[ros-users] getting a topic's md5sum without subscribing

[Ken Tossell]

On Mon, 2010-03-29 at 14:46 -0700, Josh Faust wrote:
> Do you not get warnings when doing that?  I'm pretty sure we've been 

I do get that warning.

> What output do you get when they mismatch?

I was mistaken about this -- if the messages have roughly compatible
fields (say, "string a; string b" -> "string a; string c"), then the
message goes through with no warning. If they're incompatible (say,
"string a" -> "string a; string b"), then the connection is dropped
silently.

Is there a reason why the master tracks the message type but not the MD5
hash or the message definition? I imagine having that information in the
master could be useful (prevent two publishers with different message
versions both publishing to a single topic).

Ken