Follow-up question for: If "shall / shall not" requirement is violated, then does it matter in which section (e.g. Semantics, Constraints) such requirement is located?.

ISO/IEC 9899:202x (E) working draft— December 11, 2020 N2596, 5.1.1.3 Diagnostics, 1:

A conforming implementation shall produce at least one diagnostic message (identified in an implementation-defined manner) if a preprocessing translation unit or translation unit contains a violation of any syntax rule or constraint, even if the behavior is also explicitly specified as undefined or implementation-defined. Diagnostic messages need not be produced in other circumstances.

Consequence: semantics violation does not require diagnostics.

Question: what is the (possible) rationale for "semantics violation does not require diagnostics"?

CodePudding user response：

A possible rationale is given by Rice's theorem : non-trivial semantic properties of programs are undecidable

For example, division by zero is a semantics violation; and you cannot decide, by static analysis alone of the C source code, that it won't happen...

A standard cannot require total detection of such undefined behavior, even if of course some tools (e.g. Frama-C) are sometimes capable of detecting them.

See also the halting problem. You should not expect a C compiler to solve it!

CodePudding user response：

The C99 rationale v5.10 gives this explanation:

5.1.1.3 Diagnostics

By mandating some form of diagnostic message for any program containing a syntax error or constraint violation, the Standard performs two important services. First, it gives teeth to the concept of erroneous program, since a conforming implementation must distinguish such a program from a valid one. Second, it severely constrains the nature of extensions permissible to a conforming implementation.

The Standard says nothing about the nature of the diagnostic message, which could simply be “syntax error”, with no hint of where the error occurs. (An implementation must, of course, describe what translator output constitutes a diagnostic message, so that the user can recognize it as such.) The C89 Committee ultimately decided that any diagnostic activity beyond this level is an issue of quality of implementation, and that market forces would encourage more useful diagnostics. Nevertheless, the C89 Committee felt that at least some significant class of errors must be diagnosed, and the class specified should be recognizable by all translators.

CodePudding user response：

This happens because the grammar of the C language is context-sensitive and for all the languages that are defined with context-free or more complex grammars on the Chomsky hierarchy one must do a tradeoff between the semantics of the language and its power.

C designers chose to allow much power for the language and this is why the problem of undecidability is omnipresent in C.

There are languages like Coq that try to cut out the undecidable situations and they restrict the semantics of the recursive functions (they allow only sigma(primitive) recursivity).