diff --git a/src/makefiles/Makefile.hpux b/src/makefiles/Makefile.hpux
index 1917d61..f2a8f19 100644
--- a/src/makefiles/Makefile.hpux
+++ b/src/makefiles/Makefile.hpux
@@ -43,6 +43,12 @@ else
    CFLAGS_SL = +Z
 endif
 
+
+# HP-UX libm functions on 'Integrity server' do not set errno by default,
+# for errno setting, compile with the +Olibmerrno option.
+
+CFLAGS := +Olibmerrno $(CFLAGS)
+
 # Rule for building a shared library from a single .o file
 %$(DLSUFFIX): %.o
 ifeq ($(GCC), yes)