📁 C File Handling – Read, Write, and Manage Files in C Programming

🧲 Introduction – What Is File Handling in C?

In C programming, file handling refers to the process of creating, opening, reading, writing, appending, and closing external files using standard I/O library functions (<stdio.h>). It allows C programs to interact with the filesystem, making them more useful for real-world applications like:

• Saving user data
• Loading configuration files
• Generating reports
• Sharing data with other systems

C supports both text and binary file operations using powerful functions such as fopen(), fclose(), fprintf(), fscanf(), fread(), and fwrite().

🎯 In this guide, you’ll learn:

• How to open, read, and write text and binary files
• File modes and common file I/O functions
• How to handle errors during file operations
• When to use text files vs binary files

📘 Topics Covered

📁 C File I/O Overview

C’s file I/O system relies on the FILE data type defined in <stdio.h>. You must open a file using fopen() before performing I/O operations and close it using fclose() after you’re done.

✅ Common File Opening Modes

FILE *fp = fopen("data.txt", "r");
if (fp == NULL) {
    printf("File could not be opened.\n");
}

✍️ C Write to Files

✅ Writing to a Text File

FILE *fp = fopen("output.txt", "w");
fprintf(fp, "Hello, File Handling!\n");
fputs("Writing with fputs().\n", fp);
fclose(fp);

• fprintf() – formatted output (like printf(), but to a file)
• fputs() – writes a string to the file

✅ Writing to a Binary File

FILE *fp = fopen("data.bin", "wb");
int nums[] = {1, 2, 3};
fwrite(nums, sizeof(int), 3, fp);
fclose(fp);

• fwrite() – writes raw data to a binary file

📖 C Read from Files

✅ Reading from a Text File

FILE *fp = fopen("output.txt", "r");
char line[100];
fgets(line, 100, fp);
printf("%s", line);
fclose(fp);

• fscanf() – formatted input from file
• fgets() – reads a full line from file

✅ Reading from a Binary File

FILE *fp = fopen("data.bin", "rb");
int nums[3];
fread(nums, sizeof(int), 3, fp);
fclose(fp);

• fread() – reads blocks of binary data

❗ C File Error Handling

Always verify if file operations succeed. A failed fopen() returns NULL.

FILE *fp = fopen("nonexistent.txt", "r");
if (fp == NULL) {
    perror("Error opening file");
}

✅ Error Handling Functions

📂 C Text vs Binary Files

📌 Text files are ideal for human-readable logs or configurations.
📌 Binary files are efficient for speed and storage, great for structured data like images, sound, or compiled programs.

📌 Summary – Recap & Next Steps

C file handling gives you control over external data persistence, allowing you to build programs that log, read, and transfer information effectively.

🔍 Key Takeaways:

• Use FILE * and fopen() for file access
• Use fprintf() / fputs() / fwrite() to write to files
• Use fscanf() / fgets() / fread() to read from files
• Always check for errors using ferror() or perror()
• Choose text or binary files based on readability vs performance

⚙️ Real-World Relevance:
Used in system logging, file export/import, configuration storage, database backups, save files in games, and binary serialization.

❓ Frequently Asked Questions (FAQ)

❓ How do I create a file in C?

✅ Use fopen("filename.txt", "w"). If it doesn’t exist, C will create it.

❓ What’s the difference between fprintf and fwrite?

✅ fprintf() writes formatted text (like printf()), while fwrite() writes raw binary data.

❓ How can I check if a file opened successfully?

✅ If fopen() returns NULL, the file didn’t open. Use perror() to see the error description.

❓ What happens if I open a file in "w" mode?

✅ It will overwrite the file if it exists or create a new one if it doesn’t.

❓ Can I read and write to the same file?

✅ Yes. Use "r+", "w+", or "a+" mode to enable both reading and writing.