read/write capture comments from the command line

[Martin Kaiser <lists () kaiser cx>]

Dear all,

I was asked by some people about access to capture comments from the
command line tools. We identified two use cases

- You have a capture file and want to display its capture file comment.
  Nothing but the comment, no packets etc.
- You start a capture from the command line and want to insert a comment 
  into the newly created file.

The reading should be an option to tshark ("display the capture file 
comment and exit"). Tshark would need a copy of cf_read_shb_comment() or 
a way to share it with file.c.

For writing, I added a switch -j <new comment> to both tshark and 
dumpcap. It turned out that things are most simple if I add a new field 
initial_capture_comment to the capture_options structure. Tshark can 
pass the comment to dumpcap via sync_pipe_start() and dumpcap writes the
SHB.

I have something basic that's working. Probably, a lot of plausibility 
checks for the parameters are missing. E.g. supplying a comment only 
makes sense if we capture into one pcapng file.

What do you think: Do these use cases and the approach make sense to 
you? And what letters should be used for the option? I know you've been
cleaning up the parameters lately ;-)

Best regards,

   Martin
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