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c Assignment and pointers, undefined behavior?

Note that there is a sequence point immediately before a function is called, after its arguments have been evaluated; there is also a sequence point immediately before a function returns. Thus, there are sequence points related to the evaluation of the function arguments and its return value, but...and this is crucial in this still does not tell you whether *ptr is evaluated before or after the function is called. Either is possible; both are correct; the code depends on which happens, which makes it rely on undefined behaviour.

The code is thoroughly disreputable. Do not try using it, or anything similar, in real code.

This is undefined behaviour, I believe. The standard does not stipulate when the LHS of the assignment is evaluated compared to the RHS. If *ptr is evaluated after the function is called, you will be dereferencing a null pointer; if it is evaluated before the function is called, then you get sane behaviour.