How to Read from a File in C: A Step-by-Step Guide with Practical Examples

Reading from a file is a common task in C programming, allowing you to retrieve and process data stored in external files. Whether you’re handling configuration files, reading input data, or managing logs, understanding how to read from files is essential. In this blog post, we’ll explore how to read from a file in C, provide practical examples, and discuss best practices for handling file operations.

File Handling in C: An Overview

Before diving into reading from a file, it’s important to understand the basics of file handling in C. File handling involves creating, reading, writing, and closing files using the standard input/output library (stdio.h).

Key Functions for File Handling:

1. fopen(): Opens a file for reading, writing, or appending.
2. fclose(): Closes an opened file.
3. fscanf() and fgets(): Reads data from a file.
4. fgetc(): Reads a single character from a file.

Opening a File for Reading in C

To read from a file, you first need to open it using the fopen() function. The fopen() function requires two arguments: the name of the file and the mode in which to open the file.

Syntax:

FILE *filePointer;
filePointer = fopen("filename.txt", "r");

• "r" mode is used for reading. If the file does not exist, fopen() returns NULL.

Example:

#include <stdio.h>

int main() {
    FILE *filePointer;
    filePointer = fopen("data.txt", "r");

    if (filePointer == NULL) {
        printf("Error: Could not open file.\n");
        return 1;
    }

    // File operations go here

    fclose(filePointer);
    return 0;
}

Explanation:

• The program attempts to open data.txt for reading.
• If the file cannot be opened, an error message is displayed.

Reading from a File Using fscanf()

The fscanf() function is used to read formatted input from a file. It works similarly to scanf(), but reads from a file instead of standard input.

Syntax:

fscanf(filePointer, "format_specifiers", variables);

Example: Reading Data from a File

#include <stdio.h>

int main() {
    FILE *filePointer;
    int number;
    char word[20];

    filePointer = fopen("data.txt", "r");

    if (filePointer == NULL) {
        printf("Error: Could not open file.\n");
        return 1;
    }

    while (fscanf(filePointer, "%d %s", &number, word) != EOF) {
        printf("Number: %d, Word: %s\n", number, word);
    }

    fclose(filePointer);
    return 0;
}

Explanation:

• The program reads an integer and a string from data.txt using fscanf().
• The loop continues reading until the end of the file (EOF) is reached.

Reading a Line from a File Using fgets()

For reading entire lines from a file, the fgets() function is more appropriate. It reads a string until a newline character or the specified number of characters is reached.

Syntax:

fgets(string, size, filePointer);

Example: Reading Lines from a File

#include <stdio.h>

int main() {
    FILE *filePointer;
    char line[100];

    filePointer = fopen("data.txt", "r");

    if (filePointer == NULL) {
        printf("Error: Could not open file.\n");
        return 1;
    }

    while (fgets(line, sizeof(line), filePointer)) {
        printf("%s", line);
    }

    fclose(filePointer);
    return 0;
}

Explanation:

• The program reads each line from data.txt using fgets().
• The loop continues until all lines are read.

Best Practices for Reading from Files in C

1. Always Check if the File Opened Successfully: Use fopen() and check if the file pointer is NULL to ensure the file was opened correctly.
2. Close Files After Use: Always close files with fclose() to free up system resources.
3. Handle Errors Gracefully: Implement error handling to manage cases where files cannot be opened or read.

Common Mistakes to Avoid

• Forgetting to Close the File: Not closing a file can lead to resource leaks.
• Incorrect File Path: Ensure the file path is correct and accessible by the program.
• Mismatched Format Specifiers: When using fscanf(), ensure that the format specifiers match the type of data being read.

Conclusion

Reading from a file in C is a fundamental skill that every C programmer should master. By understanding how to use fopen(), fscanf(), fgets(), and other file handling functions, you can effectively manage file operations in your programs. The examples provided in this blog post should give you a solid foundation to start reading from files in C with confidence.