Opening and Closing Files

Big idea

Variables vanish when a program ends. Files keep data on disk. You open a file with fopen, use it, and close it with fclose.

The idea

FILE *f = fopen("notes.txt", "w");
if (f == NULL) {
    printf("Couldn't open the file\n");
    return 1;
}
// … use the file …
fclose(f);

• FILE * — a pointer to a FILE, C's handle for an open file. You never look inside it; you pass it to file functions.
• "notes.txt" — the file's name (a path).
• "w" — the mode: what you want to do with it.
• fopen returns NULL if it fails — the file doesn't exist, the folder isn't writable, and so on. Always check.
• fclose(f) — finishes any pending writes and releases the file.

Modes

Careful with "w" — opening an existing file for writing wipes its contents immediately.

Try it

This writes a file, then opens it again and reads it back. The file lives in a scratch folder that's cleaned up after the run.

#include <stdio.h>

int main(void) {
    FILE *out = fopen("greeting.txt", "w");
    if (out == NULL) {
        printf("Couldn't create the file\n");
        return 1;
    }
    fprintf(out, "Hello from a file!\n");
    fclose(out);

    FILE *in = fopen("greeting.txt", "r");
    if (in == NULL) {
        printf("Couldn't open the file\n");
        return 1;
    }
    char line[100];
    fgets(line, sizeof line, in);
    printf("Read back: %s", line);
    fclose(in);
    return 0;
}

output
Read back: Hello from a file!

Watch it

What NULL looks like

#include <stdio.h>

int main(void) {
    FILE *f = fopen("does-not-exist.txt", "r");
    if (f == NULL) {
        printf("no file\n");
    } else {
        printf("opened\n");
        fclose(f);
    }
    return 0;
}

no file

perror("fopen") prints a message explaining why it failed — handy while debugging.

Common mistake

Using the file without checking for NULL. Writing to a NULL FILE * crashes the program. The check is two lines; always write it.

Think of it like

A library book: you check it out (fopen), read or annotate it, then return it (fclose). And "w" mode is borrowing a notebook — someone rubs out every page before handing it to you.

🕷️ Spidey-sense: My photo archive for the Bugle. Open the folder, drop in the shots, close it — and never open last month's folder in "w" mode. Jameson would not accept "I erased the evidence."

Key terms

• File — Named data stored on disk.
• FILE * — A handle to an open file.
• fopen / fclose — Open and close a file.
• Mode — What you'll do with the file: "r", "w", "a" (and "r+" etc. for both).