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)
  {
    ...
  }
}