am I in background?

[William LeFebvre]

(Note:  this discussion concerns only 4.2 BSD)

> I was wrong, but you misstate the correction.  It has nothing to do
> with whether the shell has the control terminal open.  It depends on
> whether the tty process group equals the process group of the process.

Quite right.  I intentionally oversimplified the situation rather than
be precise (when will I learn?).  The process only gets a TTIN when the
tty process group is not the same as this.  If the login process has
exited (causing a VHANGUP to get "sent" to all the children), the
process doing the read never gets the chance to get a TTIN, since the
VHANGUP action turned off both read and write ability for that file
descriptor.  Thus, he gets an EBADF.

> If you are ignoring or masking TTIN, you will get a real EOF however.

>From "/sys/sys/tty.c", function "ttread" (lines 1027--1041):

	/*
	 * Hang process if it's in the background.
	 */
#define bit(a) (1<<(a-1))
	while (tp == u.u_ttyp && u.u_procp->p_pgrp != tp->t_pgrp) {
		if ((u.u_procp->p_sigignore & bit(SIGTTIN)) ||
		   (u.u_procp->p_sigmask & bit(SIGTTIN)) ||
/*
		    (u.u_procp->p_flag&SDETACH) ||
*/
		    u.u_procp->p_flag&SVFORK)
			return (EIO);
		gsignal(u.u_procp->p_pgrp, SIGTTIN);
		sleep((caddr_t)&lbolt, TTIPRI);
	}

The read returns EIO, not end of file.  A simple test program will
prove this to be correct.  I leave that as an exercise to the reader.

			William LeFebvre
			Department of Computer Science
			Rice University
			<phil at Rice.arpa>
                        or, for the daring: <phil at Rice.edu>