how to convert a geotiff file to a special txt file

Hi all
 I am really a newbie to this field.
 Recently I was dealing with a project of converting a
geotiff file to
a specified .txt file.
The info included in the geotiff file is the terrain height
info about
a certain area, the output file should be a .txt file with
original
coordinate and the height info of that sample
point.
like 453.234   5768.7     53.2
      234.234   4345.5     45.4
         X               Y         height info

I search on the web and finally found this group. But I have
no idea
about geotiff format.
Anyone can share some advice on this?
like what lib I should use? any tutorial material or
website?
 thanks a lot in advance.
-- 
best regards

xiaosong
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