--- a/src/runtime/bsd-os.h
+++ b/src/runtime/bsd-os.h
@@ -75,6 +75,8 @@
 typedef ucontext_t os_context_t;
 #define SIG_MEMORY_FAULT SIGSEGV
 
+#define SIG_STOP_FOR_GC (SIGUSR2)
+
 #elif defined LISP_FEATURE_DARWIN
 #include "darwin-os.h"
 #else