RFR: 8272554: Add help message about the named pipe support of UL.

[Xin Liu]

On Fri, 20 Aug 2021 22:52:05 GMT, Xin Liu <xliu at openjdk.org> wrote:

> This patch adds a help message about the named pipe support of the Unified Logging to -Xlog:help and manpage.
> 
> For -Xlog:help, here is the new help message:
> 
> Available log outputs:
>  stdout/stderr
>  file=<filename>
>   If the filename contains %p and/or %t, they will expand to the JVM's PID and startup timestamp, respectively. The filename can be a named pipe when log rotation is disabled.  
> 
> 
> Here is the new manpage content.
> 
>  When using file=filename, specifying %p and/or %t in the file name expands to the JVM's PID and startup timestamp, respectively.  You can also configure text files to handle file rotation based on file size and a number of files to rotate.  For example, to rotate the log file every 10 MB and  keep  5
>  files  in rotation, specify the options filesize=10M, filecount=5.  The target size of the files isn't guaranteed to be exact, it's just an approximate value.  Files are rotated by default with up to 5 rotated files of target size 20 MB, unless configured otherwise.  Specifying filecount=0 means that
>  the log file shouldn't be rotated.  The filename can be a named pipe when log rotation is disabled.  There's a possibility of the pre-existing log file getting overwritten.

yes, I agree with David's comment.  here is my understanding: 
https://bugs.openjdk.java.net/browse/JDK-8272554?focusedCommentId=14443690&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-14443690

-------------

PR: https://git.openjdk.java.net/jdk/pull/5205