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<h3 class="section">5.2 Internal representation of LOGICAL variables</h3>
<p><a name="index-logical_002c-variable-representation-303"></a>
The Fortran standard does not specify how variables of <code>LOGICAL</code>
type are represented, beyond requiring that <code>LOGICAL</code> variables
of default kind have the same storage size as default <code>INTEGER</code>
and <code>REAL</code> variables. The GNU Fortran internal representation is
as follows.
<p>A <code>LOGICAL(KIND=N)</code> variable is represented as an
<code>INTEGER(KIND=N)</code> variable, however, with only two permissible
values: <code>1</code> for <code>.TRUE.</code> and <code>0</code> for
<code>.FALSE.</code>. Any other integer value results in undefined behavior.
<p>Note that for mixed-language programming using the
<code>ISO_C_BINDING</code> feature, there is a <code>C_BOOL</code> kind that can
be used to create <code>LOGICAL(KIND=C_BOOL)</code> 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 <code>LOGICAL</code> variables in GNU Fortran is identical
to C99 _Bool, except for a possible difference in storage size
depending on the kind.
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