strange little UNIX question....

[Farooq Butt]

Folks:

I've got a strange little UNIX question for you.  

The problem:

I open an fd in a child process.  The child process listens to the
user's keyboard and given a certain key ("%") issues a SIGUSR1 to its
parent that is listening on some other /dev/ttyfoo port.  The parent
has a signal handler that traps SIGUSR1.  The handler was devised to
restore the state of stdin (which I had previously converted to "raw"
so that every key I hit would immediately be read) and to exit.  Now
here's the weirdness: if I issue a signal and exit from my child
process (the child changed stdin to raw mode) _and_ ioctl stdin from
the signal handler called by the _parent_ (i.e child screws with stdin
and makes it raw, child gets a key, child signals parent and quits
leaving handler to restore sanity) then the program causes a weird
error with telnet (I/O error on /dev/ttyp1) and generally blows my
session away.  Now if I get the _child_ to restore stdin to sane
values before exiting (i.e. clean up its own mess) everything works
hunky dory.

What's up?

Isn't stdin stdin everywhere ?  Why does re-ioctl'ing stdin back to
sane modes cause life to be weird if I do it from the signal handler?
Is there a relation to processes that I am missing ? If fd's are 
indeed process-specific then how come some programs trash stdin 
for the shell (many's the time I've had to do an stty after an 
ill-behaved program)?

-fmb

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