I/O operations on files in C

I/O operations on files in C are fundamental for interacting with files, allowing programs to read data from and write data to external files on the computer’s storage system. These operations are crucial for various applications, including data processing, file management, and communication with external devices. In this overview, we’ll delve into the basic concepts, functions, and practices involved in performing I/O operations on files in the C programming language.

I/O Operations on files in C

Opening a File

The first step in performing file I/O operations is to open a file. The fopen() function is used for this purpose and takes two arguments: the filename and the mode. The mode parameter specifies the type of operation to be performed on the file, such as reading, writing, or appending.

FILE *file_pointer;
file_pointer = fopen("example.txt", "r"); // Open file for reading
if (file_pointer == NULL) {
// Handle error if file opening fails

In the above code snippet, file_pointer is a pointer to a FILE structure, which represents the opened file. If the fopen() function fails to open the file (e.g., due to non-existent file or insufficient permissions), it returns NULL, indicating an error.

Writing to a File

To write data to a file, the fprintf() function is commonly used. It works similarly to printf(), but instead of printing to the console, it writes formatted data to the specified file.

fprintf(file_pointer, "Hello, world!\n");

The above statement writes the string “Hello, world!” followed by a newline character to the file associated with file_pointer.

Reading from a File

Reading data from a file is typically done using functions like fgets() or fscanf(). These functions retrieve data from the file and store it in variables or buffers.

char data[100];
fgets(data, 100, file_pointer);

In this example, fgets() reads up to 99 characters from the file associated with file_pointer and stores them in the character array data. It stops reading either when it encounters a newline character, the specified number of characters have been read, or when the end of the file is reached.

Closing a File

After performing file operations, it’s essential to close the file using the fclose() function. This ensures that any resources associated with the file are released and any buffered data is written to the file.


Failure to close files can lead to resource leaks and potential data loss. It’s good practice to close files immediately after they are no longer needed.

Error Handling

Error handling is crucial when working with files. As mentioned earlier, functions like fopen() return NULL if an error occurs during file opening. It’s essential to check for such errors and handle them appropriately.

if (file_pointer == NULL) {
// Handle file opening error

Depending on the application’s requirements, error handling might involve displaying an error message, terminating the program, or taking alternative actions to recover from the error.

File Modes

The mode parameter passed to fopen() determines the type of operations permitted on the file. Common file modes include:

  • “r”: Open file for reading. The file must exist.
  • “w”: Open file for writing. If the file exists, its contents are truncated. If the file does not exist, it is created.
  • “a”: Open file for appending. Data is written to the end of the file. If the file does not exist, it is created.
  • “r+”: Open file for reading and writing. The file must exist.
  • “w+”: Open file for reading and writing. If the file exists, its contents are truncated. If the file does not exist, it is created.
  • “a+”: Open file for reading and appending. Data is written to the end of the file. If the file does not exist, it is created.

Binary File I/O

In addition to text files, C also supports binary file I/O operations. Binary file I/O is used when dealing with non-text files, such as images, audio, or executable files. When working with binary files, the mode parameter in fopen() is typically prefixed with “b”, indicating binary mode.

file_pointer = fopen("binary.dat", "rb");

Binary file I/O functions, such as fread() and fwrite(), are used to read from and write to binary files. These functions operate on blocks of data instead of characters.

int data[10];
fread(data, sizeof(int), 10, file_pointer);

In this example, fread() reads an array of integers from the binary file associated with file_pointer.

File Positioning

The file position indicator keeps track of the current position within the file. Functions like fseek() and ftell() are used to manipulate and query the file position indicator.

  • fseek(): Sets the file position indicator to a specified location within the file.
  • ftell(): Returns the current value of the file position indicator.
fseek(file_pointer, 0, SEEK_SET); // Move to the beginning of the file

In the above example, fseek() is used to move the file position indicator to the beginning of the file (SEEK_SET).

File Handling Best Practices

When working with files in C, it’s important to follow best practices to ensure code reliability, performance, and security:

  1. Check for Errors: Always check the return value of file I/O functions for errors and handle them appropriately.
  2. Close Files Properly: Always close files using fclose() when done with them to release resources and avoid resource leaks.
  3. Handle File Positioning: Use functions like fseek() and ftell() when necessary to navigate within files.
  4. Use Binary Mode for Binary Files: When working with binary files, use binary mode (“rb”, “wb”, “ab”, etc.) to ensure proper handling of data.
  5. Avoid Hardcoding File Paths: Instead of hardcoding file paths, use variables or command-line arguments to make the code more flexible and portable.
  6. Check File Existence: Before opening a file, check if it exists to avoid errors and unexpected behavior.
  7. Error Reporting: Provide informative error messages when file operations fail to aid in debugging and troubleshooting.


File Input/Output operations in C are essential for reading from and writing to files, enabling programs to interact with external data sources efficiently. By using functions provided by the stdio.h library and following best practices, developers can effectively incorporate file I/O functionality into their C programs, facilitating tasks like data storage, retrieval, and manipulation.