C_Macros

Software-Engineering

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# What are Macros?

A Macro is a code fragment with name (UPPERCASE normally). It is a shorthand to define longer pieces of code.

Usage: A preprocessor replaces name ⇒ code fragment

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# define vs undef

#define - create macro with name (maybe value)

• create code snippets, constants, enable conditional compilation

#undef - remove definition of macro

• reset include guards

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# simple example: .h - impl.c - main.c

Here is an example with a header file and a main program file. In this example, we define a function in the header file - we use this function in the main program.

example.h

#ifndef EXAMPLE_H  // Include guards to prevent multiple inclusion
#define EXAMPLE_H  // macro: creates EXAMPLE_H symbol - to only be used once
 
// Function declaration
int addNumbers(int a, int b);
 
#endif  // EXAMPLE_H

example.c

#include "example.h"
 
// Function definition
int addNumbers(int a, int b) {
    return a + b;
}

main.c

#include <stdio.h>
#include "example.h"  // Include the header file
 
int main() {
    // Using the function from the header file
    int result = addNumbers(5, 3);
 
    // Display the result
    printf("Result: %d\n", result);
 
    return 0;
}

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# Object-like macro

example.h

#ifndef EXAMPLE_H
#define EXAMPLE_H
 
#define MAX 100
 
// function, which uses MAX
int limitValue(int value);
 
#endif  // EXAMPLE_H

example.c

#include "example.h"
 
int limitValue(int value) {
    // MAX-define - ensures that value < 100 (MAX)
    return (value > MAX) ? MAX : value;
}

main.c

#include <stdio.h>
#include "example.h"
 
int main() {
    int x = 120;
 
    // Function, that implements MAX
    int result = limitValue(x);
    // result = 100 - 120 gets limited to 100
 
    printf("Limited value: %d\n", result);
 
    return 0;
}

# Function-like macro

example.h

#ifndef EXAMPLE_H
#define EXAMPLE_H
 
#define INCREMENT(x) ++x
#define SUM(a, b) a + b
 
#endif  // EXAMPLE_H

main.c

#include <stdio.h>
#include "example.h"
 
int main() {
    int x = 120;
 
    // Verwendung des INCREMENT-Defines
    INCREMENT(x);
 
    // Verwendung des SUM-Defines
    int sumResult = SUM(3, 5);
 
    printf("Incremented value: %d\n", x);
    printf("Sum result: %d\n", sumResult);
 
    return 0;
}