What does "Is /clock being published?" error means?

edit

Hi all,

I want to test the function of tabletop_segmentation.But when my node send a service request to /tabletop_segmentation, I always get messages indicate that "NO_CLOUD_RECEIVED".

[ INFO] [1355922807.919213857, 1093.482000000]: Tabletop detection service called; waiting for a point_cloud2 on topic /narrow_stereo_textured/points2
[ERROR] [1355922813.224946787, 1096.538000000]: Tabletop object detector: no point_cloud2 has been received

The problem is /tabletop_segmentation node receive no data from /narrow_stereo_textured/points2.(This topic exists, though)

To test /narrow_stereo_textured/points2, I started gazebo by

  roslaunch manipulation_worlds pr2_table_object.launch

And I use rviz to see what /narrow_stereo_textured/points2 published, and I can see the content by changing Fixed frame from /map to /head_mount_kinect_ir_frame.

However, when I try to rostopic echo this topic, I got

rosfuerte@rosfuerte-K53SM:~/project/nlp/libsvm-3.14$ rostopic echo /narrow_stereo_textured/points2
WARNING: no messages received and simulated time is active.
Is /clock being published?

And the segmentation fails as usual, does anyone know how to solve this problem? I want to test the function of segmentation alone. Thanks.

EDIT: This problem doesn't occur on another computer, which is also fuerte on ubuntu 12.04. I tried sudo apt-get dist-upgrade but I still got the error.