how to get the width of a string

["rion10 (sent by Nabble.com)" <lists nabble com>]

there is a entry in my window, so I want to put a string to the entry box.
if the string is too long to fit the entry, so I need cut the string to be like this:
aaaaaa...bbb, 
so, my question is how to get the width of the specify string with default font style.
Thanks for any hint!

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