C Programming File Positioning And Error Handling Fseek Ftell Feof Ferror Complete Guide

Objective:

To learn how to control the position of a file pointer within a file and handle errors that may occur during file operations.

Functions Covered:

1. fseek: Moves the file position indicator of the stream stream to a new position specified by offset relative to whence.
2. ftell: Returns the current value of the position indicator of the stream stream.
3. feof: Checks whether the end-of-file indicator associated with stream is set, returning a non-zero value if it is.
4. ferror: Checks whether the error indicator associated with stream is set, returning a non-zero value if it is.

Example 1: Using fseek and ftell

Description:

• Open a file named example.txt.
• Write some text to the file.
• Move to different positions in the file using fseek.
• Display the current file pointer position using ftell.

Step-by-Step Guide:

1. Include necessary header files.
2. Open the file.
3. Write data to the file.
4. Use fseek to move the file pointer.
5. Use ftell to get the current file pointer position.
6. Close the file.

#include <stdio.h>
#include <stdlib.h>

int main() {
    char *filename = "example.txt";
    FILE *fp;

    // Open file for writing
    fp = fopen(filename, "w");
    if (fp == NULL) {
        perror("Failed to open file");
        return EXIT_FAILURE;
    }

    // Write some data to the file
    fprintf(fp, "Hello, this is a test file.\nThis is the second line.\nAnd this is the third line.\n");

    // Close the file after writing
    fclose(fp);

    // Reopen the file for reading
    fp = fopen(filename, "r");
    if (fp == NULL) {
        perror("Failed to open file");
        return EXIT_FAILURE;
    }

    // Move the file pointer to the 17th byte (after "Hello, this is ")
    if (fseek(fp, 17L, SEEK_SET) != 0) {
        perror("Failed to seek file");
        fclose(fp);
        return EXIT_FAILURE;
    }

    // Read and print the next character from this position
    printf("Character at byte 17: '%c'\n", fgetc(fp));

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

    // Move the file pointer back to the beginning
    rewind(fp); // Equivalent to fseek(fp, 0, SEEK_SET)
    printf("After rewind, file pointer position: %ld\n", ftell(fp));

    // Close the file after reading
    fclose(fp);

    return EXIT_SUCCESS;
}

Example 2: Using feof

Description:

• Open a file named example.txt.
• Read the contents of the file line by line.
• Detect the end of the file using feof.

Step-by-Step Guide:

1. Include necessary header files.
2. Open the file.
3. Read the file line by line until the end.
4. Check if feof is set after reading.
5. Close the file.

#include <stdio.h>
#include <stdlib.h>

int main() {
    char *filename = "example.txt";
    FILE *fp;
    char buffer[100];

    // Open file for reading
    fp = fopen(filename, "r");
    if (fp == NULL) {
        perror("Failed to open file");
        return EXIT_FAILURE;
    }

    while (!feof(fp)) {
        if (fgets(buffer, sizeof(buffer), fp) != NULL) {
            printf("%s", buffer);
        } else {
            // Check if feof is set or if an error occurred
            if (feof(fp)) {
                printf("End of file reached.\n");
            } else if (ferror(fp)) {
                perror("Error reading file");
                fclose(fp);
                return EXIT_FAILURE;
            }
        }
    }

    // Close the file after reading
    fclose(fp);

    return EXIT_SUCCESS;
}

Example 3: Using ferror

Description:

• Attempt to read from a non-accessible file.
• Handle the error using ferror.

Step-by-Step Guide:

1. Include necessary header files.
2. Attempt to open a file with incorrect permissions.
3. Try to read from the inaccessible file.
4. Check if ferror is set and handle the error.
5. Close the file (if opened).

#include <stdio.h>
#include <stdlib.h>

int main() {
    char *filename = "nonexistentfile.txt";
    FILE *fp;
    char buffer[100];

    // Open file for reading
    fp = fopen(filename, "r");
    if (fp == NULL) {
        perror("Failed to open file");
        return EXIT_FAILURE;
    }

    // Try to read data from the file (which does not exist)
    if (fgets(buffer, sizeof(buffer), fp) == NULL) {
        if (ferror(fp)) {
            perror("Error reading file");
            fclose(fp);
            return EXIT_FAILURE;
        }
    }

    // Display the read content (this would not execute if an error occurred)
    printf("Read from file: %s", buffer);

    // Close the file after finishing operations
    fclose(fp);

    return EXIT_SUCCESS;
}

Example 4: Combining fseek, ftell, feof, and ferror

Description:

• Open a file named example.txt.
• Write data to the file.
• Read the file until its end, checking for errors and EOF.
• Reset the file pointer using fseek.

Step-by-Step Guide:

1. Include necessary header files.
2. Open the file for writing.
3. Write data to the file.
4. Close the file.
5. Reopen the file for reading.
6. Read data from the file.
7. Use feof, ferror, fseek, and ftell appropriately to handle different scenarios.
8. Close the file after reading.

#include <stdio.h>
#include <stdlib.h>

int main() {
    char *filename = "example.txt";
    FILE *fp;
    char buffer[100];
    int ch;

    // Open file for writing
    fp = fopen(filename, "w+");
    if (fp == NULL) {
        perror("Failed to open file");
        return EXIT_FAILURE;
    }

    // Write some data to the file
    fprintf(fp, "Line 1\nLine 2\nLine 3\n");

    // Reset the file pointer to the beginning
    rewind(fp);

    // Read data from the file
    while ((ch = fgetc(fp)) != EOF) {
        putchar(ch);
    }

    // Check if feof is set
    if (feof(fp)) {
        printf("\nEnd of file reached.\n");
        clearerr(fp); // Clear EOF flag
    }

    // Get the current file pointer position (should be at the end of the file)
    printf("Current file pointer position after reading: %ld\n", ftell(fp));

    // Reset file pointer
    fseek(fp, 0L, SEEK_SET);
    printf("After resetting, file pointer position: %ld\n", ftell(fp));

    // Read data again
    while ((ch = fgetc(fp)) != EOF) {
        putchar(ch);
    }
    printf("\n");

    // Check if ferror is set
    if (ferror(fp)) {
        perror("Error during file operation");
        fclose(fp);
        return EXIT_FAILURE;
    }

    // Close the file after operations
    fclose(fp);

    return EXIT_SUCCESS;
}

Explanation of Each Function:

1. fseek(FILE *stream, long offset, int whence):

   • stream: The file stream.
   • offset: The number of bytes to offset.
   • whence: The starting point for the offset (e.g., SEEK_SET, SEEK_CUR, SEEK_END).

2. long ftell(FILE *stream):

   • stream: The file stream.
   • Returns the current value of the position indicator of stream, or -1 on failure.

3. int feof(FILE *stream):

   • stream: The file stream.
   • Returns a non-zero value if the end-of-file indicator is set; otherwise, returns 0.

4. int ferror(FILE *stream):

   • stream: The file stream.
   • Returns a non-zero value if the error indicator is set; otherwise, returns 0.

Notes:

• Always check the return value of file operations to ensure they succeed.
• Use clearerr(FILE *stream) to reset the stream's error indicators.
• Remember to close all opened files using fclose(FILE *stream) to free associated resources.