C File I/O Tutorial | fopen, fprintf, fscanf Explained

📖 What you'll learn on this page

✅ Must-know essentials

fopen(path, "r") / "w" / "a"
Always fclose
Check fp == NULL on failure

⭐ Read if you have time

Binary mode "rb" "wb"
fprintf, fscanf, fgets
File position with fseek, ftell

The File I/O Flow

When a program exits, the values it computed are normally lost. Writing them to a file makes the data persistent.

File I/O follows three steps: (1) open (fopen) → (2) read or write (fprintf/fscanf) → (3) close (fclose).

📂

(1) fopen

Open the file

→

✏️

(2) Read/Write

fprintf / fscanf

→

🔒

(3) fclose

Close the file

    FILE *fp = fopen("data.txt", "w"); // "w" = write

    if(fp == NULL){

      printf("Could not open file\n");

      return 1;

    }

    fprintf(fp, "Hello File!\n");    // write to the file

    fclose(fp);                      // always close

Always check fopen's return value against NULL. If the file can't be opened (bad path, missing permissions), fopen returns NULL. Skip the check and your program will crash.

Writing to a File (fprintf)

fprintf is nearly identical to printf — it just takes a FILE pointer as its first argument.

    FILE *fp = fopen("scores.txt", "w");

    int scores[3] = {85, 92, 78};

    for(int i = 0; i < 3; i++){

      fprintf(fp, "Student%d: %d\n", i+1, scores[i]);

    }

    fclose(fp);

Mode "w" (write)

Creates a new file or overwrites an existing one. Any prior content is erased.

Mode "a" (append)

Appends new content at the end of an existing file.

Mode "r" (read)

Read-only. Returns NULL if the file doesn't exist.

Reading from a File (fscanf)

fscanf is the file counterpart to scanf. Check its return value to see whether the read succeeded.

    FILE *fp = fopen("numbers.txt", "r");

    if(fp == NULL) return 1;

    int val;

    while(fscanf(fp, "%d", &val) == 1){

      printf("Read: %d\n", val);

    }

    fclose(fp);

fscanf's return value is the number of items successfully read. It returns EOF at end of file, so while(fscanf(...) == 1) reads all the data.

Reading Line by Line (fgets)

fgets reads an entire line as a string. It handles lines with whitespace safely, so it's often preferred over fscanf in practice.

    FILE *fp = fopen("memo.txt", "r");

    if(fp == NULL) return 1;

    char buf[256];

    int lineNo = 1;

    while(fgets(buf, sizeof(buf), fp) != NULL){

      printf("%d: %s", lineNo, buf);

      lineNo++;

    }

    fclose(fp);

fgets is safe: it takes the buffer size (via sizeof(buf)), which prevents buffer overflows — a far safer choice than scanf("%s").

📁 Virtual File System — See file operations in action

Watch what fopen, fgets, fputs, fseek, and fclose actually do, step by step. See the file pointer position, buffer contents, and how each mode ("r", "w", "a") behaves.

🧪 Pick a scenario

Pick a scenario

        // No scenario selected
      

📄 data.txt contents

mode: - FILE* fp: NULL pos (ftell): -

buffer (buf): -

Operation log

(Pick a scenario and press ▶)

💡 Watch for: The yellow cursor (|) shows the current file pointer position. "w" empties the file on open, "a" always appends at the end, and fseek jumps to any offset.

Try It Yourself — File I/O

This example simulates writing to and reading from a file, all in your browser.

fileio.c

Output

Click Run...

💡 More ideas to try

Compare "w" (truncate) with "a" (append)
Use fprintf for formatted output
Read numbers with fscanf
Read a multi-line file line by line with fgets

Review Quiz

Check your understanding of this lesson.

Q1. What does the "r" argument to fopen mean?

Read-only

Write-only

Append mode

"r" is read mode. "w" is write and "a" is append.

Q2. What must you always do at the end of file operations?

free()

fclose()

return 0;

An opened file must be closed with fclose(). Otherwise, buffered data may never be flushed to disk.

Q3. What does fopen return when it fails?

-1

NULL

fopen returns NULL when it can't open the file. Always check the return value and handle the error.

The File I/O Flow

Writing to a File (fprintf)

Reading from a File (fscanf)

Reading Line by Line (fgets)

📁 Virtual File System — See file operations in action

Try It Yourself — File I/O

Related lessons

Review Quiz

Q1. What does the "r" argument to fopen mean?

Q2. What must you always do at the end of file operations?

Q3. What does fopen return when it fails?