C Read Files

Reading files is a fundamental skill in programming that allows you to retrieve stored data and use it in your application. In this tutorial by The Coding College, we’ll explore how to read data from files in C using functions like fscanf(), fgets(), and fgetc() with clear examples and practical tips.

Why Read Files in C?

Reading files is essential for:

  • Loading configuration settings.
  • Importing data for processing.
  • Reusing previously stored information.

By mastering file-reading techniques, you can build more versatile and user-friendly applications.

Opening a File for Reading

Before you can read data from a file, you need to open it using the fopen() function in read mode ("r").

Opening a File

Syntax:

FILE *fopen(const char *filename, const char *mode);

Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r");  // Open file in read mode
    if (file == NULL) {
        printf("Error opening file!\n");
        return 1;
    }
    fclose(file);  // Close the file after operations
    return 0;
}

Reading Functions in C

Here are the primary functions for reading data from files:

FunctionPurpose
fscanf()Reads formatted input from a file.
fgets()Reads a line of text from a file.
fgetc()Reads a single character from a file.

Reading Data Using fscanf()

The fscanf() function reads formatted data from a file, similar to scanf() for standard input.

Syntax:

int fscanf(FILE *stream, const char *format, ...);

Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("data.txt", "r");  // Open file in read mode
    if (file == NULL) {
        printf("Error opening file!\n");
        return 1;
    }

    char name[50];
    int age;

    fscanf(file, "%s %d", name, &age);  // Read data from file
    printf("Name: %s, Age: %d\n", name, age);

    fclose(file);  // Close the file
    return 0;
}

Content of data.txt:

Alice 25

Output:

Name: Alice, Age: 25

Reading Lines Using fgets()

The fgets() function reads a line of text from a file.

Syntax:

char *fgets(char *str, int n, FILE *stream);

Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r");  // Open file in read mode
    if (file == NULL) {
        printf("Error opening file!\n");
        return 1;
    }

    char line[100];
    while (fgets(line, sizeof(line), file)) {
        printf("%s", line);  // Print each line
    }

    fclose(file);  // Close the file
    return 0;
}

Content of example.txt:

Welcome to The Coding College!
Learn C programming step by step.

Output:

Welcome to The Coding College!
Learn C programming step by step.

Reading Characters Using fgetc()

The fgetc() function reads a single character from a file.

Syntax:

int fgetc(FILE *stream);

Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r");  // Open file in read mode
    if (file == NULL) {
        printf("Error opening file!\n");
        return 1;
    }

    char ch;
    while ((ch = fgetc(file)) != EOF) {  // Read until end of file
        putchar(ch);  // Print each character
    }

    fclose(file);  // Close the file
    return 0;
}

Content of example.txt:

Hello, World!

Output:

Hello, World!

Practical Example: Reading Logs

Let’s read log entries from a file and display them.

Code Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("log.txt", "r");  // Open file in read mode
    if (file == NULL) {
        printf("Error opening file!\n");
        return 1;
    }

    char line[200];
    printf("Log Entries:\n");
    while (fgets(line, sizeof(line), file)) {
        printf("%s", line);
    }

    fclose(file);  // Close the file
    return 0;
}

Content of log.txt:

Log Entry: Thu Dec 12 11:30:25 2024
Application started successfully.

Output:

Log Entries:
Log Entry: Thu Dec 12 11:30:25 2024
Application started successfully.

Best Practices for Reading Files

  1. Always Check for NULL: Ensure the file was opened successfully.
  2. Avoid Buffer Overflows: Use appropriate buffer sizes when reading data.
  3. Close Files Properly: Use fclose() to free resources.

Common Errors

File Not Found

If the file doesn’t exist, fopen() returns NULL. Always check for this.

Example:

if (file == NULL) {
    printf("Error: File not found!\n");
    return 1;
}

Data Mismatch

If the file content doesn’t match the expected format, fscanf() may fail. Validate input data when reading.

Conclusion

Reading files in C allows you to handle external data seamlessly, enhancing the functionality of your programs. With functions like fscanf(), fgets(), and fgetc(), you can efficiently read files in various formats.

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