diff --git a/minui/graphics.cpp b/minui/graphics.cpp
index c1aab41..cc02e9e 100644
--- a/minui/graphics.cpp
+++ b/minui/graphics.cpp
@@ -28,7 +28,7 @@
 #include "graphics_fbdev.h"
 #include "minui/minui.h"
 
-static GRFont* gr_font = NULL;
+static GRFont* gr_font = nullptr;
 static MinuiBackend* gr_backend = nullptr;
 
 static int overscan_percent = OVERSCAN_PERCENT;
@@ -38,7 +38,8 @@
 static uint32_t gr_current = ~0;
 static constexpr uint32_t alpha_mask = 0xff000000;
 
-static GRSurface* gr_draw = NULL;
+// gr_draw is owned by backends.
+static const GRSurface* gr_draw = nullptr;
 static GRRotation rotation = ROTATION_NONE;
 
 static bool outside(int x, int y) {
@@ -86,7 +87,7 @@
   return (out_r & 0xff) | (out_g & 0xff00) | (out_b & 0xff0000) | (gr_current & 0xff000000);
 }
 
-// increments pixel pointer right, with current rotation.
+// Increments pixel pointer right, with current rotation.
 static void incr_x(uint32_t** p, int row_pixels) {
   if (rotation % 2) {
     *p = *p + (rotation == 1 ? 1 : -1) * row_pixels;
@@ -95,7 +96,7 @@
   }
 }
 
-// increments pixel pointer down, with current rotation.
+// Increments pixel pointer down, with current rotation.
 static void incr_y(uint32_t** p, int row_pixels) {
   if (rotation % 2) {
     *p = *p + (rotation == 1 ? -1 : 1);
@@ -104,8 +105,8 @@
   }
 }
 
-// returns pixel pointer at given coordinates with rotation adjustment.
-static uint32_t* pixel_at(GRSurface* surf, int x, int y, int row_pixels) {
+// Returns pixel pointer at given coordinates with rotation adjustment.
+static uint32_t* pixel_at(const GRSurface* surf, int x, int y, int row_pixels) {
   switch (rotation) {
     case ROTATION_NONE:
       return reinterpret_cast<uint32_t*>(surf->data) + y * row_pixels + x;
@@ -172,7 +173,7 @@
 }
 
 void gr_texticon(int x, int y, GRSurface* icon) {
-  if (icon == NULL) return;
+  if (icon == nullptr) return;
 
   if (icon->pixel_bytes != 1) {
     printf("gr_texticon: source has wrong format\n");
@@ -243,7 +244,7 @@
 }
 
 void gr_blit(GRSurface* source, int sx, int sy, int w, int h, int dx, int dy) {
-  if (source == NULL) return;
+  if (source == nullptr) return;
 
   if (gr_draw->pixel_bytes != source->pixel_bytes) {
     printf("gr_blit: source has wrong format\n");
@@ -275,8 +276,7 @@
     unsigned char* src_p = source->data + sy * source->row_bytes + sx * source->pixel_bytes;
     unsigned char* dst_p = gr_draw->data + dy * gr_draw->row_bytes + dx * gr_draw->pixel_bytes;
 
-    int i;
-    for (i = 0; i < h; ++i) {
+    for (int i = 0; i < h; ++i) {
       memcpy(dst_p, src_p, w * source->pixel_bytes);
       src_p += source->row_bytes;
       dst_p += gr_draw->row_bytes;
@@ -284,15 +284,15 @@
   }
 }
 
-unsigned int gr_get_width(GRSurface* surface) {
-  if (surface == NULL) {
+unsigned int gr_get_width(const GRSurface* surface) {
+  if (surface == nullptr) {
     return 0;
   }
   return surface->width;
 }
 
-unsigned int gr_get_height(GRSurface* surface) {
-  if (surface == NULL) {
+unsigned int gr_get_height(const GRSurface* surface) {
+  if (surface == nullptr) {
     return 0;
   }
   return surface->height;
@@ -372,6 +372,10 @@
 
 void gr_exit() {
   delete gr_backend;
+  gr_backend = nullptr;
+
+  delete gr_font;
+  gr_font = nullptr;
 }
 
 int gr_fb_width() {
diff --git a/minui/include/minui/minui.h b/minui/include/minui/minui.h
index e96b7ae..ef4abe2 100644
--- a/minui/include/minui/minui.h
+++ b/minui/include/minui/minui.h
@@ -48,7 +48,13 @@
   ROTATION_LEFT = 3,
 };
 
+// Initializes the graphics backend and loads font file. Returns 0 on success, or -1 on error. Note
+// that the font initialization failure would be non-fatal, as caller may not need to draw any text
+// at all. Caller can check the font initialization result via gr_sys_font() as needed.
 int gr_init();
+
+// Frees the allocated resources. The function is idempotent, and safe to be called if gr_init()
+// didn't finish successfully.
 void gr_exit();
 
 int gr_fb_width();
@@ -57,7 +63,8 @@
 void gr_flip();
 void gr_fb_blank(bool blank);
 
-void gr_clear();  // clear entire surface to current color
+// Clears entire surface to current color.
+void gr_clear();
 void gr_color(unsigned char r, unsigned char g, unsigned char b, unsigned char a);
 void gr_fill(int x1, int y1, int x2, int y2);
 
@@ -66,16 +73,16 @@
 const GRFont* gr_sys_font();
 int gr_init_font(const char* name, GRFont** dest);
 void gr_text(const GRFont* font, int x, int y, const char* s, bool bold);
-// Return -1 if font is nullptr.
+// Returns -1 if font is nullptr.
 int gr_measure(const GRFont* font, const char* s);
-// Return -1 if font is nullptr.
+// Returns -1 if font is nullptr.
 int gr_font_size(const GRFont* font, int* x, int* y);
 
 void gr_blit(GRSurface* source, int sx, int sy, int w, int h, int dx, int dy);
-unsigned int gr_get_width(GRSurface* surface);
-unsigned int gr_get_height(GRSurface* surface);
+unsigned int gr_get_width(const GRSurface* surface);
+unsigned int gr_get_height(const GRSurface* surface);
 
-// Set rotation, flips gr_fb_width/height if 90 degree rotation difference
+// Sets rotation, flips gr_fb_width/height if 90 degree rotation difference
 void gr_rotate(GRRotation rotation);
 
 //