Behavior of select() on stdin when used on a pipe

Question

I am trying to understand an observation on behavior of select() when used on stdin, when it is receiving data from a pipe.

Basically I had a simple C program using the following code: hello.c:

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <termios.h>

int main(int argc, char *argv[])
{
  int flags, opt;
  int nsecs, tfnd;
  fd_set rfds;
  struct timeval tv;
  int retval;
  int stdin_fileno_p1 = STDIN_FILENO+1;
  char c;
  int n;

  /* Turn off canonical processing on stdin*/
  static struct termios oldt, newt;
  tcgetattr( STDIN_FILENO, &oldt);
  newt = oldt;
  newt.c_lflag &= ~(ICANON);
  tcsetattr( STDIN_FILENO, TCSANOW, &newt);

  while (1)
  {
    FD_ZERO(&rfds);
    FD_SET(STDIN_FILENO, &rfds);
    tv.tv_sec = 0;
    tv.tv_usec = 0;
    retval = select(stdin_fileno_p1, &rfds, NULL, NULL, &tv);

    if ( retval && (retval!=-1) )
    {
      n = read(STDIN_FILENO, &c, 1);
      write(STDOUT_FILENO, &c, 1);
    }
    else printf("No Data\n");
    usleep(100000);
  }
  tcsetattr( STDIN_FILENO, TCSANOW, &oldt);
}

If I ran the program as follows I could see characters echoing when I type keys on while the program is running. When keys are not pressed, it displays "No Data" as expected.

./hello

However, if use the program as follows, the program never gets to a state where is displays "No Data". Instead last character "c" is repeatedly displayed.

echo -n abc | ./hello

I'm a bit puzzled by this observation, and would be grateful if you could help me to understand the observed behavior.

Answer 1

The problem is that your program does not detect an end-of-file condition when it reads from the STDIN_FILENO descriptor. After echo has written the c character it will close its end of the pipe, which will cause the select in your program to return immediately and your read to return 0 as an indication that no more data will ever be available from that descriptor. Your program doesn't detect that condition. Instead it just calls write with whatever character was left in the buffer by the last successful read and then repeats the loop.

To fix, do if (n==0) break; after the read.