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Leo Simon
Asked: 2018-06-25 22:29:31 +0800 CST

disper -l on 18.04 generates a segmentation fault. Are there alternatives?

  • 772

I've used disper -l for many generations of ubuntu in order to determine which monitor I'm attached to, and whether an external monitor with either vga or hdmi is attached. Having now upgraded to 18.04, I find that this command is generating a segmentation fault. The problem seems to be around for a while, as reported in this bug report but nobody seems to be doing anything about it. Is there another program that has the same functionality? I've looked at xdpyinfo which has a lot of information, but not what I need, which is enough resolution information to distinguish between monitors, plus information about whether multiple monitors are attached, and by what connection.

Thanks!

multiple-monitors command-line display hdmi

3 Answers

  1. Best Answer

    I can confirm this bug 1655275, it seems to be caused by some anti-Meltdown or anti-Spectre fixes (or something similar as CVE-2017-1000364 with Scilab) since my apport crash log in comment #9 contains strange lines:

    SegvAnalysis:
 Segfault happened at: 0x7f1233f14add <XQueryExtension+45>: mov    0x968(%rdi),%rax
 PC (0x7f1233f14add) ok
 source "0x968(%rdi)" (0xffffffffd63a1188) not located in a known VMA region (needed readable region)!
 destination "%rax" ok
SegvReason: reading unknown VMA

    I have never used disper, but I can recommend xrandr as alternative.

    • 4

  2. I can confirm the bug, too.

    But good news! This seems extraordinarily easy to fix.

    I generated a patched version (available on Github if interested),

    git clone <URL>
cd disper
make all
sudo make install

    But please don't unconditionally trust random sources (e.g. me).

    Read below for a complete guide on how to do this for yourself, instead!

    Cherry picking AUR into debian upstream

    Cloning and adding remote

    Clone the Debian source (source here),

    git clone https://salsa.debian.org/python-team/applications/disper.git
cd disper

    Add AUR upstream as remote,

    git remote add AUR https://github.com/apeyser/disper
git fetch --all

    git remote -v

    [output]

    AUR https://github.com/apeyser/disper (fetch)
AUR https://github.com/apeyser/disper (push)
origin  https://salsa.debian.org/python-team/applications/disper.git (fetch)
origin  https://salsa.debian.org/python-team/applications/disper.git (push)

    Yes, they're different repositories -- not merely different branches -- but for this cherry-picking patch it's surprisingly convenient to just pretend, for a while.

    Just don't merge the "branches" unless you have a good reason, lest you cause an unecessary mess.

    Checking out the master's

    "Rename" (create new branches) their respective master (for your own record),

    git checkout -b AUR-master AUR/master
git checkout -b debian-master debian/master

    [output]

    Branch 'AUR-master' set up to track remote branch 'master' from 'AUR'.
Switched to a new branch 'AUR-master'
Switched to a new branch 'debian-master'

    Sanity check on that,

    git branch -vv

    [output]

      AUR-master    4c36710 [AUR/master] Update README
* debian-master e3ca840 d/control: Remove ancient X-Python-Version field
  debian/master e3ca840 [origin/debian/master] d/control: Remove ancient X-Python-Version field

    Seems good!

    You can explore and view the differences like so,

    git log --graph --decorate --oneline debian-master
git log --graph --decorate --oneline AUR-master
git diff debian-master AUR-master

    Identifying new commits

    What we're interested in is that this,

    git log debian-master -1

    [output]

    commit e3ca840a48208a79a5c96da1bd9f6f1b845f5773 (HEAD -> debian-master, origin/debian/master, origin/HEAD, debian/master)
Author: Ondřej Nový <[email protected]>
Date:   Mon May 14 08:10:02 2018 +0200

    d/control: Remove ancient X-Python-Version field

    does not include the recent patch here,

    git log AUR-master -4

    [output]

    commit 4c367108b370d57ce178863fed7640bb846d3186 (AUR/master, AUR-master)
Author: Alexander Peyser <[email protected]>
Date:   Thu Aug 23 17:16:42 2018 +0200

    Update README

commit 4c635d65244bcb2a86a9da86d00007de96a23626 (tag: disper-0.3.1.1)
Author: Alexander Peyser <[email protected]>
Date:   Thu Aug 23 16:02:23 2018 +0200

    apply XOpenDisplay return type patch

commit 01132563030b9230e8f007794c3269dd913c3052
Author: Alexander Peyser <[email protected]>
Date:   Thu Aug 23 16:17:07 2018 +0200

    Update INSTALL and README

commit d0498b7892b36d183d09b3d905296ee7456ff24d (tag: disper-0.3.1)
Author: [email protected] <>
Date:   Tue Apr 23 12:01:43 2013 +0200

    also update Debian version number

    Three (3) new commits! The second one is the fixing patch.

    Cherry-picking (what we want)

    Switch to debian-master and branch a local version (to not confuse the merge with the upstreams),

    git checkout debian-master
git checkout -b local

    git cherry-pick 0113256..4c36710

    [output]

    [local d05e2bd] apply XOpenDisplay return type patch
 Author: Alexander Peyser <[email protected]>
 Date: Thu Aug 23 16:02:23 2018 +0200
 1 file changed, 12 insertions(+), 2 deletions(-)
error: could not apply 4c36710... Update README
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add <paths>' or 'git rm <paths>'
hint: and commit the result with 'git commit'

    Of course we expected a merge conflict...

    But did you really expect it to be only README? Let's fix it!

    git mergetool  # or, vimdiff README (or whatever)
diff -NaurbB README* --color
git cherry-pick --continue
rm README.orig

    I merged in the nice new section of README and sanity checked, then continued with an unchanged commit message.

    Bump the version

    Identify where the version is kept,

    rg 0.3.1  # or, grep -rn 0.3.1 (or whatever)
git blame debian/changelog

    So you can add a dummy release section to debian/changelog (for your own record), or not.

    vim debian/changelog
git add debian/changelog

    I copied 0.3.1-3 to a new 0.3.1-4 section and wrote a few words.

    Test it!

    Does it work?

    make <Tab>
make all -n
make install -n

    Seems good to me. Let's install,

    make all
sudo make install
~~

Confirm that it works,

~~~bash
disper -l

    Let's commit and tag it (for future reference),

    git commit -v
git tag local/0.3.1-4 -a

    I really recommend a tag or something and a version bump. Be nice to future you.

    Happy hacking.

    • 3

  3. Disper is no longer in Debian and will probably soon disappear of Ubuntu.

    I wrote a very very simple remplacement and published it here: https://github.com/fidergo-stephane-gourichon/simplest_external_screen_config

    Here's it's README:

    Very simple external display management for the window system that is called "X" or "X11".

    It does not handle the general case.

    Intended use case: plug a screen, to your machine's output. Call this script, optionally with a word telling the intended use:

    • mirror to have both display cover (roughly) the same area of visualizable video memory
    • left, right, above or below, to cover a relatively positioned area outside of main display

    For example:

    simplest_external_screen_config
simplest_external_screen_config mirror
simplest_external_screen_config left
simplest_external_screen_config right
simplest_external_screen_config above
simplest_external_screen_config below

    I have put installation instructions for non-developers to the README.me there.

    • 0