Provide and document ctermid_r function.

lib/libc/gen/ctermid.3

@@ -42,6 +42,8 @@
 .Fd #include <stdio.h>
 .Ft char *
 .Fn ctermid "char *buf"
+.Ft char *
+.Fn ctermid_r "char *buf"
 .Sh DESCRIPTION
 The
 .Fn ctermid
@@ -64,6 +66,17 @@ file
 .Aq Pa stdio.h )
 bytes long.
 .Pp
+.Fn ctermid_r
+provides the same functionality as
+.Fn ctermid
+except that if
+.Ar buf
+is a 
+.Dv NULL
+pointer, 
+.Dv NULL
+is returned.
+.Pp
 The current implementation simply returns
 .Ql /dev/tty .
 .Sh RETURN VALUES

lib/libc/gen/ctermid.c

@@ -29,6 +29,8 @@
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
+ * 
+ * $FreeBSD$
  */
 
 #if defined(LIBC_SCCS) && !defined(lint)
@@ -40,8 +42,7 @@ static char sccsid[] = "@(#)ctermid.c	8.1 (Berkeley) 6/4/93";
 #include <string.h>
 
 char *
-ctermid(s)
-	char *s;
+ctermid(char *s)
 {
 	static char def[] = _PATH_TTY;
 
@@ -51,3 +52,10 @@ ctermid(s)
 	}
 	return(def);
 }
+
+
+char *
+ctermid_r(char *s)
+{
+	return (s) ? ctermid(s) : NULL;
+}