While reading output, more than 2000 bytes can force
earlier  bytes  to  be  "forgotten".   This  may  be
changed with the  function  match_max.   (Note  that
excessively  large  values can slow down the pattern
matcher.)  If patlist  is  full_buffer,  the  corre-
sponding  body  is  executed if match_max bytes have
been received and no other  patterns  have  matched.
Whether  or not the full_buffer keyword is used, the
forgotten     characters     are     written      to
expect_out(buffer).

             taken from 'man expect'

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I've found that match_max can cause expect_out(buffer) to become undefined.
Redefine it after calling match_max to get round this

             set expect_out(buffer) {}
             set expect_out(buffer) {}

As noted by Cameron Laird here: http://phaseit.net/claird/comp.lang.tcl/expect.html

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[Category Expect]