diff --git a/src/platform.h b/src/platform.h
index 36494c13..ad02575b 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -40,6 +40,7 @@ typedef unsigned __int64 uint64_t;
 
 #else
 #  include <inttypes.h>
+#  include <unistd.h>  // Used by sysconf(_SC_NPROCESSORS_ONLN)
 #endif
 
 #if !defined(_WIN32) && !defined(_WIN64) // Linux - Unix