diff --git a/lib/Makefile.am b/lib/Makefile.am
index 8738b97..2faca1f 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -17,7 +17,7 @@ libcomedi_la_SOURCES = \
 	buffer.c calib.c cmd.c comedi.c data.c dio.c error.c \
 	filler.c get.c ioctl.c range.c sv.c timed.c timer.c
 
-libcomedi_la_CFLAGS = $(COMEDILIB_CFLAGS)
+libcomedi_la_CFLAGS = $(COMEDILIB_CFLAGS) -D_REENTRANT
 libcomedi_la_LDFLAGS = -Wl,--version-script=$(srcdir)/version_script
 
 calib_yacc.c: $(srcdir)/calib_yacc.y