Fundamentals of Conditional Compilation

It does not affect the runtime behavior but determines what gets included in the final binary. This is useful for adapting code to different platforms, configurations, or versions. Typical use cases include:

Debug builds: insert logs or debug information only during development.
Platform-specific code: use #ifdef _WIN32 for Windows, __linux__ for Linux, etc.
Feature toggling: enable or disable modules based on configuration settings.

The preprocessor evaluates conditions based on defined macros.Expressions in #if and #elif are evaluated similarly to C expressions but with limited operators (arithmetic, logical, comparison).

main


              12345678910
            
#include <stdio.h>
#define FEATURE_ENABLED

int main() 
{
#ifdef FEATURE_ENABLED
    printf("Feature is active!\n");
#endif
    return 0;
}

This code prints the message only if FEATURE_ENABLED is defined. If not, the line is completely removed before compilation. You can also use multiple conditional branches, which is great for version control and managing different builds.

main


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#include <stdio.h>
// Choose 1, 2 or another value
#define VERSION 2 

int main()
{
    #if VERSION == 1
        printf("Old version\n");
    #elif VERSION == 2
        printf("Current version\n");
    #else
        printf("Unknown version\n");
    #endif
    return 0;
}

This code works like a switch...case or if() statement, but the key difference is that branching happens before runtime. Unlike runtime checks, it lets you include or exclude large chunks of code, making the final program lighter.

You can think of conditional compilation like a set of electrical switches in your code. Each #if, #ifdef, or #ifndef acts like a switch turning code on or off before the compiler even sees it.

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Section 3. Chapter 1

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Course Content

C Preprocessing

1. Introduction to Preprocessing

Introduction to Compilation Stages Role of the Preprocessor Preprocessing Commands

2. Macros

Symbolic Constant Macro With Arguments Advanced Usage of Symbolic Constant Cancel Macro Stringify Operators

3. Conditional compilation

Fundamentals of Conditional Compilation Core Directives Practical Use Cases Pitfalls and Best Practices

4. Compiler Directives and Advanced Control

Communicating with the Compiler Pack GCC Other Built-in Macros Every C Programmer Should Know Precise Control for Debugging and Code Generation

Fundamentals of Conditional Compilation

Debug builds: insert logs or debug information only during development.
Platform-specific code: use #ifdef _WIN32 for Windows, __linux__ for Linux, etc.
Feature toggling: enable or disable modules based on configuration settings.

The preprocessor evaluates conditions based on defined macros.Expressions in #if and #elif are evaluated similarly to C expressions but with limited operators (arithmetic, logical, comparison).

main


              12345678910
            
#include <stdio.h>
#define FEATURE_ENABLED

int main() 
{
#ifdef FEATURE_ENABLED
    printf("Feature is active!\n");
#endif
    return 0;
}

main


              123456789101112131415
            
#include <stdio.h>
// Choose 1, 2 or another value
#define VERSION 2 

int main()
{
    #if VERSION == 1
        printf("Old version\n");
    #elif VERSION == 2
        printf("Current version\n");
    #else
        printf("Unknown version\n");
    #endif
    return 0;
}

Everything was clear?

Thanks for your feedback!

Section 3. Chapter 1