mesg

#include<stdio.h>
#include<sys/types.h>
#include<stdlib.h>
#include<sys/stat.h>
int main(int ac,char *av[])
{
 struct stat settings;
 int perms;
 if(fstat(0,&settings)==-1)
 {
  perror("Can't stat stdin");
  exit(0);
 }
 perms=(settings.st_mode&07777);
 if(ac==1)
 {
  printf("is%c\n",(perms&S_IWGRP)?'y':'n');
  exit(1);
 }
 if(ac==2&&av[1][0]=='n')
  perms&=~S_IWGRP;
 else if(ac==2&&av[1][0]=='y')
  perms|=S_IWGRP;
 else
 {
  fprintf(stderr,"usage:mesg[y|n]\n");
  exit(2);
 }
 if(fchmod(0,perms)==-1)
 {
  perror("Can't change mesg status");
  exit(3);
 }
 return 0;
}