What\'s the Difference Between \'Ill-formed; No Diagnostic Required\' and Undefined Behavior in C ?

Difference Between "Ill-formed; No Diagnostic Required" and Undefined Behavior

In C , there are various categories of irregular program behavior, including undefined, ill-formed, and implementation-defined. While undefined behavior implies that the program is erroneous, ill-formed programs are simply not well-formed.

The term "ill-formed; no diagnostic required" suggests a program that violates syntax or semantic rules, making it erroneous. However, the absence of a diagnostic requirement indicates that it is similar to undefined behavior. This inconsistency arises because the C standard attempts to require diagnostics for all errors, while the C standard considers all errors as undefined behavior.

The distinction between "ill-formed; no diagnostic required" and undefined behavior ultimately lies in the specific context. The former is more commonly found with programming errors, while the latter is associated with runtime issues. However, both cases imply that the program's behavior is unpredictable and potentially erroneous.

Therefore, although the wording may differ, "ill-formed; no diagnostic required" and undefined behavior essentially convey the same meaning: the program cannot be considered well-formed and the implementation's behavior is undefined.

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