Printing source of a C program itself

In C programming, a self-printing program (also called a quine) is a program that prints its own source code as output. This can be achieved using the __FILE__ macro along with file I/O operations to read and display the current source file.

Syntax

FILE *fopen(__FILE__, "r");

The __FILE__ macro is a preprocessor macro that returns the name of the current source file as a string literal. This allows us to open and read our own source code at runtime.

Example: Understanding __FILE__ Macro

First, let's see how the __FILE__ macro works −

#include <stdio.h>

int main() {
    printf("Current file: %s<br>", __FILE__);
    return 0;
}

Current file: main.c

Example: Complete Self-Printing Program

Now let's create a program that prints its entire source code −

Note: This example uses file operations and will work when compiled and run locally. Online compilers may have limitations with file access.

#include <stdio.h>

int main(void) {
    char c;
    FILE *file = fopen(__FILE__, "r");
    
    if (file == NULL) {
        printf("Error: Cannot open source file<br>");
        return 1;
    }
    
    while ((c = fgetc(file)) != EOF) {
        putchar(c);
    }
    
    fclose(file);
    return 0;
}

How It Works

• The __FILE__ macro expands to the current source file name at compile time
• fopen(__FILE__, "r") opens the source file in read mode
• fgetc() reads characters one by one from the file
• putchar() prints each character to the console
• The loop continues until EOF (End of File) is reached

Key Points

• Always check if fopen() returns NULL to handle file opening errors
• Use fclose() to properly close the file after reading
• The program reads its own source code character by character and prints it exactly
• This technique works because the executable knows the path to its source file

Conclusion

Self-printing programs in C demonstrate the power of the __FILE__ macro and file I/O operations. While mainly used for educational purposes, they showcase how programs can access and manipulate their own source code at runtime.