diff --git a/src/s/sco5.h b/src/s/sco5.h
index 418efaf75ab..5eea7955ee5 100644
--- a/src/s/sco5.h
+++ b/src/s/sco5.h
@@ -132,6 +132,13 @@ Boston, MA 02111-1307, USA.  */
 /* SCO Unix has Posix signals, but in 3.2.5 something broken that causes
  * all keyboard-quit signals to be lost after the first one. */
 #undef POSIX_SIGNALS
+
+#define SIGMASKTYPE long
+
+#ifndef NOT_C_CODE
+extern SIGMASKTYPE sigprocmask_set;
+#endif /* not NOT_C_CODE */
+
 #define sigblock(sig)					\
      (sigprocmask_set = SIGEMPTYMASK | (sig),		\
       sigprocmask (SIG_BLOCK, &sigprocmask_set, NULL))