Difference between r+ and w+ in fopen()

Both r+ and w+ can read and write to a file. However, r+ doesn't delete the content of the file and doesn't create a new file if such file doesn't exist, whereas w+ deletes the content of the file and creates it if it doesn't exist.

The main difference is w+ truncate the file to zero length if it exists or create a new file if it doesn't. While r+ neither deletes the content nor create a new file if it doesn't exist.

Try these codes and you will understand:

#include <stdio.h>int main(){   FILE *fp;   fp = fopen("test.txt", "w+");   fprintf(fp, "This is testing for fprintf...\n");   fputs("This is testing for fputs...\n", fp);   fclose(fp);}  

and then this

#include <stdio.h>int main(){   FILE *fp;   fp = fopen("test.txt", "w+");   fclose(fp);}   

If you will open test.txt, you will see that all data written by the first program has been erased.
Repeat this for r+ and see the result.
Here is the summary of different file modes (1 => ture, 0 => false):

This diagram will be faster to read next time. Maybe someone else will find that helpful too. This is clearly explained the difference between.