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Mach's [[IPC]] mechanism allows for getting access to a message's sequence
number.

This can be used for serializing requests in a multithreaded environment.

* [GNU Mach Reference Manual: 4.2.6 Message
  Receive](http://www.gnu.org/software/hurd/gnumach-doc/Message-Receive.html)
* [GNU Mach Reference Manual: 4.3.6 Receive
  Rights](http://www.gnu.org/software/hurd/gnumach-doc/Receive-Rights.html)