CC. Undefined Behavior

According to the [ Fortran 2018 ] and [ Fortran 2023 ] language standards, the behavior of a program is undefined when it violates a normative requirement of the language or executes a construct for which the standard specifies that a value, association status, pointer, object, or effect is undefined. Unlike the C Standard, the Fortran standard does not provide a dedicated annex enumerating undefined behaviors. Instead, such cases are specified throughout the document by means of normative requirements (typically expressed using shall) and statements indicating that a value, variable, pointer, association status, allocation status, or other entity is undefined; that the effect is undefined; or that a program is not standard-conforming.

The "Guideline" column in the following table identifies the coding practices that address each instance of undefined behavior (UB). The "Description" column provides a short description of the undefined behavior, optionally indicating a quoted or closely paraphrased from the Fortran 2018 and Fortran 2023 standards.