diff --git a/libc/include/sys/ucontext.h b/libc/include/sys/ucontext.h
index 68b218e..72b8adb 100644
--- a/libc/include/sys/ucontext.h
+++ b/libc/include/sys/ucontext.h
@@ -320,6 +320,7 @@
 #define REG_RA 1
 #define REG_SP 2
 #define REG_TP 4
+#define REG_S0 8
 #define REG_A0 10
 
 typedef unsigned long __riscv_mc_gp_state[NGREG];