nan: link to SO answer

Author: cirosantilli

c/nan.c

@@ -1,3 +1,5 @@
+/* https://stackoverflow.com/questions/18118408/what-is-difference-between-quiet-nan-and-signaling-nan/55648118#55648118 */
+
 #include <assert.h>
 #include <math.h> /* NAN, isnan */
 #include <stdio.h>

cpp/nan.cpp

@@ -1,3 +1,5 @@
+// https://stackoverflow.com/questions/18118408/what-is-difference-between-quiet-nan-and-signaling-nan/55648118#55648118
+//
 // C++ has some NaN functionality that C does not have,
 // we will illustrate those here.
 

glibc/interactive/feenableexcept.cpp

@@ -1,3 +1,5 @@
+// https://stackoverflow.com/questions/18118408/what-is-difference-between-quiet-nan-and-signaling-nan/55648118#55648118
+//
 // In C++ for signling_NaN.
 //
 // In GCC 8.2, sNaN is only done in -O0 and completely skipped in -O3.