fopen() for existing file in write mode in C

The fopen() function in C is used to open files for various operations. When opening an existing file in write mode, it's important to understand how different modes affect the file's content.

Syntax

FILE *fopen(const char *filename, const char *mode);

fopen() for an Existing File in Write Mode

When using fopen() with write mode on an existing file −

• 'w' mode: Creates a new file if it doesn't exist, or truncates (empties) an existing file before writing
• 'w+' mode: Same as 'w' but allows both reading and writing
• 'a' mode: Appends new content to the end of an existing file
• 'wx' mode: Fails if the file already exists (exclusive creation)

Example 1: Write Mode ('w') - Truncates Existing File

This example shows how 'w' mode overwrites an existing file's content −

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

int main() {
    FILE *file = fopen("test.txt", "w");
    
    if (file == NULL) {
        printf("Could not open file
");
        return 1;
    }
    
    fprintf(file, "New content replaces old content");
    printf("Write operation successful
");
    fclose(file);
    
    return 0;
}

Write operation successful

Note: If "test.txt" had previous content like "Old data", it would be completely replaced with "New content replaces old content".

Example 2: Append Mode ('a') - Preserves Existing Content

This example demonstrates how 'a' mode adds content to the end of an existing file −

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

int main() {
    FILE *file = fopen("test.txt", "a");
    
    if (file == NULL) {
        printf("Could not open file
");
        return 1;
    }
    
    fprintf(file, " - Appended text");
    printf("Append operation successful
");
    fclose(file);
    
    return 0;
}

Append operation successful

Note: If "test.txt" contained "Original content", it would become "Original content - Appended text".

Example 3: Exclusive Write Mode ('wx') - Fails if File Exists

This example shows how 'wx' mode prevents overwriting existing files −

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

int main() {
    FILE *file = fopen("test.txt", "wx");
    
    if (file == NULL) {
        printf("File already exists or could not create file
");
        return 1;
    }
    
    fprintf(file, "This is a new file");
    printf("File created and written successfully
");
    fclose(file);
    
    return 0;
}

File already exists or could not create file

Comparison of Write Modes

Conclusion

Choose the appropriate write mode based on your needs: 'w' for complete file replacement, 'a' for appending content, or 'wx' for safe file creation without overwriting existing files.