Any way, here is a quick tip that will allow your c program to easily add text to the end of a file without ever having to deal with the file in your code. This is accomplished by a simple system call, take a look at the following example.

#include
int main() {
  system("ls >> 1.txt");
}

This simple code will perform the ls command and redirect its output to the file 1.txt. If 1.txt does not exist it will create the file. If the file exists it will append the output from ls to the file.