diff --git a/config.def.h b/config.def.h
index 8db8983..9ac3e9b 100644
--- a/config.def.h
+++ b/config.def.h
@@ -56,6 +56,9 @@ static const bool ALPHA_LAYER = false;
  *   3 means use 3% of total memory which is about 245MiB on 8GiB machine.
  *   0 or less means disable cache.
  * 100 means use all available memory (but not above CACHE_SIZE_LIMIT).
+ *
+ * NOTE: higher cache size means better image reloading performance, but also
+ * higher memory usage.
  */
 static const int CACHE_SIZE_MEM_PERCENTAGE = 3;          /* use 3% of total memory for cache */
 static const int CACHE_SIZE_LIMIT = 256 * 1024 * 1024;   /* but not above 256MiB */
diff --git a/image.c b/image.c
index beedcbd..ad2a254 100644
--- a/image.c
+++ b/image.c
@@ -49,14 +49,14 @@ enum { DEF_WEBP_DELAY = 75 };
 
 static int calc_cache_size(void)
 {
-	int cache;
-	long pages, page_size;
+	long cache, pages = -1, page_size = -1;
 
 	if (CACHE_SIZE_MEM_PERCENTAGE <= 0)
 		return 0;
-
+#ifdef _SC_PHYS_PAGES /* _SC_PHYS_PAGES isn't POSIX */
 	pages = sysconf(_SC_PHYS_PAGES);
 	page_size = sysconf(_SC_PAGE_SIZE);
+#endif
 	if (pages < 0 || page_size < 0)
 		return CACHE_SIZE_FALLBACK;
 	cache = (pages/100) * CACHE_SIZE_MEM_PERCENTAGE;