The following is a demonstration of the fddist command,


Here fddist is run for a few seconds on an idle workstation,

   Tracing reads and writes... Hit Ctrl-C to end.
   ^C
   EXEC: dtrace           PID: 3288
   
              value  ------------- Distribution ------------- count
                  0 |                                         0
                  1 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 2
                  2 |                                         0
   
   EXEC: mozilla-bin      PID: 1659
   
              value  ------------- Distribution ------------- count
                  3 |                                         0
                  4 |@@@@@@@@@@                               28
                  5 |                                         0
                  6 |@@@@@@@@@@@@@@@                          40
                  7 |@@@@@@@@@@@@@@@                          40
                  8 |                                         0
   
   EXEC: Xorg             PID: 1532
   
              value  ------------- Distribution ------------- count
                 22 |                                         0
                 23 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 57
                 24 |                                         0

The above displays the usage pattern for process file descriptors.
We can see the Xorg process (PID 1532) has made 57 reads or writes to
it's file descriptor 23. 

The pfiles(1) command can be used to help determine what file 
descriptor 23 actually is.