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[[Category:Exception]]
[[Category:Code Generation]]
[[Category:Language]]

== Overview ==
This article documents exception handling mechanisms at implementation level, including runtime and code generation.

== Typical execution/trace stack operation in a routine ==
Once or routine with old variable is slightly different.
See following pseudocode:

<eiffel>
-- entry

new_exset
	do
		-- eif_stack: Create a new vector EX_CALL on eif_stack.
	end


if not setjmp then

	-- Routine body

else

	exresc

	do

		-- eif_trace: Check the stack of eif_trace top element must be EN_FAIL or EN_RES
		
		-- eif_trace: Mark top node as rescued

		-- eif_trace: Create new EN_ILVL item on eif_trace, indicating entering a new level

		-- eif_stack: Create new EX_RESC vector on eif_stack.

	end

	Rescue_body

		-- If there is retry clause, it calls 
		
	exret

	do

		-- eif_stack: Check top vector of eif_stack is EX_RESC

		-- eif_stack: Override top item of eif_stack with the vector of current routine (saved as local). Change the type of the vector to EX_RETY

		-- eif_trace: Pop off eif_trace, which must be EN_ILVL item

		-- eif_trace: unwind_trace, pop items from eif_trace until EN_ILVL item is found. Restore `exdata' at previous level.

	end

		-- If no retry is called, call at the end:
		
	exfail

	do

		-- eif_stack: pop EX_RESC from eif_stack

		-- eif_trace: pop EN_ILVL from eif_trace, leaving call failures created by `draise' on stack.

		-- create last_exception (ROUTINE_FAILURE)

		-- eif_stack: call `backtrack' which will pop eif_stack until find the top most jmpbuf.

		-- Call longjmp with the found jmpbuf.

	end

end

exok

do

	-- eif_stack: Pop eif_stack until a EX_CALL, EX_RESC, EX_RETY or EX_OSTK is found and popped.

	-- eif_trace: unwind_trace, pop item from eif_trace until we find EN_ILVL item. Restore `exdata' at previous level.

end
</eiffel>

== Raise an exception (draise, eraise, com_raise, ) ==

*  Create a new item on eif_trace, with the type the exception.

*  Pop and push back eif_stack in order to get information. Fill those information into `exdata'.

*  Setup information for top node in eif_trace.

*  In make_exception, when building trace, traverse eif_stack, and push corresponding items into eif_trace, until jmp pointer is found. In the meantime, if the item is EX_RESC, we change it to EN_OLVL and put into eif_trace.

*  Continue buidling trace from the rest element of eif_stack until root node is reached.

*  Call `back_track' to really pop off items in eif_stack and call longjmp when the first jmp pointer is found.

== System signal handler ==

*  Push an EN_ILVL item on eif_trace

*  Push an EX_HDLR item on eif_stack and setup a jmpbuf

*  Call exception handler and be ready to longjmp back and call eraise to populate the exception.

Return to Exception mechanism internals.

Retrieved from "https://dev.eiffel.com/Exception_mechanism_internals"

View source for Exception mechanism internals

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