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File Positioning

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In this section, we will explore how to manipulate the position of the file pointer in C. This is crucial for tasks such as reading from or writing to specific locations within a file. Understanding file positioning allows for more efficient file operations and is essential for handling large files or implementing complex file manipulation logic.

Key Concepts

  1. File Pointer: A file pointer is an internal pointer that keeps track of the current position within a file.
  2. Functions for File Positioning:
    • fseek()
    • ftell()
    • rewind()

Functions for File Positioning

fseek()

The fseek() function is used to move the file pointer to a specific location within a file.

Syntax:

int fseek(FILE *stream, long int offset, int whence);
  • stream: Pointer to the file object.
  • offset: Number of bytes to move the file pointer.
  • whence: Position from where offset is added. It can be:
    • SEEK_SET: Beginning of the file.
    • SEEK_CUR: Current position of the file pointer.
    • SEEK_END: End of the file.

Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r");
    if (file == NULL) {
        perror("Error opening file");
        return -1;
    }

    // Move file pointer to the 10th byte from the beginning
    fseek(file, 10, SEEK_SET);

    // Read and print the character at the new position
    char ch = fgetc(file);
    printf("Character at position 10: %c\n", ch);

    fclose(file);
    return 0;
}

ftell()

The ftell() function returns the current position of the file pointer.

Syntax:

long int ftell(FILE *stream);
  • stream: Pointer to the file object.

Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r");
    if (file == NULL) {
        perror("Error opening file");
        return -1;
    }

    // Move file pointer to the 10th byte from the beginning
    fseek(file, 10, SEEK_SET);

    // Get the current position of the file pointer
    long int position = ftell(file);
    printf("Current position: %ld\n", position);

    fclose(file);
    return 0;
}

rewind()

The rewind() function sets the file pointer to the beginning of the file.

Syntax:

void rewind(FILE *stream);
  • stream: Pointer to the file object.

Example:

#include <stdio.h>

int main() {
    FILE *file = fopen("example.txt", "r");
    if (file == NULL) {
        perror("Error opening file");
        return -1;
    }

    // Move file pointer to the 10th byte from the beginning
    fseek(file, 10, SEEK_SET);

    // Rewind the file pointer to the beginning
    rewind(file);

    // Read and print the character at the new position
    char ch = fgetc(file);
    printf("Character at the beginning: %c\n", ch);

    fclose(file);
    return 0;
}

Practical Exercises

Exercise 1: Reading a Specific Line from a File

Task: Write a program that reads and prints the third line from a file named data.txt.

Solution:

#include <stdio.h>

int main() {
    FILE *file = fopen("data.txt", "r");
    if (file == NULL) {
        perror("Error opening file");
        return -1;
    }

    char buffer[256];
    int line_number = 1;

    while (fgets(buffer, sizeof(buffer), file)) {
        if (line_number == 3) {
            printf("Third line: %s", buffer);
            break;
        }
        line_number++;
    }

    fclose(file);
    return 0;
}

Exercise 2: Appending Data at a Specific Position

Task: Write a program that appends the string "INSERTED" at the 5th byte of a file named data.txt.

Solution:

#include <stdio.h>
#include <string.h>

int main() {
    FILE *file = fopen("data.txt", "r+");
    if (file == NULL) {
        perror("Error opening file");
        return -1;
    }

    // Move file pointer to the 5th byte from the beginning
    fseek(file, 5, SEEK_SET);

    // Insert the string "INSERTED"
    fputs("INSERTED", file);

    fclose(file);
    return 0;
}

Common Mistakes and Tips

  • Incorrect whence Value: Ensure you use the correct whence value (SEEK_SET, SEEK_CUR, SEEK_END) when using fseek().
  • Checking Return Values: Always check the return values of file operations to handle errors gracefully.
  • Buffer Size: When reading lines with fgets(), ensure the buffer size is sufficient to hold the entire line.

Summary

In this section, we covered the basics of file positioning in C, including how to use fseek(), ftell(), and rewind() functions. We also provided practical examples and exercises to reinforce the concepts. Understanding file positioning is essential for efficient file manipulation and is a valuable skill for any C programmer.

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