===== Rootless GPIO access: /dev/gpiomem =====
///dev/gpiomem// provides user with root-free access to the GPIO registers. Opening ///dev/gpiomem// device file and calling //mmap()// function will map the GPIO registers' memory map area to the user's pointer.\\
This driver is available in below's Ubuntu kernel versions:
  * ODROID-C2: 3.14.79-105 kernel version or later
  * ODROID-C0/C1+: 3.10.104-182 kernel version or later
  * ODROID-XU4: 4.9.x kernel version or later

==== How to enable gpiomem driver ====
///dev/gpiomem// driver is included as module by default. In order to use gpiomem driver, it is necessary to configure account access permission.
<code>
$ sudo addgroup gpio
$ sudo usermod -a -G gpio odroid    
$ cat /etc/udev/rules.d/90-gpiomem.rules 
</code>
  * **ODROID-C0/C1+/C2**
<code>
SUBSYSTEM=="meson-gpiomem", GROUP="gpio", MODE="0660"
</code>
  * **ODROID-XU4**
<code>
SUBSYSTEM=="exynos-gpiomem", GROUP="gpio", MODE="0660"
</code>
==== Example ====
Hardkernel's wiringPi library which is for GPIO control uses ///dev/gpiomem// device file for rootless gpio control. Usage is very simple. Just open ///dev/gpiomem// device file instead of ///dev/mem//.
  * [[https://github.com/hardkernel/wiringPi/blob/master/wiringPi/wiringPi.c#L2868-L2879|($HK_WIRINGPI)/wiringPi/wiringPi.c]]
<code>
  // Open the master /dev/memory device
  if (access("/dev/gpiomem",0) == 0) {
    if ((fd = open ("/dev/gpiomem", O_RDWR | O_SYNC | O_CLOEXEC) ) < 0)
      return wiringPiFailure (WPI_ALMOST, "wiringPiSetup: Unable to open /dev/gpiomem: %s\n", strerror (errno)) ;
  }
  else {
    if (geteuid () != 0)
      (void)wiringPiFailure (WPI_FATAL, "wiringPiSetup: Must be root. (Did you forget sudo?)\n") ;

    if ((fd = open ("/dev/mem", O_RDWR | O_SYNC | O_CLOEXEC) ) < 0)
      return wiringPiFailure (WPI_ALMOST, "wiringPiSetup: Unable to open /dev/mem: %s\n", strerror (errno)) ;
  }
</code>

===== Rootless GPIO SYS-filesystem: /sys/class/gpio =====
<code>
$ sudo addgroup gpio
$ sudo usermod -a -G gpio odroid    
$ cat /etc/udev/rules.d/90-odroid-sysfs.rules 
</code>

<code>
SUBSYSTEM=="gpio", KERNEL=="gpiochip*", ACTION=="add", PROGRAM="/bin/sh -c 'chown root:gpio /sys/class/gpio/export /sys/class/gpio/unexport ; chmod 220 /sys/class/gpio/export /sys/class/gpio/unexport'"
SUBSYSTEM=="gpio", KERNEL=="gpio*", ACTION=="add", PROGRAM="/bin/sh -c 'chown root:gpio /sys%p/active_low /sys%p/direction /sys%p/edge /sys%p/value ; chmod 660 /sys%p/active_low /sys%p/direction /sys%p/edge /sys%p/value'"
</code>