Preprocessor Definitions

• Preprocessor definitions are a tool in C. It is important to know how and when to use them, as well as the implications they have. Note that C++ has many tools such as inline functions that may be better suited compared to a preprocessor definition such as a macro.

Macros

• A macro is implemented by using #define. Put simply, a macro will quite literally replace any text of your choosing with another piece of text. Consider the following code snippet:

  #define TWO (2) #define ARRAY_SIZE(a) (sizeof(a)/sizeof(*a)) //stuff in between static uint16_t test_array[] = {1,2,3}; int main() { uint8_t something = TWO + TWO; uint8_t array_size = ARRAY_SIZE(test_array); }

  will evaluate to

  #define TWO (2) #define ARRAY_SIZE(a) (sizeof(a)/sizeof(*a)) //stuff in between static uint16_t test_array[] = {1,2,3}; int main() { uint8_t something = (2) + (2); uint8_t array_size = (sizeof(test_array)/(sizeof(*test_array)); }

  It simply replaces everything that was defined with your new term (i.e. TWO quite literally evaluated to (2)).

• A macro can be quite useful in the case of ARRAY_SIZE, where you do not have to know what type of pointer something is to use it.

• IMPLEMENTATION NOTES

  • Always put brackets around macros, as not doing so can result in weird things happening if the user forgets a bracket etc.

  • The convention is to make macros in ALL CAPS.

• However, using a macro may have significant drawbacks

  • Nothing you type in a macro is ‘safe’, and everything is directly swapped out for what you defined. A simple typo can cause an error.

  • You also cannot step through a macro with a debugger.

    • Further reading on a debugger: The Debugger

Other preprocessor definitions

• #ifdef and #ifndef are quite popular, you can use this to toggle features or do whatever really. Consider the following code snippet

  //features.h #define MY_FEATURE //main.c int main() { #ifdef MY_FEATURE //Do something #else //Do something else #endif }

  • Beware for those who intend to do work in the automotive industry, I believe MISRA C does not permit this design pattern in released code. Still really good for debugging etc, since you can just do #define DEBUG and #ifdef DEBUG to make your code much cleaner instead of removing and adding comments

• #error is good to provide compile-time errors. Further reading on #error can be found here: https://www.techonthenet.com/c_language/directives/error.php