Files

So far you’ve mainly worked with data in memory. But when the program ends, that information disappears.

Files appear to preserve data between executions.

In this lesson you will learn:

• what a file is
• what FILE* is
• how to open, read, write, and close files
• common opening modes
• why it’s always important to verify and close correctly

What is a file?

A file is a way of storing information on disk so it continues to exist after the program ends.

That allows, for example:

• saving results
• loading previously saved data
• maintaining persistent information

The FILE* type

In C, files are handled through the FILE* type.

That type represents an open file on which the program can operate.

Opening a file

fopen is used.

FILE* file = fopen("data.txt", "w");

Common modes

• "r" → open for reading
• "w" → open for writing
• "a" → open for appending at the end

Important: verify if it opened correctly

It’s not enough to call fopen. You need to verify if the result is different from NULL.

if (file != NULL) {
    /* use file */
}

Writing to a file

fprintf(file, "Hello World!\n");

Reading from a file

With fscanf

fscanf(file, "%d", &number);

With fgets

fgets(line, 100, file);

Closing the file

fclose(file);

Closing the file is fundamental.

Why? Because it helps ensure data is saved correctly and frees the resource the program was using.

Complete writing example

#include <stdio.h>

int main() {
    FILE* file = fopen("greeting.txt", "w");

    if (file != NULL) {
        fprintf(file, "Hello World!\n");
        fclose(file);
    }

    return 0;
}

Conceptual reading example

#include <stdio.h>

int main() {
    FILE* file = fopen("data.txt", "r");
    int number;

    if (file != NULL) {
        fscanf(file, "%d", &number);
        printf("Read: %d\n", number);
        fclose(file);
    }

    return 0;
}

Common mistakes when starting

1. Not verifying if fopen returned NULL

That can make the program try to use a file that actually didn’t open.

2. Forgetting fclose

It’s a very bad practice to leave files open.

3. Confusing reading with writing

It’s not the same to open with "r" as with "w".

Summary

• a file allows storing information permanently
• in C it’s handled with FILE*
• fopen opens the file
• fprintf, fscanf, and fgets allow operating on it
• fclose closes the file
• it’s always worth verifying if the file opened correctly

Final idea

Files make a program stop being something purely momentary.

Thanks to them, data can survive execution and software starts behaving in a much more realistic way.