在函数中定义/使用宏 - 好的风格/做法？

Question

I folks, I have a question for the C/C++ Syntax / Coding-Style Gurus out there:

The situation: I am working on an STM32 Microcontroller and in some functions I have to do a lot of bit shifting, logical operations (and, or, xor) on bits, set/clear bits and writing / reading this code is really painfull.

An example: lets asume I had to turn an device on with a digital Output, and I have 5 sensors to check, where each of the sensors has its own dedicated digital Input. Some of them need to be TRUE, others need to be FALSE

A (Pseudo-)Code would eventually look like this: Note: this code is obviously nonsense/nothing really functional, it's just for explaining the principle.

#define SENSOR1 0
#define SENSOR2 1
#define SENSOR3 2
#define SENSOR4 3
#define SENSOR5 4

void DeviceOn(void) {
   uint8_t DIOPort = ReadDIOPort();
   if((DIOPort & (1 << SENSOR1)) && (!(DIOPort & (1 << SENSOR2))) && (DIOPort & (1 << SENSOR3))) {
      turnOnDevice();
   } else {
      turnOffDevice();
   }
}

As you see, the if-condition becomes pretty ugly and if there are more signals involved it becomes more and more unreadable.

My idea was to define macros inside the function, only used by / within that function, to make the code more readable. This would look like this.

void DeviceOn(void) {
       #define __POWER_ISOK (DIOPort & (1 << SENSOR1))
       #define __SAFETY_ISOK (!(DIOPort & (1 << SENSOR2)))
       #define __LIGHT_ISON (DIOPort & (1 << SENSOR3))
       #define __COFFEEMUG_ISFULL (DIOPort & (1 << SENSOR4))
       uint8_t DIOPort = ReadDIOPort();
       if(__POWER_ISOK && __SAFETY_ISOK && __LIGHT_ISON && __COFFEEMUG_ISFULL) {
         turnOnDevice();
       } else {
         turnOffDevice();
       }
    }

IMHO the if-condition is much more readable as the first example.

The question is: is this considered to be "good coding style" or is that an absoulte no-go and other developers will start to avoid me and consider me to be the worst coder in the whole universe?

I want the macros to be defined within the function, because they are ONLY used in this one function, and it makes documentation easier: the one who has to read this code just has to scroll up to the start of the function and does not have to search in the C-File or even the Header-File for the definition. The Definitions of the bits in the Digital Input Status Variable are defined gobally (at the beginning of the C-File) as they are used in other functions, too.

Of course I'd define any macros gobally, that are used in multiple functions. Of course I could also define all macros gobally, but I think that it makes the code more readable, if those who are used only in one function, are defined in that function.

Technically I don't see any issues, as the preprocessor should replace the macros in the build process, or am I wrong?

Answer 1

Marcos don't have scopes, and their definition won't end at the end of the function.

Defining macros inside of functions such that they remain defined after the function is a bad practice because programmers expect definitions to end when the scope ends. You could undefine the macros at the end of the function. That is a good practice when the macros is needed in the first place which isn't often.

Unnecessary use of macros is a bad practice. In this case, it is a better practice to use constant variables instead. Variables have a type system, and scope, both of which make it easier to write a correct program.

PS Names that contain two consecutive underscores are reserved to the language implementation. Don't define them yourself.

Answer 2

Alternate solution:

#include <stdbool.h>

void DeviceOn(void)
{
    uint8_t DIOPort = ReadDIOPort();

    bool PowerIsOK       =    DIOPort & 1 << SENSOR1;
    bool SafetyIsOK      = ! (DIOPort & 1 << SENSOR2);
    bool LightIsOn       =    DIOPort & 1 << SENSOR3;
    bool CoffeeMugIsFull =    DIOPort & 1 << SENSOR4;

    if (PowerISOK && SafetyIsOK && LightIsOn && CoffeeMugIsFull)
        turnOnDevice();
    else
        turnOffDevice();
}