1 files changed, 15 insertions, 0 deletions
diff --git a/missing/isnan.c b/missing/isnan.c
index 5846947f50..ed10bf5cd6 100644
--- a/missing/isnan.c
+++ b/missing/isnan.c
@@ -2,6 +2,20 @@
 
 #include "ruby/missing.h"
 
+/*
+ * isnan() may be a macro, a function or both.
+ * (The C99 standard defines that isnan() is a macro, though.)
+ * http://www.gnu.org/software/automake/manual/autoconf/Function-Portability.html
+ *
+ * macro only: uClibc
+ * both: GNU libc
+ *
+ * This file is compile if no isnan() function is available.
+ * (autoconf AC_REPLACE_FUNCS detects only the function.)
+ * The macro is detected by following #ifndef.
+ */
+
+#ifndef isnan
 static int double_ne(double n1, double n2);
 
 int
@@ -15,3 +29,4 @@ double_ne(double n1, double n2)
 {
     return n1 != n2;
 }
+#endif