diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index 5e52e78..0440871 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -12,6 +12,11 @@
 #endif /* __STDC__ */
 #endif /* _MSC_VER */
 
+#ifdef _OSF_SOURCE
+/* OSF1 5.1 doesn't make this available with XOPEN_SOURCE_EXTENDED defined */
+extern double copysign(double, double);
+#endif
+
 /* Call is_error when errno != 0, and where x is the result libm
  * returned.  is_error will usually set up an exception and return
  * true (1), but may return false (0) without setting up an exception.
diff --git a/Objects/floatobject.c b/Objects/floatobject.c
index 340b0e7..fc4dd21 100644
--- a/Objects/floatobject.c
+++ b/Objects/floatobject.c
@@ -16,6 +16,11 @@
 extern double pow(double, double);
 #endif
 
+#ifdef _OSF_SOURCE
+/* OSF1 5.1 doesn't make this available with XOPEN_SOURCE_EXTENDED defined */
+extern int finite(double);
+#endif
+
 /* Special free list -- see comments for same code in intobject.c. */
 #define BLOCK_SIZE	1000	/* 1K less typical malloc overhead */
 #define BHEAD_SIZE	8	/* Enough for a 64-bit pointer */