Add Data to the End of a File in C

To add data to the end of a file in C, you can open the file in append mode using fopen() with the mode "a" or "a+". This allows you to write new data without altering the existing content in the file.

Example 1: Appending Text to a File using fopen() with “a” Mode

In this example, we will open an existing file in append mode ("a") and add a line of text at the end of the file.

main.c

</>
Copy
#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "a"); // Open file in append mode
    if (fp == NULL) {
        perror("Error opening file");
        return 1;
    }
    fprintf(fp, "This is a new line added to the file.\n");
    fclose(fp);
    return 0;
}

Explanation:

  1. FILE *fp: Declares a file pointer to manage file operations.
  2. fopen("example.txt", "a"): Opens example.txt in append mode. The "a" mode allows new data to be written at the end of the file without modifying its existing content.
  3. if (fp == NULL): Checks if the file was opened successfully; if not, it prints an error message.
  4. fprintf(fp, "..."): Writes a new line of text into the file.
  5. fclose(fp): Closes the file to ensure that all data is properly saved.

Output:

This is a new line added to the file.

Example 2: Appending Data to a File Using fopen() with “a+” Mode

In this example, we will open a file in append mode with read access ("a+"), append data, and then read back the contents of the file to verify the changes.

main.c

</>
Copy
#include <stdio.h>

int main() {
    FILE *fp = fopen("example.txt", "a+"); // Open file in append and read mode
    if (fp == NULL) {
        perror("Error opening file");
        return 1;
    }
    // Append new data to the file
    fprintf(fp, "Appending a second line to the file.\n");

    // Reset the file position to the beginning to read the contents
    fseek(fp, 0, SEEK_SET);

    char ch;
    // Read and print the entire file content
    while ((ch = fgetc(fp)) != EOF) {
        putchar(ch);
    }
    
    fclose(fp);
    return 0;
}

Explanation:

  1. FILE *fp: Declares a file pointer for file operations.
  2. fopen("example.txt", "a+"): Opens example.txt in append and read mode. The "a+" mode allows appending data while also providing the ability to read the file.
  3. fprintf(fp, "..."): Appends a new line of text to the file.
  4. fseek(fp, 0, SEEK_SET): Resets the file pointer to the beginning so that the file content can be read from the start.
  5. while ((ch = fgetc(fp)) != EOF): Reads each character until the end of the file and prints it using putchar(ch).
  6. fclose(fp): Closes the file after all operations are complete.

Output:

Contents of example.txt will include all previous data followed by:
Appending a second line to the file.

Conclusion

In this tutorial, we learned multiple approaches to add data to the end of a file in C:

  1. Example 1: Demonstrated how to append text using fopen() in "a" mode.
  2. Example 2: Showed how to use fopen() in "a+" mode to append data and then read the entire file.