Clarify the return values for SSL_get_shared_curve.

Reviewed-by: NMatt Caswell <matt@openssl.org>

doc/ssl/SSL_CTX_set1_curves.pod

@@ -45,11 +45,12 @@ B<curves> array is in the form of a set of curve NIDs in preference
 order. It can return zero if the client did not send a supported curves
 extension.
 
-SSL_get1_shared_curve() returns shared curve B<n> for B<ssl>. If B<n> is
--1 then the total number of shared curves is returned, which may be
-zero. Other than for diagnostic purposes, most applications will only
-be interested in the first shared curve so B<n> is normally set to zero.
-If the value B<n> is out of range zero is returned.
+SSL_get_shared_curve() returns shared curve B<n> for a server-side
+SSL B<ssl>. If B<n> is -1 then the total number of shared curves is
+returned, which may be zero. Other than for diagnostic purposes,
+most applications will only be interested in the first shared curve
+so B<n> is normally set to zero. If the value B<n> is out of range,
+NID_undef is returned.
 
 SSL_CTX_set_ecdh_auto() and SSL_set_ecdh_auto() set automatic curve
 selection for server B<ctx> or B<ssl> to B<onoff>. If B<onoff> is 1 then 
@@ -84,8 +85,12 @@ return 1 for success and 0 for failure.
 
 SSL_get1_curves() returns the number of curves, which may be zero.
 
-SSL_get1_shared_curve() returns the NID of shared curve B<n> of zero if there
-is no shared curve B<n> or the number of shared curves if B<n> is -1.
+SSL_get_shared_curve() returns the NID of shared curve B<n> or NID_undef if there
+is no shared curve B<n>; or the total number of shared curves if B<n>
+is -1.
+
+When called on a client B<ssl>, SSL_get_shared_curve() has no meaning and
+returns -1.
 
 =head1 SEE ALSO
 

ssl/t1_lib.c

@@ -504,11 +504,12 @@ int tls1_check_curve(SSL *s, const unsigned char *p, size_t len)
 	return 0;
 	}
 
-/* Return nth shared curve. If nmatch == -1 return number of
- * matches. For nmatch == -2 return the NID of the curve to use for
- * an EC tmp key.
+/*
+ * Return |nmatch|th shared curve or NID_undef if there is no match.
+ * For nmatch == -1, return number of  matches
+ * For nmatch == -2, return the NID of the curve to use for
+ * an EC tmp key, or NID_undef if there is no match.
  */
-
 int tls1_shared_curve(SSL *s, int nmatch)
 	{
 	const unsigned char *pref, *supp;
@@ -542,10 +543,11 @@ int tls1_shared_curve(SSL *s, int nmatch)
 	 */
 	if (!tls1_get_curvelist(s, (s->options & SSL_OP_CIPHER_SERVER_PREFERENCE) != 0,
 			&supp, &num_supp))
-		return 0;
+		/* In practice, NID_undef == 0 but let's be precise. */
+		return nmatch == -1 ? 0 : NID_undef;
 	if(!tls1_get_curvelist(s, !(s->options & SSL_OP_CIPHER_SERVER_PREFERENCE),
 			&pref, &num_pref))
-		return 0;
+		return nmatch == -1 ? 0 : NID_undef;
 	k = 0;
 	for (i = 0; i < num_pref; i++, pref+=2)
 		{
@@ -567,7 +569,8 @@ int tls1_shared_curve(SSL *s, int nmatch)
 		}
 	if (nmatch == -1)
 		return k;
-	return 0;
+	/* Out of range (nmatch > k). */
+	return NID_undef;
 	}
 
 int tls1_set_curves(unsigned char **pext, size_t *pextlen,