Why are arguments which do not match the conversion specifier in printf undefined behavior?

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printf only works as described by the standard if you use it correctly. If you use it incorrectly, the behaviour is undefined. Why should the standard define what happens when you use it wrong?

Concretely, on some architectures floating point arguments are passed in different registers to integer arguments, so inside printf when it tries to find an int matching the format specifier it will find garbage in the corresponding register. Since those details are outside the scope of the standard there is no way to deal with that kind of misbehaviour except to say it’s undefined.

For an example of how badly it could go wrong, using a format specifier of "%p" but passing a floating point type could mean that printf tries to read a pointer from a register or stack location which hasn’t been set to a valid value and could contain a trap representation, which would cause the program to abort.

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