Array of macros in c -- is it possible

Question

I was wondering if it is possible to create something like an array of macros. I've implemented the following code which works:

struct led_cmds_
{ 
    ioport_pin_t *commands[LED_COUNT] ; 
};
struct led_cmds_ the_led_cmd_ ; 

void populate() {
    the_led_cmd_.commands[0] = SPECIFICPIN(0);
}

and in main:

int main(void) 
{
    //.....  
    populate();
    LED_On(the_led_cmd_.commands[0]); 
}

SPECIFICPIN(x) is macro defined as:

#define SPECIFICPIN(X) (LED##X##_PIN)

What I was hoping for is a way to is a way to do something like this:

#define ioport_pin_t* ARR_LED[LED_COUNT] \
for (int j = 0; j < LED_COUNT; j++) ARR_LED[j] = SPECIFICPIN(j);

and then only need to call the following when I want to use the specific pin

LED_On(ARR_LED[some_number])

when I try to do that I get an ARR_LED undeclared (first use in this function) error.

When I try to call SPECIFICPIN(x) where x is an int iterator in a for loop for example, I get an error saying something like 'LEDx_PIN' undeclared...

Don't know if you're using the gcc toolchain, but you need to use C99 for avr-gcc (and it's support libraries) anyway - typically combined with `-Os`. Why not stick with inline functions? — Brett Hale, Apr 08 '16 at 07:03
the compiler knows nothing about macros because they're replaced by the *preprocessor* before the compiler is called. Therefore array of macros is impossible — phuclv, Apr 08 '16 at 07:11
When working with hardware registers, why would you want an array of pointers? That's fishy code. — Lundin, Apr 08 '16 at 07:57
Also, there exists no hardware with variable amount of I/O ports. Any integrated circuit must have a fixed amount of pins, you cannot add and remove them on the fly. Simply declare an array corresponding to the worst case, then use as many of the array items as you want. — Lundin, Apr 08 '16 at 08:16

tofro · Accepted Answer · 2016-04-08T07:14:07.783

You need to work on your terminology. An array of macros is not possible. Macros are no data type, but rather pure text replacement before your program is actually compiled.

I guess " populate an array using macros " is what you want to do. But it is not possible to do that in a compile-time loop - What you seem to want to achieve with your ioport_pin_t macro attempt. Macros do not have the capability to expand to more instances of text elements than you have initially given. There is no such feature as looping at compile time through macro expansions and do repetitive expansion of macros.

Your for loop loops at run-time, while the macro is being expanded at compile-time. Once you have made yourself aware what is done by the preprocessor what is done by the compiler, and what is done at run-time by the finished program, you will see that will not work.

Something like

#define P(X) {(LED##X##_PIN)}

ioport_pin_t *commands[LED_COUNT] = {
  P(0), P(1), P(2),......}

#undefine P

Would be the closest thing possible to what you seem to want. Note the main use of the pre-processor is not to save you typing effort - You would be better off using copy & paste in your editor, achieve the same thing and have clearer code.

copy and paste violates the DRY rule, and it's [very easy to make mistake](https://software.intel.com/en-us/blogs/2015/04/22/the-last-line-effect) — phuclv, Apr 08 '16 at 07:19
If copy & paste *clarifies* what your program is doing (repeatedly initializing array members) it doesn't violate any rule IMHO. At least you can immediately see see what is happening, you are not polluting the pre-processors name space with (preferrably) short #defines. And: It is *generally* very easy to make mistakes in C..... — tofro, Apr 08 '16 at 09:26
Thanks for the help. I wound up making a bunch of preprocessor directives like this: #if LED_COUNT == n add to array at position n -1, undefine LED_COUNT, then define it again as n-1 then call #elif LED_COUNT == n-1 and do that down to 0 — swamp_law, Apr 09 '16 at 20:04

score 0 · Answer 2 · answered Apr 08 '16 at 07:24

An array as tofro's answer is the way to go. However in cases that couldn't be solved simply with an array then there's another way with switch

#define SPECIFICPIN(X) (LED##X##_PIN)

void setpin(int pin, int value)
{
    switch (pin)
    {
    case 1:
        SPECIFICPIN(1) = value;
        doSomething(); // if needed
        break;
    case x: ...
    default: ...
    }
}

Array of macros in c -- is it possible

2 Answers2

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