const
diff --git a/src/libthread/threadimpl.h b/src/libthread/threadimpl.h
index a658903..3364465 100644
--- a/src/libthread/threadimpl.h
+++ b/src/libthread/threadimpl.h
@@ -44,7 +44,7 @@
 
 #if defined(__arm__)
 int getmcontext(mcontext_t*);
-void setmcontext(mcontext_t*);
+void setmcontext(const mcontext_t*);
 #define	setcontext(u)	setmcontext(&(u)->uc_mcontext)
 #define	getcontext(u)	getmcontext(&(u)->uc_mcontext)
 #endif