"undefined behavior - behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this document imposes no requirements"
It says "for which" and not "for the whole program". So the "interpretation" that a program becomes invalid or other requirements are affected was never supported by the text in the C standard. This was justified by some with arguments such as: "no requirements" includes behavior that can go back in time or otherwise break the rules of the real world. Of course, if you start to interpret a standard text in this creative way, you can justify anything. A reasonable interpretation must assume some general properties of the application domain it applies to (and in C there also more specific rules about how requirements are to be interpreted in 5.1) and must assume that wording such as "for which" has actual meaning that needs to be taken into account and can not simply be ignored. In C23 "Note 3" was added that aims to clarify that such nonsensical interpretations are not intended:
"Note 3 to entry: Any other behavior during execution of a program is only affected as a direct consequence of the concrete behavior that occurs when encountering the erroneous or non-portable program construct or data. In particular, all observable behavior (5.1.2.4) appears as specified in this document when it happens before an operation with undefined behavior in the execution of the program"
FrostKiwi|1 year ago
uecker|1 year ago
"undefined behavior - behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this document imposes no requirements"
It says "for which" and not "for the whole program". So the "interpretation" that a program becomes invalid or other requirements are affected was never supported by the text in the C standard. This was justified by some with arguments such as: "no requirements" includes behavior that can go back in time or otherwise break the rules of the real world. Of course, if you start to interpret a standard text in this creative way, you can justify anything. A reasonable interpretation must assume some general properties of the application domain it applies to (and in C there also more specific rules about how requirements are to be interpreted in 5.1) and must assume that wording such as "for which" has actual meaning that needs to be taken into account and can not simply be ignored. In C23 "Note 3" was added that aims to clarify that such nonsensical interpretations are not intended:
"Note 3 to entry: Any other behavior during execution of a program is only affected as a direct consequence of the concrete behavior that occurs when encountering the erroneous or non-portable program construct or data. In particular, all observable behavior (5.1.2.4) appears as specified in this document when it happens before an operation with undefined behavior in the execution of the program"