doubt in catch_errors for gdb-6.4/6.6

Hi, 

can any one please explain me 
What is happening when we go through the 
following code path.

version:  gdb-6.4/gdb-6.6 


info_threads_command -> print_stack_frame ->
catch_errors  -> TRY_CATCH (macro )


I am mainly getting trouble in the MACRO itslef.



Thanks
RAM


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