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I have been learning about daemons and writing them in C. I want to make the daemon receive the global key press signals in Ubuntu, and write the received key-presses to a separate file.

Is there any way that be achieved? If so, how?

Thomas Dickey
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Be Wake Pandey
  • 131

1 Answers1

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The most direct way would be reading event devices exposed in /dev/input/. Read the kernel document on Linux input subsystem, from which you could learn how to manipulate those devices in C.

There're several packages for you to quickly test your input devices. For example you can use the following tools after apt-get install input-utils on Ubuntu.

lsinput: enumerate input devices connected to the system.

$ sudo lsinput
/dev/input/event0
   bustype : BUS_HOST
   vendor  : 0x0
   product : 0x3
   version : 0
   name    : "Sleep Button"
   phys    : "PNP0C0E/button/input0"
   bits ev : EV_SYN EV_KEY

/dev/input/event1
   bustype : BUS_HOST
   vendor  : 0x0
   product : 0x5
   version : 0
   name    : "Lid Switch"
   phys    : "PNP0C0D/button/input0"
   bits ev : EV_SYN EV_SW

/dev/input/event2
   bustype : BUS_HOST
   vendor  : 0x0
   product : 0x1
   version : 0
   name    : "Power Button"
   phys    : "LNXPWRBN/button/input0"
   bits ev : EV_SYN EV_KEY

/dev/input/event3
   bustype : BUS_I8042
   vendor  : 0x1
   product : 0x1
   version : 43841
   name    : "AT Translated Set 2 keyboard"
   phys    : "isa0060/serio0/input0"
   bits ev : EV_SYN EV_KEY EV_MSC EV_LED EV_REP

Here, /dev/input/event3 is a keyboard device you might want to watch.

input-events: Read and show events from input device. It takes an argument of device number, say, 3 for /dev/input/event3.

$ sudo input-events 3
/dev/input/event3
   bustype : BUS_I8042
   vendor  : 0x1
   product : 0x1
   version : 43841
   name    : "AT Translated Set 2 keyboard"
   phys    : "isa0060/serio0/input0"
   bits ev : EV_SYN EV_KEY EV_MSC EV_LED EV_REP

waiting for events
09:27:02.961576: EV_MSC MSC_SCAN 50
09:27:02.961576: EV_KEY KEY_M (0x32) released
09:27:02.961576: EV_SYN code=0 value=0
09:27:03.833925: EV_MSC MSC_SCAN 58
09:27:03.833925: EV_KEY KEY_CAPSLOCK (0x3a) released
09:27:03.833925: EV_SYN code=0 value=0
09:27:09.344765: EV_MSC MSC_SCAN 30
09:27:09.344765: EV_KEY KEY_A (0x1e) pressed
09:27:09.344765: EV_SYN code=0 value=0
09:27:09.446881: EV_MSC MSC_SCAN 30
09:27:09.446881: EV_KEY KEY_A (0x1e) released
09:27:09.446881: EV_SYN code=0 value=0
09:27:09.548389: EV_MSC MSC_SCAN 48
09:27:09.548389: EV_KEY KEY_B (0x30) pressed
09:27:09.548389: EV_SYN code=0 value=0
09:27:09.640385: EV_MSC MSC_SCAN 48
09:27:09.640385: EV_KEY KEY_B (0x30) released
09:27:09.640385: EV_SYN code=0 value=0
09:27:09.781365: EV_MSC MSC_SCAN 46
09:27:09.781365: EV_KEY KEY_C (0x2e) pressed
09:27:09.781365: EV_SYN code=0 value=0
09:27:09.853143: EV_MSC MSC_SCAN 46
09:27:09.853143: EV_KEY KEY_C (0x2e) released
09:27:09.853143: EV_SYN code=0 value=0
...
yaegashi
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