How to compare working copy file with older version of that file?

I can't seem to figure out how to compare the working version
of a file with an old revision of that file. Here's the log
for the file:

   $ svn log matrixConfig.h
   ------------------------------------------------------------------------
   r2068 | grante | 2008-05-22 18:02:04 -0500 (Thu, 22 May 2008) | 2 lines
   
   Disable all ciphers except RC4-SHA so that ssldump could
   decrypt application data.
   ------------------------------------------------------------------------
   r2057 | grante | 2008-05-21 11:00:19 -0500 (Wed, 21 May 2008) | 3 lines
   
   Merge in changes that came with the SSH beta release.
   
   
   ------------------------------------------------------------------------
   r1973 | grante | 2008-05-06 13:05:23 -0500 (Tue, 06 May 2008) | 3 lines
   
   Branch in which MatrixSSL/SSH incorporation is being done.
   
   
   ------------------------------------------------------------------------

According to the manual (p34), this should work, shouldn't it?

   $ svn diff -c 2068 matrixConfig.h
   svn: File not found: revision 1972, path '/ecos/ecosapps/trunk/matrixssl/src/matrixConfig.h'

What does /ecos/ecosapps/trunk/matrixssl/src/matrixConfig.h
have to do with anything? That's not the path of the file in
question (and I didn't ask about revision 1972).

According to the manual, this also should work but doesn't:

   $ svn diff -r 2057:2068 matrixConfig.h
   svn: File not found: revision 1972, path '/ecos/ecosapps/trunk/matrixssl/src/matrixConfig.h'

How do I ask for the differences between either the working
verion and an older version or between two older versions of a
file?

If I edit the file and then do "svn diff matrixConfig.h" it
works fine, but anything where I specify a version falls down.

-- 
Grant Edwards                   grante             Yow! Will this never-ending
                                  at               series of PLEASURABLE
                               visi.com            EVENTS never cease?
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