Question

I would like to stuff an ‘A’ character back into stdin using ungetc on receipt of SIGUSR1. Imagine that I have a good reason for doing this.

When calling foo(), the blocking read in stdin is not interrupted by the ungetc call on receipt of the signal. While I didn’t expect this to work as is, I wonder if there is a way to achieve this – does anyone have suggestions?

void handler (int sig)
{
  ungetc ('A', stdin);
}

void foo ()
{
  signal (SIGUSR1, handler);

  while ((key = fgetc (stdin)) != EOF)
  {
    ...
  }
}

Answer 1

Rather than try to get ungetc() to unblock a blocking fgetc() call via a signal, perhaps you could try not having fgetc() block to begin with and wait for activity on stdin using select().

---

By default, the line discipline for a terminal device may work in canonical mode. In this mode, the terminal driver doesn’t present the buffer to userspace until the newline is seen (Enter key is pressed).

To accomplish what you want, you can set the terminal into raw (non-canonical) mode by using tcsetattr() to manipulate the termios structure. This should case the blocking call to fgetc() to immediately return the character inserted with ungetc().

void handler(int sig) {
   /* I know I shouldn't do this in a signal handler,
    * but this is modeled after the OP's code.
    */
   ungetc('A', stdin);
}

void wait_for_stdin() {
   fd_set fdset;
   FD_ZERO(&fdset);
   FD_SET(fileno(stdin),&fdset);
   select(1, &fdset, NULL, NULL, NULL);
}

void foo () {
   int key;
   struct termios terminal_settings;

   signal(SIGUSR1, handler);

   /* set the terminal to raw mode */
   tcgetattr(fileno(stdin), &terminal_settings);
   terminal_settings.c_lflag &= ~(ECHO|ICANON);
   terminal_settings.c_cc[VTIME] = 0;
   terminal_settings.c_cc[VMIN] = 0;
   tcsetattr(fileno(stdin), TCSANOW, &terminal_settings);

   for (;;) {
      wait_for_stdin();
      key = fgetc(stdin);
      /* terminate loop on Ctrl-D */
      if (key == 0x04) {
         break;
      }      
      if (key != EOF) {
         printf("%c\n", key);
      }
   }
}

NOTE: This code omits error checking for simplicity.

Clearing the ECHO and ICANON flags respectively disables echoing of characters as they are typed and causes read requests to be satisfied directly from the input queue. Setting the values of VTIME and VMIN to zero in the c_cc array causes the read request (fgetc()) to return immediately rather than block; effectively polling stdin. This causes key to get set to EOF so another method for terminating the loop is necessary. Unnecessary polling of stdin is reduced by waiting for activity on stdin using select().

Executing the program, sending a SIGUSR1 signal, and typing
t e s t results in the following output¹:

A
t
e
s
t

^{1) tested on Linux}