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

Question

In both C (n1570 7.21.6.1/10) and C++ (by inclusion of the C standard library) it is undefined behavior to provide an argument to printf whose type does not match its conver

Answer 1

Some compilers may implement variable-format arguments in a way that allows the types of arguments to be validated; since having a program trap on incorrect usage may be better than possibly having it output seemingly-valid-but-wrong information, some platforms may choose to do that.

Because the behavior of traps is outside the realm of the C Standard, any action which might plausibly trap is classified as invoking Undefined Behavior.

Note that the possibility of implementations trapping based on incorrect formatting means that behavior is considered undefined even in cases where the expected type and the actual passed type have the same representation, except that signed and unsigned numbers of the same rank are interchangeable if the values they hold are within the range which is common to both [i.e. if a "long" holds 23, it may be output with "%lX" but not with "%X" even if "int" and "long" are the same size].

Note also that the C89 committee introduced a rule by fiat, which remains to this day, which states that even if "int" and "long" have the same format, the code:

long foo=23;
int *u = &foo;
(*u)++;

invokes Undefined Behavior since it causes information which was written as type "long" to be read as type "int" (behavior would also be Undefined if it was type "unsigned int"). Since a "%X" format specifier would cause data to be read as type "unsigned int", passing the data as type "long" would almost certainly cause the data to be stored somewhere as "long" but subsequently read as type "unsigned int", such behavior would almost likely violate the aforementioned rule.