diff --git a/ChangeLog b/ChangeLog
index 516be8a96c..55319d8848 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2016-05-10  Joseph Myers  <joseph@codesourcery.com>
+
+	[BZ #20043]
+	* posix/unistd.h [__USE_XOPEN && !__USE_XOPEN2K] (cuserid): New
+	prototype.
+
 2016-05-10  Florian Weimer  <fweimer@redhat.com>
 
 	[BZ #20017]
diff --git a/posix/unistd.h b/posix/unistd.h
index 405638ce79..4722f7fc5c 100644
--- a/posix/unistd.h
+++ b/posix/unistd.h
@@ -1138,11 +1138,15 @@ extern void swab (const void *__restrict __from, void *__restrict __to,
 #endif
 
 
-/* The Single Unix specification demands this prototype to be here.
-   It is also found in <stdio.h>.  */
+/* Prior to Issue 6, the Single Unix Specification required these
+   prototypes to appear in this header.  They are also found in
+   <stdio.h>.  */
 #if defined __USE_XOPEN && !defined __USE_XOPEN2K
 /* Return the name of the controlling terminal.  */
 extern char *ctermid (char *__s) __THROW;
+
+/* Return the name of the current user.  */
+extern char *cuserid (char *__s);
 #endif