Re: [PATCH] n_tty_read: check for hanging tty while waiting for input

From: Peter Hurley
Date: Wed Feb 18 2015 - 11:39:07 EST

On 02/18/2015 10:56 AM, Aristeu Rozanski wrote:
> Hi Peter,
> On Wed, Feb 18, 2015 at 10:40:10AM -0500, Peter Hurley wrote:
>> The child is not receiving SIGHUP because /dev/ttyS0 was not set as the
>> controlling terminal by ioctl(TIOCSCTTY), which is failing (probably
>> with errno == EPERM). You need to check the return value and errno.
>> To set the controlling tty, the calling process must be a session leader;
>> ie., have called setsid() before ioctl(TIOCSCTTY). Check the return value
>> for that too.
>> FWIW, the idiom for starting a session leader is for the parent to
>> fork a child and exit and for the child to become the session leader with
>> setsid() and establish its controlling tty either with ioctl(TIOCSCTTY)
>> or simply opening the first tty.
>> The reason for this idiom is that setsid() will fail for an existing
>> group leader (because otherwise a group leader could abandon existing
>> members of its process group, leaving them without a group leader in
>> a different session).
>> I highly recommend Ch 34 of Michael Kerrisk's book, "The Linux Programming
>> Interface", especially if this is not a toy project.
> Actually wrote this trying to reproduce a problem a customer is seeing
> in a commercial application, but clearly I need to read it. Specifically
> about the console behavior, would you recommend the same book?

Ch 34 is about controlling ttys and job control, so it covers the two-tier
process hierarchy, foreground and background process groups, and job control
signals. Personally, I think the book is invaluable.

Unfortunately, consoles are not well documented anywhere.

> Every time I need details like this I fail to find any reference online.
> Thanks for your help

No problem.

Peter Hurley

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