CHECK(NRFX_SAADC_ENABLED) resolves to (0), so you're "commenting out" the code. If NRFX_SAADC_ENABLED is 1, then NRFX_CHECK(NRFX_SAADC_ENABLED) resolves to (1), so you end up with #if (1) so the code is not "commented out".

No, that's just a function-style macro. The "if statement" is the #if macro, but of course it is a preprocessor directive, not an if statement.

The NRFX_CHECK macro might be for flexibility so that in case you want to enable/disable regardless of the value of NRFX_SAADC_ENABLED (and possibly other macros), you can do so in just one place.

Recall that macro substitution is a fairly simple replacement mechanism. So if you have a function-style macro like this:

Code:

#define ADD(x, y) ((x) + (y))

and you use it like this:

Code:

printf("%d\n, ADD(1, 2));

what happens is that after preprocessing, it is as if you had written:

Code:

printf("%d\n, ((1) + (2)));

The "extra" parentheses are there because you might write something like this:

Code:

printf("%d\n, ADD(1, 2 + 3) * 4);

which becomes:

Code:

printf("%d\n, ((1) + (2 + 3)) * 4);

whereas if you had omitted the parentheses it would become:

Code:

printf("%d\n, 1 + 2 + 3 * 4);

which is probably not what you wanted due to how precedence comes into play.

So if we go back to NRFX_CHECK, you can see that it is exceedingly simple (and if you don't care about the flexibility or whatever else the author had in mind, you arguably don't need it). NRFX_CHECK(0) resolves to (0) because for this particular macro substition, 0 is module_enabled, and the rule says that the macro is replaced by (module_enabled), i.e., it becomes (0). That's it. Don't overthink.

If you really really cannot understand, then consider this function:

Code:

int foo(int module_enabled)
{
    return module_enabled;
}

We now call this function:

Code:

printf("%d\n", foo(0));

What is the output, and why? The function-style macro works along similiar lines, though you must keep in mind that it is a macro, not a function.