clearing stdin obtained from a pipe

pjr · 03-14-2010, 09:20 AM

I have a small program that reads stdin from a pipe using fgets. Now fgets blocks for the first line but after that it will not block.

THE CODE, my_echo.c -

int main(int argc, char **argv)
{ char buf [2000] ;
char* pc ;
printf("hello\n") ;
while (1)
{ buf[0] = (char) 0 ;
pc = fgets(buf, sizeof(buf), stdin);
if (pc != NULL)
printf("%s\n",buf);
}
return 0;
}

HOW ITS CALLED
* In terminal window 1: ./my_echo < my_fifo

* In terminal window 2: echo "1234" > my_fifo

* In terminal window 1: prints hello then 1234.

* Checking with ksysguard or top shows that my_echo
is consuming 40% of CPU time.
Adding a few printf's shows that the gets is not
blocking and returns a null pointer.

* In terminal window 2: echo "qwerty" > my_fifo

* In terminal window 1 qwerty prints.

MY PROBLEM : I want a read function that does in fact block so my program does not tie up CPU time, read does not block. Any ideas anyone?

PTrenholme · 03-14-2010, 01:45 PM

The problem may be that you're using a while (1) with no termination condition (i.e., an "infinite CPU loop"). Consider something like this:

Code:

int main(int argc, char **argv) {
   char buf [2000] ;
   char* pc ;
   int waiting=0;
   printf("Hello\n") ;
// Quit if no input is received within 60 seconds.
   while (waiting < 60) {
      buf[0] = (char) 0 ;
      pc = fgets(buf, sizeof(buf), stdin);
      if (pc != NULL) {
         printf("%s\n",buf);
         waiting = 0;
      }
      else {
        ++waiting;
        sleep(1);
     }
   }
   printf("Goodbye.\n")
   return 0;
}

pjr · 03-14-2010, 11:09 PM

Hi,
thanks for the solution, but in the much larger program I am writing I need to find a way to make the pipe read block when the pipe is empty.

PTrenholme · 03-16-2010, 06:43 PM

Ah, well, on my Fedora system, a man 7 pipe gives a nice overview of pipe operations. That overview suggests that, if you create your pipe using a mkfifo call, then the read will be blocked when the pipe is empty.

ntubski · 03-17-2010, 08:20 AM

I think the problem is that echo "1234" > my_fifo sends an EOF after finishing, perhaps something like this would work?

Code:

exec 3> my_fifo # opens my_fifo for writing
echo "1234" >&3
echo "qwerty" >&3

pjr · 03-18-2010, 07:58 AM

Hi ntubski,
thanks for the thoughts here but I really need the my_echo program to read from the fifo, and get blocked if the fifo is empty.

ntubski · 03-18-2010, 09:22 AM

It is blocked when the fifo is empty, it's not blocked when the fifo gets an EOF. That's why I suggested not closing the fifo from the other end...

pjr · 03-20-2010, 09:39 PM

You are right and it does work. Thanks!