diff --git a/runtime/pb/c/c_runtime.c b/runtime/pb/c/c_runtime.c
index e97ff3bdbe174b89d3866ad75d43df39fdedeae8..b540ce1cf0b83e660d43b870a7bedb02b94a8baf 100644
--- a/runtime/pb/c/c_runtime.c
+++ b/runtime/pb/c/c_runtime.c
@@ -10,6 +10,12 @@  *
  * Conventions follow docs/specs/c_accel_strategy.md.
  */
 
+/* strdup is POSIX, not C99; declare we want the POSIX surface from
+ * <string.h> before any system header pulls it in. */
+#ifndef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809L
+#endif
+
 #include <module.h>
 #include <lauxlib.h>