Internal representation of LOGICAL variables - The GNU Fortran Compiler
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### 5.2 Internal representation of LOGICAL variables

The Fortran standard does not specify how variables of `LOGICAL` type are represented, beyond requiring that `LOGICAL` variables of default kind have the same storage size as default `INTEGER` and `REAL` variables. The GNU Fortran internal representation is as follows.

A `LOGICAL(KIND=N)` variable is represented as an `INTEGER(KIND=N)` variable, however, with only two permissible values: `1` for `.TRUE.` and `0` for `.FALSE.`. Any other integer value results in undefined behavior.

Note that for mixed-language programming using the `ISO_C_BINDING` feature, there is a `C_BOOL` kind that can be used to create `LOGICAL(KIND=C_BOOL)` variables which are interoperable with the C99 _Bool type. The C99 _Bool type has an internal representation described in the C99 standard, which is identical to the above description, i.e. with 1 for true and 0 for false being the only permissible values. Thus the internal representation of `LOGICAL` variables in GNU Fortran is identical to C99 _Bool, except for a possible difference in storage size depending on the kind.