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    SubjectRe: [RFC] The SIGINFO signal from BSD
    On Wed, Nov 05, 2014 at 02:31:12PM -0500, Austin S Hemmelgarn wrote:
    > >
    > >SIGINFO prints the status of the process to the terminal; BSD cp, for example,
    > >shows show much data it's copied:
    > >
    > > $ cp large_file /dev/null
    > > <press ^t>
    > > load: 1.39 cmd: cp 85837 [running] 3.91r 0.00u 0.98s 8% 2340k
    > > large_file -> /dev/null 15%
    > >
    > >As you see, it shows the current load, pid, process status, memory usage, as
    > >well as how much of the file has been copied. Many other BSD tools print similar
    > >statistics (mv, tar, dd, sleep, fetch, etc.).
    >
    > You have to understand however, that the reason that SIGINFO works like that
    > on *BSD is that the kernel and core userspace are developed together,
    > whereas on Linux, they are maintained entirely separately. Outside of core
    > userspace components, using SIGINFO that way on *BSD is just convention.

    Actually, the first line:

    load: 1.39 cmd: cp 85837 [running] 3.91r 0.00u 0.98s 8% 2340k

    is actually printed by the kernel. It's actually something which is
    implemented in the BSD N_TTY line displine. We never implemented it
    (at least when I was maintaining the tty subsystem) mostly out of
    laziness. Part of the reason is that the main reason was that main
    reason why people (at least systems programmers / kernel programers
    like me) used ^T was to debug an apparently hung system, and for
    Linux, we had a much more powerful system using the magic-sysrq key.

    Changing various userspace utilities to set up a signal handler for
    SIGINFO and then printing some extra information, such as:

    large_file -> /dev/null 15%

    is a much more recent innovation (at least, newer than BSD 4.3 in the
    early 90's, which is the last time I hacked on BSD :-), and is largely
    separate from the question of implementing ^T in the N_TTY line
    displine.

    In a world where we have a GUI desktop, I suspect implementing ^T is
    much less interesting, but if someone were to submit a patch to at
    least make ^T send a SIGINFO, I can't think of a reason why it
    wouldn't be accepted. (BTW, if you're going to do this, note that ^T
    could be remapped to any control character via stty; so to do this we
    would need to define an extra index in c_cc[] array in the struct
    termios.)

    Cheers,

    - Ted


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