Add basic backlight percentage support

At the moment linux only, but will add support for OpenBSD as well.

Makefile

@@ -6,6 +6,7 @@ include config.mk
 
 REQ = util
 COM =\
+	components/backlight\
 	components/battery\
 	components/cpu\
 	components/datetime\

README

@@ -6,6 +6,7 @@ slstatus is a suckless status monitor for window managers that use WM_NAME
 
 Features
 --------
+- Backlight percentage
 - Battery percentage/state/time left
 - CPU usage
 - CPU frequency

components/backlight.c

@@ -0,0 +1,32 @@
+/* See LICENSE file for copyright and license details. */
+#include "../util.h"
+
+#if defined(__linux__)
+	#include <limits.h>
+
+	#define BRIGHTNESS_MAX "/sys/class/backlight/%s/max_brightness"
+	#define BRIGHTNESS_CUR "/sys/class/backlight/%s/brightness"
+
+	const char *
+	backlight_perc(const char *card)
+	{
+		char path[PATH_MAX];
+		int max, cur;
+
+		if (esnprintf(path, sizeof (path), BRIGHTNESS_MAX, card) < 0 ||
+			pscanf(path, "%d", &max) != 1) {
+			return NULL;
+		}
+
+		if (esnprintf(path, sizeof (path), BRIGHTNESS_CUR, card) < 0 ||
+			pscanf(path, "%d", &cur) != 1) {
+			return NULL;
+		}
+
+		if (max == 0) {
+			return NULL;
+		}
+
+		return bprintf("%d", cur * 100 / max);
+	}
+#endif

config.def.h

@@ -12,6 +12,8 @@ static const char unknown_str[] = "n/a";
 /*
  * function             description                     argument (example)
  *
+ * backlight_perc       backlight percentage            device name
+ *                                                      (intel_backlight)
  * battery_perc         battery percentage              battery name (BAT0)
  *                                                      NULL on OpenBSD
  * battery_state        battery charging state          battery name (BAT0)

slstatus.h

@@ -1,5 +1,8 @@
 /* See LICENSE file for copyright and license details. */
 
+/* backlight */
+const char *backlight_perc(const char *);
+
 /* battery */
 const char *battery_perc(const char *);
 const char *battery_state(const char *);

util.h

@@ -1,4 +1,6 @@
 /* See LICENSE file for copyright and license details. */
+#include <stddef.h>
+
 extern char buf[1024];
 
 #define LEN(x) (sizeof (x) / sizeof *(x))