diff --git a/mk/unittest.mk b/mk/unittest.mk
index ea397f21b5..737840dd5d 100644
--- a/mk/unittest.mk
+++ b/mk/unittest.mk
@@ -1,19 +1,22 @@
 #
-# Copyright 2022 the Pacemaker project contributors
+# Copyright 2022-2024 the Pacemaker project contributors
 #
 # The version control history for this file may have further details.
 #
 # This source code is licensed under the GNU General Public License version 2
 # or later (GPLv2+) WITHOUT ANY WARRANTY.
 #
 
 AM_CPPFLAGS = -I$(top_builddir)/include		\
 	      -I$(top_srcdir)/include		\
 	      -I$(top_srcdir)/lib/common
 
 AM_CFLAGS = -DPCMK__UNIT_TESTING
+# Add -fno-builtin and -fno-inline to allow mocking realloc.
+AM_CFLAGS += -fno-builtin
+AM_CFLAGS += -fno-inline
 
 AM_LDFLAGS = $(LDFLAGS_WRAP)
 
 LDADD = $(top_builddir)/lib/common/libcrmcommon_test.la \
 	-lcmocka