Add backlight

README.md

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+# PinePhone Toolkit
+A collection of tools and utility apps for the PINE64 PinePhone.
+
+## Components
+### backlight
+Get / Set backlight brightness.
+
+## Building
+```
+$ meson build
+$ cd build
+$ ninja
+```

meson.build

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+project('PinePhone Toolkit', 'c')
+executable('pptk-backlight', ['src/backlight.c'])

src/backlight.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+static char *argv0 = "./backlight";
+static char *backlight_file = "/sys/devices/platform/backlight/backlight/backlight/brightness";
+
+void print_usage()
+{
+    printf("usage: %s COMMAND [VALUE]\n", argv0);
+    printf("  COMMAND = {get, set}\n");
+    printf("    get ... prints current backlight level\n");
+    printf("    set ... sets backlight level to VALUE\n");
+    printf("  VALUE = [0..10] ... backlight amount\n");
+}
+
+void print_usage_and_fail()
+{
+    print_usage();
+    exit(EXIT_FAILURE);
+}
+
+void command_get()
+{
+    FILE *fp = fopen(backlight_file, "r");
+    if (fp == NULL) {
+        fprintf(stderr, "Cannot open file '%s' for reading.\n", backlight_file);
+        exit(EXIT_FAILURE);
+    }
+    int buffer_length = 255;
+    char buffer[buffer_length];
+    while (fgets(buffer, buffer_length, fp)) {
+        printf("%s", buffer);
+    }
+    fclose(fp);
+}
+
+void command_set(int value)
+{
+    if (setuid(0)) {
+        fprintf(stderr, "Cannot set root permissions via setuid(0).\n");
+        exit(EXIT_FAILURE);
+    }
+
+    FILE *fp = fopen(backlight_file, "w+");
+    if (fp == NULL) {
+        fprintf(stderr, "Cannot open file '%s' for writing.\n", backlight_file);
+        exit(EXIT_FAILURE);
+    }
+    fprintf(fp, "%d\n", value);
+    fclose(fp);
+}
+
+int main(int argc, char *argv[])
+{
+    if (argc < 1) {
+        print_usage_and_fail();
+    }
+    argv0 = argv[0];
+    if (argc < 2) {
+        print_usage_and_fail();
+    }
+    char *command = argv[1];
+    if ((strcmp(command, "get") == 0) && (argc == 2)) {
+        command_get();
+    } else if ((strcmp(command, "set") == 0) && (argc == 3)) {
+        int value = atoi(argv[2]);
+        if (value < 0 || value > 10) {
+            print_usage_and_fail();
+        }
+        command_set(value);
+    } else {
+        print_usage_and_fail();
+    }
+    exit(EXIT_SUCCESS);
+}