Is there a dummy lvalue that can be used when nothing is to be executed?

Question

Consider the following C99 function:

void port_pin_set(const bool value, const uint8_t pin_mask)
{
    if (value) {
        PORT |= pin_mask;
    } else {
        PORT &= ~pin_mask;
    }
}

With PORT being a define, for example:

#define PORT (P1OUT)

Is there a way to redefine PORT so that:

• it is accepted as an lvalue,
• the function does not do anything.

What I want to do is to keep the function source as is, while compiling it to do nothing.

Edit: I am aware that using such an lvalue might not be the best solution. I am not looking for the best solution for this specific problem, I am interested in the language itself. This is a theoretical question, not a pragmatic one.

Answer 1

C99 has compound literals that can be used for such a task. Their syntax is a cast followed by an initializer:

#define PORT ((int){0})

should do it in your case, only that you might get some warnings about unused values or so.

Any decent compiler will optimize assignments to such compound literal out.

Answer 2

You can define a variable of the same type as your P1OUT (say, unsigned char), make it available in the header, and define it in one of the sources, like this:

Header:

extern unsigned char dummy_P1OUT;
#define PORT (dummy_P1OUT)

C file:

unsigned char dummy_P1OUT;

Usages:

PORT &= ~pin_mask;

if (PORT & pin_mask) {
    blink_led();
}

Answer 3

This should do it in C99:

#ifdef PORT
#undef PORT
#endif

#define PORT int x = 0; x

for your case. But I suggest you put the whole code between #ifdef's instead of redefining PORT:

void port_pin_set(const bool value, const uint8_t pin_mask)
{
#ifdef BUILD_PORT_PIN_SET
    if (value) {
        PORT |= pin_mask;
    } else {
        PORT &= ~pin_mask;
    }
#endif
}