File Handling in C

# CHAPTER 17

File Handling in C #

1. Introduction #

2. Learning Objectives #

• Open files in different modes (read, write, append).

• Use a FILE pointer.

• Write text to files using fprintf() and fputs().

• Read text from files using fscanf() and fgets().

• Close files properly to prevent data corruption.

3. The FILE Pointer and fopen() #

Common Modes:

• "r" : Read mode. (Returns NULL if file doesn't exist).

• "w" : Write mode. (Creates a new file. If the file exists, it erases all contents!).

• "a" : Append mode. (Creates a new file or adds data to the end of an existing file).

#include <stdio.h>

int main() {
    FILE *filePtr;
    
    // Open file in Write mode
    filePtr = fopen("data.txt", "w");
    
    if (filePtr == NULL) {
        printf("Error opening file.\n");
        return 1;
    }
    
    printf("File opened successfully!\n");
    // We will write code here next...
    
    // Always close the file!
    fclose(filePtr); 
    return 0;
}

4. Writing to a File #

FILE *filePtr = fopen("data.txt", "w");

if (filePtr != NULL) {
    fprintf(filePtr, "Hello, File!\n");
    fprintf(filePtr, "My age is %d\n", 25);
    fclose(filePtr);
}

5. Reading from a File #

Reading a full line using fgets():

FILE *filePtr = fopen("data.txt", "r");
char buffer[100];

if (filePtr != NULL) {
    // Read up to 99 characters until a newline or EOF is reached
    while (fgets(buffer, sizeof(buffer), filePtr) != NULL) {
        printf("%s", buffer); // Print the line to the screen
    }
    fclose(filePtr);
} else {
    printf("File not found!\n");
}

6. Mini Project: Notes Storage System #

#include <stdio.h>

int main() {
    FILE *file;
    char note[200];

    // Use "a" (append) so we don't delete older notes!
    file = fopen("my_notes.txt", "a");

    if (file == NULL) {
        printf("Could not open file!\n");
        return 1;
    }

    printf("=== MY NOTES SYSTEM ===\n");
    printf("Enter a new note (max 200 chars): ");
    
    // Read the line including spaces
    fgets(note, sizeof(note), stdin);

    // Write it to the file
    fprintf(file, "- %s", note);
    
    printf("Note saved to my_notes.txt successfully!\n");

    fclose(file);
    return 0;
}

7. Memory-Level Explanation #

When you write using fprintf(), you are actually writing to the RAM buffer. It is only when you call fclose() (or the buffer fills up) that the OS "flushes" the buffer and physically writes the data to the hard drive. If you forget to call fclose() and the program crashes, your data may be lost!

8. Common Mistakes #

• Forgetting fclose(): Results in memory leaks and missing data in the file.

• Using "w" instead of "a": You wanted to add a line, but "w" wiped the whole 10,000-line database!

• Not checking for NULL: If you try to read a file that doesn't exist, fopen returns NULL. If you pass NULL to fgets, the program will crash (Segmentation Fault).

9. Exercises #

1. 1. Write a program that asks for the user's name and age, and writes it to users.txt.

1. 2. Write a second program that opens users.txt in "r" mode and prints its contents to the screen.

10. MCQ Quiz with Answers #

11. Interview Questions #

• Q: Explain the difference between text mode ("w") and binary mode ("wb") in C file handling.

• Q: Why is it dangerous to ignore the return value of fopen()?

• Q: Explain the concept of the file buffer and why fflush() or fclose() is necessary.

12. Summary #

13. Next Chapter Recommendation #