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<h1 class="menu-title">Notes</h1>
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<h1 id="linux-input"><a class="header" href="#linux-input">Linux input</a></h1>
<p>Some notes on using <code>/dev/input/*</code> device driver files.</p>
<h2 id="mousex--mice"><a class="header" href="#mousex--mice">mouseX / mice</a></h2>
<p>These device files are created by the <a href="https://elixir.bootlin.com/linux/latest/source/drivers/input/mousedev.c#L842">mousedev</a> driver.</p>
<ul>
<li><code>/dev/input/mouseX</code> represents the input stream for a <em>SINGLE</em> mouse device.</li>
<li><code>/dev/input/mice</code> represents the merged input stream for <em>ALL</em> mouse devices.</li>
</ul>
<p>The data stream consists of <code>3 bytes</code> per <code>event</code>. An event is encoded as <code>(BTN, X, Y)</code>.</p>
<ul>
<li><code>BTN</code> button pressed</li>
<li><code>X</code> movement in x-direction <code>-1 -> left</code> and <code>1 -> right</code></li>
<li><code>Y</code> movement in y-direction <code>-1 -> down</code> and <code>1 -> up</code></li>
</ul>
<p>The raw data stream can be inspected as follows.</p>
<pre><code class="language-bash">sudo cat /dev/input/mice | od -tx1 -w3 -v
</code></pre>
<h2 id="eventx"><a class="header" href="#eventx">eventX</a></h2>
<p>These device files are created by the <a href="https://elixir.bootlin.com/linux/latest/source/drivers/input/evdev.c#L1337">evdev</a> driver.</p>
<ul>
<li><code>/dev/input/eventX</code> represents the generic input event interface a <em>SINGLE</em> input device.</li>
</ul>
<p>Input events are encoded as given by the <code>input_event</code> struct below. Reading
from the <code>eventX</code> device file will always yield whole number of input events.</p>
<pre><code class="language-c">struct input_event {
struct timeval time;
unsigned short type;
unsigned short code;
unsigned int value;
};
</code></pre>
<p>On most 64bit machines the raw data stream can be inspected as follows.</p>
<pre><code class="language-bash">sudo cat /dev/input/event4 | od -tx1 -w24 -v
</code></pre>
<h2 id="identifying-device-files"><a class="header" href="#identifying-device-files">Identifying device files.</a></h2>
<p>To find out which device file is assigned to which input device the following
file <code>/proc/bus/input/devices</code> in the proc filesystem can be consulted.</p>
<p>This yields entries as follows and shows which <code>Handlers</code> are assigned to which
<code>Name</code>.</p>
<pre><code>I: Bus=0018 Vendor=04f3 Product=0033 Version=0000
N: Name="Elan Touchpad"
...
H: Handlers=event15 mouse0
...
</code></pre>
<h2 id="example-toying-with-devinputeventx"><a class="header" href="#example-toying-with-devinputeventx">Example: Toying with <code>/dev/input/eventX</code></a></h2>
<p>Once compiled, the example should be run as <code>sudo ./event /dev/input/eventX</code>.</p>
<pre><code class="language-c">#include <stdio.h>
#include <fcntl.h>
#include <assert.h>
#include <unistd.h>
#include <time.h>
#include <sys/time.h>
#include <linux/input-event-codes.h>
struct input_event {
struct timeval time;
unsigned short type;
unsigned short code;
unsigned int value;
};
const char* type(unsigned short t) {
static char buf[32];
const char* fmt = "0x%x";
switch (t) {
#define FMT(TYPE) case TYPE: fmt = #TYPE"(0x%x)"; break
FMT(EV_SYN);
FMT(EV_KEY);
FMT(EV_REL);
FMT(EV_ABS);
#undef FMT
}
snprintf(buf, sizeof(buf), fmt, t);
return buf;
}
const char* code(unsigned short c) {
static char buf[32];
const char* fmt = "0x%x";
switch (c) {
#define FMT(CODE) case CODE: fmt = #CODE"(0x%x)"; break
FMT(BTN_LEFT);
FMT(BTN_RIGHT);
FMT(BTN_MIDDLE);
FMT(REL_X);
FMT(REL_Y);
#undef FMT
}
snprintf(buf, sizeof(buf), fmt, c);
return buf;
}
const char* timefmt(const struct timeval* t) {
assert(t);
struct tm* lt = localtime(&t->tv_sec); // Returns pointer to static tm object.
static char buf[64];
strftime(buf, sizeof(buf), "%H:%M:%S", lt);
return buf;
}
int main(int argc, char* argv[]) {
assert(argc == 2);
int fd = open(argv[1], O_RDONLY);
assert(fd != -1);
struct input_event inp;
while (1) {
int ret = read(fd, &inp, sizeof(inp));
assert(ret == sizeof(inp));
printf("time: %s type: %s code: %s value: 0x%x\n",
timefmt(&inp.time), type(inp.type), code(inp.code), inp.value);
}
}
</code></pre>
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