diff --git a/lib/url.c b/lib/url.c
index 8947d8e3b95..511c0a7e20b 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -18,10 +18,13 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301  USA
  */
 
+/* _GNU_SOURCE must stay defined for the rest of this file. On musl libc
+ * the feature-test macros are re-evaluated by every system header, so
+ * undefining it here would hide the clock_gettime()/ctime_r() prototypes
+ * that ut.h and dprint.h (via <time.h>) rely on, breaking the build. */
 #define _GNU_SOURCE
 #include <stdio.h>
 #include <string.h>
-#undef _GNU_SOURCE
 
 #include "../mem/mem.h"
 #include "../ut.h"
diff --git a/modules/mathops/math_funcs.c b/modules/mathops/math_funcs.c
index 696e6b2dad0..29a205dbc14 100644
--- a/modules/mathops/math_funcs.c
+++ b/modules/mathops/math_funcs.c
@@ -32,11 +32,10 @@
 #include <stdlib.h>
 #include <math.h>
 
-#ifdef _ADDED_XOPEN
-#undef _ADDED_XOPEN
-#undef _XOPEN_SOURCE
-#undef _GNU_SOURCE
-#endif
+/* The feature-test macros above must stay defined for the OpenSIPS headers
+ * below.  musl evaluates them in every system header, so undefining them
+ * here leaves the translation unit with none of them set and takes ctime_r()
+ * out of <time.h>, which dprint.h needs. */
 
 #include <errno.h>
 #include <ctype.h>
diff --git a/modules/mathops/mathops.c b/modules/mathops/mathops.c
index 461f5d32072..ec7d9c4a18e 100644
--- a/modules/mathops/mathops.c
+++ b/modules/mathops/mathops.c
@@ -32,11 +32,10 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#ifdef _ADDED_XOPEN
-#undef _ADDED_XOPEN
-#undef _XOPEN_SOURCE
-#undef _GNU_SOURCE
-#endif
+/* The feature-test macros above must stay defined for the OpenSIPS headers
+ * below.  musl evaluates them in every system header, so undefining them
+ * here leaves the translation unit with none of them set and takes ctime_r()
+ * out of <time.h>, which dprint.h needs. */
 
 #include "../../sr_module.h"
 #include "../../dprint.h"