Preprocessor Macros & Conditional Compilation

#include 

/* Object-like macro: named constant */
#define CAN_MAX_DLC      8u
#define VREF_MV          3300u
#define ADC_RESOLUTION   4096u

/* Function-like macro: inline code substitution */
/* Problem: no type checking; side effects if argument has side effects */
#define MAX(a, b)        ((a) > (b) ? (a) : (b))

/* Bug: MAX(i++, j) expands to ((i++) > (j) ? (i++) : (j)) — i incremented twice! */
/* Fix: use inline function instead (preferred per MISRA Dir 4.9 Advisory) */

/* Correct pattern: use parentheses around every argument and the whole expression */
#define BITS_TO_BYTES(n)  (((n) + 7u) / 8u)

/* Register access macros (acceptable: hardware-specific, no side-effect risk) */
#define REG_SET_BIT(reg, bit)    ((reg) |=  (1u << (bit)))
#define REG_CLR_BIT(reg, bit)    ((reg) &= ~(1u << (bit)))
#define REG_TST_BIT(reg, bit)    (((reg) >> (bit)) & 1u)

/* Stringification and token pasting */
#define ASSERT_MSG(cond, msg)    _Static_assert((cond), msg)
#define CONCAT(a, b)             a##b

/* Safer: inline function for MAX */
static inline uint32_t u32_max(uint32_t a, uint32_t b) {
    return (a > b) ? a : b;
}

/* Variant management: compile different code per hardware/feature variant */

/* Feature flags defined in variant-specific header or build system */
/* In CMakeLists.txt: add_compile_definitions(ECU_VARIANT_GATEWAY=1)    */

#ifndef ECU_VARIANT_GATEWAY
  #define ECU_VARIANT_GATEWAY   0
#endif
#ifndef ECU_VARIANT_MOTOR       
  #define ECU_VARIANT_MOTOR     0
#endif
#ifndef FEATURE_DOIP_ENABLED
  #define FEATURE_DOIP_ENABLED  0
#endif

/* Use #if (not #ifdef) so missing definition is caught as error */
#if (ECU_VARIANT_GATEWAY == 1)
  #include "Gateway_Config.h"
  #define CAN_CHANNEL_COUNT    4u
#elif (ECU_VARIANT_MOTOR == 1)
  #include "Motor_Config.h"
  #define CAN_CHANNEL_COUNT    2u
#else
  #error "No ECU variant defined — set ECU_VARIANT_GATEWAY or ECU_VARIANT_MOTOR"
#endif

/* Debug output: compiled out in RELEASE build */
#if defined(BUILD_DEBUG) && (BUILD_DEBUG == 1)
  #define DBG_PRINT(fmt, ...)  printf("[DBG] " fmt, ##__VA_ARGS__)
#else
  #define DBG_PRINT(fmt, ...)  /* empty — zero overhead in release */
#endif

/* Compile-time assertion for configuration validation */
_Static_assert(CAN_CHANNEL_COUNT >= 1u, "Must have at least 1 CAN channel");
_Static_assert(CAN_CHANNEL_COUNT <= 8u, "CAN_CHANNEL_COUNT exceeds hardware limit");