Let's explore conditional compilation in C with detailed examples, including the use of #ifdef, #ifndef, #else, #endif, and #undef. We'll also incorporate bash scripting and environment variables to demonstrate dynamic control over compilation.

Example: Simple Conditional Compilation

// main.c

#include <stdio.h>

// Uncomment the following line to enable debugging
#define DEBUG

int main() {
#ifdef DEBUG
    printf("Debugging is enabled.\n");
#else
    printf("Debugging is disabled.\n");
#endif

    return 0;
}

In this example, the #ifdef DEBUG checks if the symbol DEBUG is defined. If defined, the code within the #ifdef block is included during compilation; otherwise, the code within the #else block is included. If the DEBUG line is uncommented, the output will be:

Debugging is enabled.

Example: Using Bash and Environment Variables

// main.c

#include <stdio.h>

#ifdef ENABLE_FEATURE
#define FEATURE "Feature is enabled."
#else
#define FEATURE "Feature is disabled."
#endif

int main() {
    printf("%s\n", FEATURE);
    return 0;
}

Bash script (compile_and_run.sh):

#!/bin/bash

# Enable or disable the feature by setting the environment variable
export ENABLE_FEATURE=true

# Compile the C code
gcc main.c -o my_program

# Run the compiled program
./my_program

If you run the bash script with ENABLE_FEATURE=true, the output will be:

Feature is enabled.

If you run the bash script with ENABLE_FEATURE=false, the output will be:

Feature is disabled.

This demonstrates how you can control the compilation behavior using environment variables.

Example: Conditional Compilation with #else and #undef

// main.c

#include <stdio.h>

// Uncomment the following line to enable debugging
#define DEBUG

int main() {
#ifdef DEBUG
    printf("Debugging is enabled.\n");
#else
    printf("Debugging is disabled.\n");
#endif

#ifdef RELEASE
    printf("Release mode is enabled.\n");
#else
    printf("Release mode is disabled.\n");
#endif

// Undefine DEBUG for the rest of the program
#undef DEBUG

#ifdef DEBUG
    printf("This line won't be printed because DEBUG is undefined.\n");
#endif

    return 0;
}

In this example, DEBUG is initially defined, but then it's undefined using #undef DEBUG. After the #undef directive, attempting to use DEBUG will result in a compilation error.

Compilation and Execution

To compile and run these examples, you can use the following bash commands:

# Compile and run Example 1
gcc main.c -o example1
./example1

# Compile and run Example 2
chmod +x compile_and_run.sh
./compile_and_run.sh

# Compile and run Example 3
gcc main.c -o example3
./example3

These examples illustrate how conditional compilation can be utilized in different scenarios, allowing you to control the compilation process based on preprocessor directives and external factors such as environment variables.