What is the complete usage of predefined macros and conditional compilation in C language?

What is the complete usage of predefined macros and conditional compilation in C language?

Predefined Macros:

1. Standard Predefined Macros

   c
   void print_predefined_macros() {
       printf("File: %s\n", __FILE__);
       printf("Line: %d\n", __LINE__);
       printf("Date: %s\n", __DATE__);
       printf("Time: %s\n", __TIME__);
       printf("Function: %s\n", __func__);
       #ifdef __STDC_VERSION__
           printf("C Standard: %ld\n", __STDC_VERSION__);
       #endif
   }

2. Compiler-Specific Macros

   c
   void compiler_specific_code() {
       #ifdef __GNUC__
           printf("GCC compiler\n");
       #elif defined(__clang__)
           printf("Clang compiler\n");
       #elif defined(_MSC_VER)
           printf("MSVC compiler\n");
       #endif
   }

3. Platform Detection

   c
   void platform_specific_code() {
       #ifdef _WIN32
           printf("Windows platform\n");
       #elif defined(__linux__)
           printf("Linux platform\n");
       #elif defined(__APPLE__)
           printf("macOS platform\n");
       #endif
   }

Conditional Compilation:

1. Basic Conditional Compilation

   c
   #ifdef DEBUG
       #define LOG(x) printf x
   #else
       #define LOG(x)
   #endif
   
   void function() {
       LOG(("Debug message\n"));
   }

2. Multiple Conditions

   c
   #if defined(UNIX) && !defined(DEBUG)
       #define OPTIMIZED_CODE
   #endif
   
   #if VERSION >= 2
       #include "new_features.h"
   #else
       #include "legacy_features.h"
   #endif

3. Errors and Warnings

   c
   #if SIZE < 0
       #error "Size must be positive"
   #endif
   
   #if !defined(VERSION)
       #warning "VERSION not defined, using default"
       #define VERSION "1.0"
   #endif

Advanced Usage:

1. Macro Stringification

   c
   #define STRINGIFY(x) #x
   #define TOSTRING(x) STRINGIFY(x)
   
   printf("Line: %s\n", TOSTRING(__LINE__));

2. Macro Concatenation

   c
   #define CONCAT(a, b) a##b
   int CONCAT(var, 1) = 10;  // var1

3. Variadic Macros

   c
   #define LOG(fmt, ...) printf("[LOG] " fmt "\n", ##__VA_ARGS__)
   
   LOG("Value: %d", 42);
   LOG("Simple message");

Practical Applications:

1. Debug Macros

   c
   #ifdef DEBUG
       #define DEBUG_PRINT(fmt, ...) \
           printf("[DEBUG %s:%d] " fmt "\n", __FILE__, __LINE__, ##__VA_ARGS__)
   #else
       #define DEBUG_PRINT(fmt, ...)
   #endif
   
   void example_function() {
       DEBUG_PRINT("Processing data: %d", 100);
   }

2. Version Control

   c
   #define VERSION_MAJOR 2
   #define VERSION_MINOR 5
   #define VERSION_PATCH 1
   
   #define VERSION_STRING \
           TOSTRING(VERSION_MAJOR) "." \
           TOSTRING(VERSION_MINOR) "." \
           TOSTRING(VERSION_PATCH)
   
   printf("Version: %s\n", VERSION_STRING);

3. Cross-Platform Code

   c
   #ifdef _WIN32
       #define PATH_SEPARATOR '\\'
       #define PATH_SEPARATOR_STR "\\"
   #else
       #define PATH_SEPARATOR '/'
       #define PATH_SEPARATOR_STR "/"
   #endif
   
   void join_path(char *dest, const char *dir, const char *file) {
       sprintf(dest, "%s%s%s", dir, PATH_SEPARATOR_STR, file);
   }

4. Feature Toggles

   c
   #define FEATURE_A_ENABLED 1
   #define FEATURE_B_ENABLED 0
   
   void process_data() {
       #if FEATURE_A_ENABLED
           feature_a_process();
       #endif
       
       #if FEATURE_B_ENABLED
           feature_b_process();
       #endif
   }

Best Practices:

1. Header Guards

   c
   #ifndef HEADER_H
   #define HEADER_H
   
   // Header content
   
   #endif

2. Include Once

   c
   #pragma once
   
   // Header content

3. Macro Scope Control

   c
   #undef TEMP_MACRO
   #define TEMP_MACRO(x) (x * 2)
   
   int result = TEMP_MACRO(5);
   #undef TEMP_MACRO

Important Considerations:

1. Macro Side Effects

   c
   #define SQUARE(x) ((x) * (x))
   int i = 2;
   int result = SQUARE(i++);  // Undefined behavior

2. Macro Parentheses

   c
   #define MUL(a, b) ((a) * (b))
   int result = MUL(2 + 3, 4);  // Correct: (2 + 3) * 4 = 20

3. Nested Conditional Compilation

   c
   #ifdef DEBUG
       #ifdef VERBOSE
           #define LOG(x) printf x
       #else
           #define LOG(x)
       #endif
   #else
       #define LOG(x)
   #endif