How do vararg functions find out the number of arguments in machine code?

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The trick is that you tell them somehow else. For printf you have to supply a format string which even contains type information (which might be incorrect though). The way to supply this information is mainly user-contract and often error-prone.

As for calling conventions: Usually the arguments are pushed onto the stack from left to right and then the backjump address at last. The calling routine clears the stack. So there is no technical need for the called routine to know the number of parameters.

EDIT: In C++0x there is a safe way (even typesafe!) to call variadic functions!

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