Change sigTTOU/TTIN to IGNORE by default

[Malcolm Wallace]

> But we're not changing any of the terminal flags, simply setting
> the same ones again.
> I'm totally stumped!     Any ideas, anyone?

According to the manual page,

       tcgetattr() ...              This  function may be invoked
       from  a  background   process;   however,   the   terminal
       attributes  may  be  subsequently  changed by a foreground
       process.

In other words, the attributes belong to the *terminal*, not to the
process.

Modifying your C prog a little to print the flag values, and also to
ensure that the TOSTOP flag is unset:

int main(void) {
    struct termios tio;
    if (tcgetattr(1, &tio) < 0)          { exit(1); }
    printf("0x%x\n",tio.c_lflag);
    tio.c_lflag &= ~TOSTOP;
    printf("0x%x\n",tio.c_lflag);
    if (tcsetattr(1, TCSANOW, &tio) < 0) { exit(1); }
    printf("0x%x\n",tio.c_lflag);
    printf("hello world\n");
    exit(0);
}

The output of ./a.out is
    0x8a1b
    0x8a1b
    0x8a1b
    hello world
and ./aout & is
    0x8a11
    0x8a11
    [pid] Suspended.
Hence, we can see that the TOSTOP flag is not being changed.  It is
correctly 'off' all the way through.

But if you actually set the flag using
    tio.c_lflag |= TOSTOP;
instead, you will see the output change.  With ./a.out & it becomes
    0x8a11
    0x8b11
    [pid] Suspended.
Note the flag is now set correctly, and with *another* invocation of
./a.out & immediately following, you see just:
    [pid] Suspended.
Now, not even the debugging output is shown, because the previous
run of the program set the attribute TOSTOP on the terminal.

So it seems that there is possibly a feature/bug in the C library
such that calling tcsetattr() always suspends a backgrounded process,
regardless of the setting of TOSTOP.  As yet, I haven't been able to
find a workaround.  :-(

Regards,
    Malcolm