Why pre-increment operator gives rvalue in C?

Question

In C++, pre-increment operator gives lvalue because incremented object itself is returned, not a copy. But in C, it gives rvalue. Why?

Answer 1

C99 says in the footnote (of section $6.3.2.1),

The name ‘‘lvalue’’ comes originally from the assignment expression E1 = E2, in which the left operand E1 is required to be a (modifiable) lvalue. It is perhaps better considered as representing an object ‘‘locator value’’. What is sometimes called ‘‘rvalue’’ is in this International Standard described as the ‘‘value of an expression’’.

Hope that explains why ++i in C, returns rvalue.

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As for C++, I would say it depends on the object being incremented. If the object's type is some user-defined type, then it may always return lvalue. That means, you can always write i++++++++ or ++++++i if type of i is Index as defined here:

Undefined behavior and sequence points reloaded