Explain append mode operation of files in C language

In C programming, append mode is a file opening mode that allows you to add new data to the end of an existing file without overwriting its current contents. When you open a file in append mode, the file pointer is automatically positioned at the end of the file.

Syntax

FILE *fp;
fp = fopen("filename.txt", "a");

Where "a" represents the append mode. If the file doesn't exist, a new file will be created. If the file exists, new data will be added after the existing content.

Key Features of Append Mode

• File pointer is positioned at the end of the file
• Existing content is preserved
• New data is added after existing content
• Creates a new file if it doesn't exist
• Only allows write operations

Example 1: Basic Append Operation

This example demonstrates how to append text to a file −

#include <stdio.h>

int main() {
    FILE *fp;
    
    /* Open file in append mode */
    fp = fopen("sample.txt", "a");
    
    if (fp == NULL) {
        printf("Error opening file!<br>");
        return 1;
    }
    
    /* Append data to file */
    fprintf(fp, "This line will be appended.<br>");
    fprintf(fp, "Another line appended.<br>");
    
    /* Close the file */
    fclose(fp);
    
    printf("Data appended successfully!<br>");
    return 0;
}

Example 2: Reading and Appending Data

This example shows how to read existing content and then append new data −

#include <stdio.h>

int main() {
    FILE *fp;
    char buffer[100];
    
    /* First, read existing content */
    fp = fopen("data.txt", "r");
    if (fp != NULL) {
        printf("Existing content:<br>");
        while (fgets(buffer, sizeof(buffer), fp) != NULL) {
            printf("%s", buffer);
        }
        fclose(fp);
    }
    
    /* Now append new content */
    fp = fopen("data.txt", "a");
    if (fp == NULL) {
        printf("Error opening file for append!<br>");
        return 1;
    }
    
    fprintf(fp, "New line 1<br>");
    fprintf(fp, "New line 2<br>");
    
    fclose(fp);
    printf("\nData appended successfully!<br>");
    return 0;
}

Example 3: Append Mode vs Write Mode

This example demonstrates the difference between append mode and write mode −

#include <stdio.h>

int main() {
    FILE *fp;
    
    /* Create initial file with write mode */
    fp = fopen("test.txt", "w");
    if (fp != NULL) {
        fprintf(fp, "Initial content<br>");
        fclose(fp);
        printf("Initial file created.<br>");
    }
    
    /* Append to the file */
    fp = fopen("test.txt", "a");
    if (fp != NULL) {
        fprintf(fp, "Appended content<br>");
        fclose(fp);
        printf("Content appended.<br>");
    }
    
    /* Read and display final content */
    fp = fopen("test.txt", "r");
    if (fp != NULL) {
        char line[100];
        printf("\nFinal file content:<br>");
        while (fgets(line, sizeof(line), fp) != NULL) {
            printf("%s", line);
        }
        fclose(fp);
    }
    
    return 0;
}

Common File Modes Comparison

Key Points

• Always check if file opening was successful by comparing with NULL
• Use fclose() to properly close the file after operations
• Append mode preserves existing data, unlike write mode which truncates
• File position cannot be changed with fseek() in append mode on some systems

Conclusion

Append mode in C is essential for adding new data to existing files without losing original content. It's commonly used for logging, data collection, and maintaining records where preservation of existing data is crucial.