Use the XLib DPMS extension from C wrappers instead of subprocess calls

xdg/display/display.go

@@ -1,48 +1,74 @@
 package display
 
+// #cgo CFLAGS: --std=gnu99 -pedantic -Wall
+// #cgo LDFLAGS: -lX11 -lXext
+// #include "dpms.h"
+// #include <stdlib.h>
+// #include <stdbool.h>
+import "C"
 import (
-	"os/exec"
+	"unsafe"
-	"regexp"
+	"errors"
-	"strings"
+	"os"
 )
 
 
-// Xset monitors/controls the display using xset
+// Xset monitors/controls the display using the DPMS X extension
 type Xset struct {}
 
 // Suspend the display
 func (Xset) Suspend() error {
-	cmd := exec.Command("xset",
+	xDisplayName := C.CString(tryGetXDisplay())
-		"dpms",
+	defer C.free(unsafe.Pointer(xDisplayName))
-		"force",
+
-		"suspend",
+	err := C.turn_display_off(xDisplayName)
-	)
+	if err != C.int(0) {
-	return cmd.Run()
+		return convertErrorIntToGoError(err)
+	}
+
+	return nil
+}
+
+func tryGetXDisplay() string {
+	display := os.Getenv("DISPLAY")
+	if display == "" {
+		panic(errors.New("DISPLAY envvar not set"))
+	}
+	return display
+}
+
+func convertErrorIntToGoError(err C.int) error {
+	errorMsg := ""
+	switch err {
+	case C.BADDISPLAY:
+		errorMsg = "Failed to open X display"
+	case C.NODPMSEXTENSION:
+		errorMsg = "DPMS extension is not available"
+	case C.NOTDPMSCAPABLE:
+		errorMsg = "Display is not DPMS capable"
+	case C.UNKNOWNDPMSMODE:
+		errorMsg = "Reported DPMS state unknown"
+	case C.DPMSDISABLED:
+		errorMsg = "DPMS is disabled"
+	default:
+		errorMsg = "Unknown error from dpms.h"
+	}
+	return errors.New(errorMsg)
 }
 
 // IsOn determines if the display is on
 func (Xset) IsOn() (bool, error) {
-	cmd := exec.Command("xset", "q")
+	xDisplayName := C.CString(tryGetXDisplay())
-	outputBytes, err := cmd.CombinedOutput()
+	defer C.free(unsafe.Pointer(xDisplayName))
-	if err != nil {
+	var isDisplayOn C.bool
-		return false, err
+
-	}
+	err := C.is_display_on(xDisplayName, &isDisplayOn)
-	return determineIfDisplayIsOn(string(outputBytes)), nil
+	if err != C.int(0) {
+		return false, convertErrorIntToGoError(err)
 	}
 
-func determineIfDisplayIsOn(xsetOutput string) bool {
+	if isDisplayOn {
-	if ! strings.Contains(xsetOutput, "Monitor") {
+		return true, nil
-		// after booting xset q is missing this line
-		// lacking better ideas assume display is on
-		return true
 	}
-
+	return false, nil
-	re := regexp.MustCompile(`Monitor is (\w+)`)
-	matches := re.FindStringSubmatch(xsetOutput)
-	if len(matches) >= 2 {
-		if matches[1] == "On" {
-			return true
-		}
-	}
-	return false
 }

xdg/display/dpms.c

@@ -0,0 +1,130 @@
+/*
+* Based on xorg-xset
+* https://github.com/freedesktop/xorg-xset/blob/master/xset.c
+*/
+
+#include "dpms.h"
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <unistd.h>
+#include <stdbool.h>
+#include <string.h>
+
+#include <X11/Xlib.h>
+#include <X11/extensions/dpms.h>
+
+
+int check_dpms_extension(Display* disp)
+{
+    int err = 0;
+    int dummy;
+
+    if (DPMSQueryExtension(disp, &dummy, &dummy))
+    {
+        if (!DPMSCapable(disp))
+            err = NOTDPMSCAPABLE;
+    }
+    else
+        err = NODPMSEXTENSION;
+
+    return err;
+}
+
+/*
+* status->state can result in:
+*  - DPMSModeOn
+*  - DPMSModeStandby
+*  - DPMSModeSuspend
+*  - DPMSModeOff
+*
+* anything else means unknown state
+*/
+int check_dpms_status(Display* disp, CARD16* state)
+{
+    BOOL dpmsEnabled;
+
+    int error = check_dpms_extension(disp);
+    if (!error)
+    {
+        DPMSInfo(disp, state, &dpmsEnabled);
+        if (!dpmsEnabled)
+            return DPMSDISABLED;
+    }
+    else
+        return error;
+
+    return 0;
+}
+
+int is_display_on(const char* xDisplayName, bool* isOn)
+{
+    Display* disp = XOpenDisplay(xDisplayName);
+    if (disp == NULL)
+        return BADDISPLAY;
+
+    CARD16 state;
+    int returnError = 0;
+    int error = check_dpms_status(disp, &state);
+
+    if (!error)
+    {
+        switch (state)
+        {
+        case DPMSModeOn:
+            *isOn = true;
+            break;
+        case DPMSModeStandby: case DPMSModeSuspend: case DPMSModeOff:
+            *isOn = false;
+            break;
+        default:
+            returnError = UNKNOWNDPMSMODE;
+            break;
+        }
+    }
+    else if (error == DPMSDISABLED)
+        *isOn = true;
+    else if (error)
+        returnError = error;
+
+    XCloseDisplay(disp);
+    return returnError;
+}
+
+/*
+* Citing the original xorg-xset:
+*
+* The calls to usleep below are necessary to
+* delay the actual DPMS mode setting briefly.
+* Without them, it's likely that the mode will be
+* set between the Down and Up key transitions, in
+* which case the Up transition may immediately
+* turn the display back on.
+*/
+int dpms_force_off(Display* disp)
+{
+    int error = check_dpms_extension(disp);
+    if (!error)
+    {
+        DPMSEnable(disp);
+        usleep(100000);
+        DPMSForceLevel(disp, DPMSModeOff);
+    }
+    return error;
+}
+
+
+int turn_display_off(const char* xDisplayName)
+{
+    Display* disp = XOpenDisplay(xDisplayName);
+    if (disp == NULL)
+        return BADDISPLAY;
+
+    int returnError = 0;
+    int error = dpms_force_off(disp);
+    if (error)
+        returnError = error;
+
+    XCloseDisplay(disp);
+    return returnError;
+}

xdg/display/dpms.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <stdbool.h>
+
+
+#define BADDISPLAY -1
+#define DPMSDISABLED -2
+#define NODPMSEXTENSION -3
+#define NOTDPMSCAPABLE -4
+#define UNKNOWNDPMSMODE -5
+
+
+int turn_display_off(const char* xDisplayName);
+int is_display_on(const char* xDisplayName, bool* isOn);