Add Readme

README.md

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+# GPIO_SYS
+
+Gpio driver using sys file systeme.g.:
+
+```shell
+echo "4" > /sys/class/gpio/export
+echo "out" > /sys/class/gpio/gpio4/direction
+echo "1"> /sys/class/gpio/gpio4/value
+echo "4" > /sys/class/gpio/unexport
+```
+
+## Installation
+
+```shell
+make install
+```
+
+Installs `libgpio_sys.a` and `libgpio_sys.so` in the directory `$(PREFIX)/lib/`. It also installs the include header file `gpio.h` in the directory `$(PREFIX)/include/gpio_sys/`. The default value for `PREFIX` is: `PREFIX=usr`. To use another location for installation changing the value of the `PREFIX` variable is needed. E.g.:
+
+```shell
+PREFIX=/usr/local/ make install
+```
+
+## Uninstallation
+
+```shell
+make install
+```
+
+Removes the files installed with the `install` make target.  
+> **NOTE:** If `PREFIX` variable was changed during installation process the same value needs to be set for uninstallation. E.g.:
+
+```shell
+PREFIX=/usr/local/ make install
+```
+
+## Example
+
+```C
+    #include <stdio.h>
+    #include <stdlib.h>
+    #include <unistd.h>
+    #include <syslog.h>
+    #include <stdbool.h>
+    #include <gpio_sys/gpio.h>
+
+
+    static const struct gpio_sys gpio_4 = {
+        .pin = 4,
+        .direction = 0, // out
+    };
+
+    int main(void)
+    {
+        int res, cnt;
+        int value;
+
+        openlog("gpio_sys", LOG_CONS | LOG_PID | LOG_NDELAY, LOG_LOCAL1);
+
+        res = gpio_open(&gpio_4);
+        if(res != EXIT_SUCCESS) {
+            syslog(LOG_ERR, "Unable to open gpio %d\n", gpio_4.pin);
+            return res;
+        }
+
+        gpio_write(&gpio_4, 1);
+        for(cnt = 0; cnt < 10; cnt++) {
+            if(cnt % 2) {
+                gpio_write(&gpio_4, 1);
+            } else {
+                gpio_write(&gpio_4, 0);
+            }
+            value = gpio_read(&gpio_4);
+            if(value < 0) {
+                syslog(LOG_ERR, "Unable to read from gpio %d\n", gpio_4.pin);
+                return EXIT_FAILURE;
+            } else {
+                printf("Gpio 4: %d\n", value);
+            }
+            sleep(1);
+        }
+        gpio_close(&gpio_4);
+
+        return EXIT_SUCCESS;
+    }
+```
+
+## Additional make targets
+
+| Target          | Meaning                                                     |
+| --------------- | ----------------------------------------------------------- |
+| all             | Build the code, assemble the static and shared library.     |
+| install         | Install the libraries and necessary include header files.   |
+| uninstall       | Uninstall the libraries and necessary include header files. |
+| clean           | Clean up build artifacts.                                   |
+| build_unit_test | Build the unit tests.                                       |
+| exec_unit_test  | Execute the unit tests.                                     |
+| coverage        | Determine code coverage of the unit tests.                  |
+| check           | Static code analysis (cppcheck).                            |