Communicating with the Compiler

Each #pragma directive can vary widely, but most follow a general pattern. The instruction being given is specified by the directive-name. The optional arguments depend on the specific directive and the compiler in use.

header


              1
            
#pragma directive-name [optional arguments]

Include Once

The old way to ensure the header is included only once is using #ifndef / #define guards. While still widely used and fully portable, it requires boilerplate and manual work.

main

config


              12345678
            
#include <stdio.h>
#include "config.h"
#include "config.h" // Reinclusion — NO duplication will occur

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

Ensures that a header file is included in the compilation only once, even if it is included multiple times in different places. It is an alternative (and more modern) approach to the traditional #ifndef pattern.

main

config


              12345678
            
#include <stdio.h>
#include "config.h"
#include "config.h" // Reinclusion — NO duplication will occur

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

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Thanks for your feedback!

Section 4. 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

Communicating with the Compiler

header


              1
            
#pragma directive-name [optional arguments]

Include Once

The old way to ensure the header is included only once is using #ifndef / #define guards. While still widely used and fully portable, it requires boilerplate and manual work.

main

config


              12345678
            
#include <stdio.h>
#include "config.h"
#include "config.h" // Reinclusion — NO duplication will occur

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

main

config


              12345678
            
#include <stdio.h>
#include "config.h"
#include "config.h" // Reinclusion — NO duplication will occur

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

Everything was clear?

Thanks for your feedback!

Section 4. Chapter 1