demos: ensure DISPLAY envar is valid

If the DISPLAY environment variable is unset or null, the cube demo will
emit "Cannot find a compatible Vulkan installable client driver (ICD)."
That message is not helpful when attempting to resolve the issue, since
the problem is with the envar, not the ICD.

Change-Id: Idf145cf0da906a4a8352b5bb136f88d9c1bca90d

2 files changed, 27 insertions, 0 deletions

diff --git a/demos/cube.c b/demos/cube.c
index e5b10b05..5fb79c9a 100644
--- a/demos/cube.c
+++ b/demos/cube.c
@@ -2504,6 +2504,12 @@ static void demo_create_window(struct demo *demo) {
 }
 #elif defined(VK_USE_PLATFORM_XLIB_KHR)
 static void demo_create_xlib_window(struct demo *demo) {
+    const char *display_envar = getenv("DISPLAY");
+    if (display_envar == NULL || display_envar[0] == '\0') {
+        printf("Environment variable DISPLAY requires a valid value.\nExiting ...\n");
+        fflush(stdout);
+        exit(1);
+    }
 
     XInitThreads();
     demo->display = XOpenDisplay(NULL);
@@ -3764,6 +3770,13 @@ static void demo_init_connection(struct demo *demo) {
     xcb_screen_iterator_t iter;
     int scr;
 
+    const char *display_envar = getenv("DISPLAY");
+    if (display_envar == NULL || display_envar[0] == '\0') {
+        printf("Environment variable DISPLAY requires a valid value.\nExiting ...\n");
+        fflush(stdout);
+        exit(1);
+    }
+
     demo->connection = xcb_connect(NULL, &scr);
     if (xcb_connection_has_error(demo->connection) > 0) {
         printf("Cannot find a compatible Vulkan installable client driver "
diff --git a/demos/cube.cpp b/demos/cube.cpp
index 1cd32052..f436cd56 100644
--- a/demos/cube.cpp
+++ b/demos/cube.cpp
@@ -971,6 +971,13 @@ Demo::Demo()
         xcb_screen_iterator_t iter;
         int scr;
 
+        const char *display_envar = getenv("DISPLAY");
+        if (display_envar == nullptr || display_envar[0] == '\0') {
+            printf("Environment variable DISPLAY requires a valid value.\nExiting ...\n");
+            fflush(stdout);
+            exit(1);
+        }
+
         connection = xcb_connect(nullptr, &scr);
         if (xcb_connection_has_error(connection) > 0) {
             printf(
@@ -2438,6 +2445,13 @@ Demo::Demo()
 #elif defined(VK_USE_PLATFORM_XLIB_KHR)
 
     void Demo::create_xlib_window() {
+        const char *display_envar = getenv("DISPLAY");
+        if (display_envar == nullptr || display_envar[0] == '\0') {
+            printf("Environment variable DISPLAY requires a valid value.\nExiting ...\n");
+            fflush(stdout);
+            exit(1);
+        }
+
         XInitThreads();
         display = XOpenDisplay(nullptr);
         long visualMask = VisualScreenMask;