On Sat, Jul 15, 2017 at 8:34 AM, Brandon Allbery <allber...@gmail.com>

> Ctrl-C sends SIGINT to all processes in the terminal's foreground process
> group. There is no concept of "current process" on a terminal; Unixlike
> systems are multitasking, and all processes in the process group have
> access to the terminal, and there is no way for the OS to consider one of
> them to be "the current process". (Indeed, it is possible, if unfortunate,
> for multiple processes to be reading from the terminal at the same time.)
> That only one such process can be considered by users to be "current" is
> merely convention, and one that is easy to violate.
> (Shells make it more difficult by forcing command pipelines into separate
> process groups, and controlling them by sending signals to the process
> group; if running in the foreground, it will be assigned as the terminal's
> current process group. If a process in such process group *not* in the
> terminal's foreground process group tries to read from the terminal, it
> gets SIGTTIN; if it tries to write, it gets SIGTTOU. If these are not
> specifically handled by the process, they turn into SIGSTOP. This allows
> shells to maintain the illusion that there is a "current process", provided
> that only the first process in a pipeline reads from the terminal.)

If you want to find out about this in even more gory detail, you can look
up the arguments about supporting SIGSTAT (Ctrl-T, as supported on *BSD and
OS X) on Linux. It involves a number of gory hacks to maintain the pretense
that there is a "current process", all of which are unreliable, fragile,
and require quite a lot of ugliness in the kernel.

AFAIK there's no convenient way to look up the terminal's foreground
process group; you need to dig around in kernel memory to find the tty
structure. "who -u" makes a guess though.

brandon s allbery kf8nh                               sine nomine associates
allber...@gmail.com                                  ballb...@sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net

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