File Handling in C Programming

In C programming, file handling is essential for performing input and output (I/O) operations involving data storage and retrieval from external files. Unlike standard input/output functions (scanf, printf), which work with the console, file handling allows programs to read from and write to files on disk.

1. Why Use File Handling?

• To store data permanently
• To process large data not manageable through console I/O
• To transfer data between programs
• To log program events and results

2. Basic File Operations in C

The following are the most common operations performed when working with files in C:

Opening a File

Files are opened using the fopen() function. It requires the filename and mode (such as read, write, append, etc.).

Modes include:

• "r" – open for reading
• "w" – open for writing (creates new file or truncates)
• "a" – open for appending
• "rb", "wb", "ab" – same as above, but for binary files

Reading from a File

To read data from a file, use functions like fscanf(), fgets(), or fread().

Writing to a File

To write data to a file, use fprintf(), fputs(), or fwrite().

Closing a File

Always close files using the fclose() function to ensure that all buffers are flushed and system resources are released.

3. File Pointers

All file operations use a file pointer of type FILE *. This pointer acts as a handler to interact with the file represented as FILE *fp;

4. Text Files vs. Binary Files

Text Files:

• Store data in readable ASCII characters.
• Use fprintf() and fscanf() for reading/writing.
• Suitable for logs, config files, etc.

Binary Files:

• Store data in binary (machine-readable) format.
• Use fwrite() and fread() functions.
• More compact and efficient for complex data types (like structures).

5. File Input/Output Functions

fprintf() and fscanf()

Used for formatted text I/O to/from a file.

• fprintf(FILE *fp, format, variables...) – writes formatted output to a file
• fscanf(FILE *fp, format, variables...) – reads formatted input from a file

fread() and fwrite()

Used for binary data input/output.

• fread(void *ptr, size_t size, size_t count, FILE *fp) – reads binary data into memory
• fwrite(const void *ptr, size_t size, size_t count, FILE *fp) – writes binary data from memory to file

These functions are more suitable for writing structures or large blocks of memory.

6. Common File Handling Functions in C

7. Error Handling in File Operations

File operations can fail due to various reasons, such as:

• File does not exist
• File cannot be accessed (permissions)
• Disk errors

Common techniques for error handling:

• Check if fopen() returns NULL:

  if (fp == NULL) {
      // handle error
  }
  

• Use perror() to print a descriptive error message.

• Use feof(fp) to check for end-of-file.

• Use ferror(fp) to check if a file error occurred during an operation.

These checks ensure that your program can respond to file-related problems gracefully, instead of crashing or producing incorrect output.

8. Best Practices in File Handling

• Always check if the file opened successfully.
• Always close the file using fclose() after the operation is complete.
• Use binary mode when working with raw memory data (e.g., structures).
• Flush output buffers using fflush() if necessary.
• Avoid hardcoding file names and use relative paths if possible.

9. Summary

File handling in C is a vital skill for building programs that interact with external data. By understanding how to open, read, write, and close both text and binary files, and how to handle errors effectively, you can build robust, data-driven applications. Whether you're logging program output or reading configuration files, these file operations form the backbone of persistent data management in C.