//Here's a simple solution for mesg, which flips the permission of accepting
//messages on the current terminal window, using fstat() and fchmod(). 
//These are the same as stat() and chmod() but work with file
//descriptors instead of file names. File descriptor 0 refers to the 
//terminal.

#include <sys/stat.h>

int main() {
  struct stat info;
  fstat(0, &info);
  if (info.st_mode & S_IWGRP) 
    fchmod(0, info.st_mode & ~S_IWGRP);
  else 
    fchmod(0, info.st_mode | S_IWGRP);
}