Programming

   while(fgets(buf, 4096, fin)) //keep reading line-by-line into buffer
       {
               if(strncmp(buf, "@HWI", 4) == 0) //if buffer starts with '@HWI' 
               {
                       buf[0]='#';  //set the first char to '#'
                       fputs(buf, fout);   //and print the whole line
                       if(fgets(buf, 4096, fin) == NULL) continue;   //read next line; check for failure or eof
                       fputs(buf, fout);  //print the line 
               }
       }

unsigned long size=(1024L*1024L*64L); // 64 megabytes
char *buf=malloc(size);