diff --git a/math_private.h b/math_private.h
--- a/math_private.h
+++ b/math_private.h
@@ -20,16 +20,24 @@
 #include <cfloat>
 #include <stdint.h>
 #include <sys/types.h>
 
 #include "mozilla/EndianUtils.h"
 
 #include "fdlibm.h"
 
+/*
+ * Emulate FreeBSD internal double types.
+ * Adapted from https://github.com/freebsd/freebsd-src/search?q=__double_t
+ */
+
+typedef double      __double_t;
+typedef __double_t  double_t;
+
 /*
  * The original fdlibm code used statements like:
  *	n0 = ((*(int*)&one)>>29)^1;		* index of high word *
  *	ix0 = *(n0+(int*)&x);			* high word of x *
  *	ix1 = *((1-n0)+(int*)&x);		* low word of x *
  * to dig two 32 bit words out of the 64 bit IEEE floating point
  * value.  That is non-ANSI, and, moreover, the gcc instruction
  * scheduler gets it wrong.  We instead use the following macros.