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This is the context: I am running Debian GNU/Linux and I switch reguarly with desktop environments ("DE" for the next).
My question is simple : I want to know which operation, syscalls or even functions used when I press the keyboard key "Print Screen".

Does the way changes with DE? I.e. do Mate, Gnome, KDE, LXDE or Xfce (etc) used a particular call of their own code or is there a generic syscall?

I think the answer (if any) is not Debian relative but more X or Wayland, is not it?

Thank you in advance for you advices and answers :)

PS: I precise that I read a good part of X lib source code, but did not find something useful.

Vadim Kotov
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Tiger-222
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  • Do you mean the whole sequence from pressing the key to having stored the image in the clipboard, or just the one that copies the screen content into a temporary bitmap? (I'm not sure I know the answer, but it would certainly help to understand which part you are asking for) – Mats Petersson Aug 05 '13 at 19:19
  • I am interested in the one that copies the screen content into a temporary bitmap, whatever the WM/DE. – Tiger-222 Aug 05 '13 at 19:24

2 Answers2

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Print screen itself is definitely not a syscall, but the kernel daemon which gets key presses definitely causes a routine to execute that uses what you would call a "syscall." I put that in quotes, because printscreen probably causes a program to run which is already in kernel space, which means there won't be any system calls to the kernel since you're already there (unless the window manager actually runs at user space, which isn't true for Mac OSX or windows, and I'm assuming for linux as well).

How does it work? It probably works by copying the current display from the screen buffer (region of ram which is DMA'd to your graphics card), and then transforming the pixel representation into a bit map.

KrisSodroski
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  • Indeed, I would know the process of copying the current display from the screen buffer, whatever the WM/DE. – Tiger-222 Aug 05 '13 at 19:25
  • You'd have to attach a debugger to the keyboard_event handler in the kernel. I tried looking at X11's code for print screen but couldn't find it. – KrisSodroski Aug 05 '13 at 19:29
  • Ehm, the Window Manager in all forms of Unix (Linux, MacOS, etc) is definitely a User mode Process - the graphics drive may well run in kernel mode, but the WM is a user-mode process. – Mats Petersson Aug 05 '13 at 19:29
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The basic principle can be found in the xwd tool.

The code isn't that bad to read. In the simple scenario, it used XGetImage, but if the screen has multiple visual regions, it gets more complex, but the fundamental principle is to use XGetPixel to get screen pixels and XPutPixel to store in the temporary image.

What happens when you press PrtScrn is the same thing, except it may be some other application that starts. Exactly what application depends on what the graphics package is in the distribution (Gnome, KDE, Unity, etc). But internally, they will do something very similar.

Edit:

As Peter points out, if the windowing system is "compositing" (that is, each window draws its own content off-screen, and the graphics hardware combines the output via composition), then the screen capture is required to ask the composition system to render the output off-screen, and then copy that.

Mats Petersson
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  • Interesting, I viewed source code of several tools, but not that one. Thank you, I will take a look! – Tiger-222 Aug 05 '13 at 19:51
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    Compositing window managers might do this differently. See, for example the compiz screenshot plugin. OpenGL-based: http://bazaar.launchpad.net/~mniess/ubuntu/precise/compiz/fix-screenshot/view/head:/plugins/screenshot/src/screenshot.cpp – Peter Aug 05 '13 at 19:52