fix lseek() wrapper error handling

libvips/iofuncs/streamo.c

@@ -35,6 +35,8 @@
  *
  * - filename encoding
  * - need printf()-style output? eg. for CSV write ... see dbuf.c
+ * - make vfuncs just call write() and seek() (for tiff), all other processing
+ *   in the invocation ... cf. streami
  * - test we can really change all behaviour in the subclass ... add callbacks
  *   as well to make it simpler for language bindings
  */

libvips/iofuncs/util.c

@@ -1054,7 +1054,8 @@ vips__seek( int fd, gint64 pos, int whence )
 }
 #else /*!OS_WIN32*/
 	if( (new_pos = lseek( fd, pos, whence )) == (off_t) -1 ) {
-		vips_error_system( "vips__seek", "%s", _( "unable to seek" ) );
+		vips_error_system( errno, "vips__seek", 
+			"%s", _( "unable to seek" ) );
 		return( -1 );
 	}
 #endif /*OS_WIN32*/