diff options
Diffstat (limited to 'gcc/fortran/gfortran.texi')
| -rw-r--r-- | gcc/fortran/gfortran.texi | 38 |
1 files changed, 32 insertions, 6 deletions
diff --git a/gcc/fortran/gfortran.texi b/gcc/fortran/gfortran.texi index af1d2961085..0512cb9e6fc 100644 --- a/gcc/fortran/gfortran.texi +++ b/gcc/fortran/gfortran.texi @@ -181,7 +181,7 @@ Part I: Invoking GNU Fortran Part II: Language Reference * Fortran 2003 and 2008 status:: Fortran 2003 and 2008 features supported by GNU Fortran. -* Compiler Characteristics:: KIND type parameters supported. +* Compiler Characteristics:: User-visible implementation details. * Extensions:: Language extensions implemented by GNU Fortran. * Intrinsic Procedures:: Intrinsic procedures supported by GNU Fortran. * Intrinsic Modules:: Intrinsic modules supported by GNU Fortran. @@ -914,14 +914,13 @@ made and you should only use it for experimental purposes. @node Compiler Characteristics @chapter Compiler Characteristics -@c TODO: Formulate this introduction a little more generally once -@c there is more here than KIND type parameters. - -This chapter describes certain characteristics of the GNU Fortran compiler, -namely the KIND type parameter values supported. +This chapter describes certain characteristics of the GNU Fortran +compiler, that are not specified by the Fortran standard, but which +might in some way or another become visible to the programmer. @menu * KIND Type Parameters:: +* Internal representation of LOGICAL variables:: @end menu @@ -965,6 +964,33 @@ imaginary part are a real value of the given size). It is recommended to use the @code{SELECT_*_KIND} intrinsics instead of the concrete values. +@node Internal representation of LOGICAL variables +@section Internal representation of LOGICAL variables +@cindex logical, variable representation + +The Fortran standard does not specify how variables of @code{LOGICAL} +type are represented, beyond requiring that @code{LOGICAL} variables +of default kind have the same storage size as default @code{INTEGER} +and @code{REAL} variables. The GNU Fortran internal representation is +as follows. + +A @code{LOGICAL(KIND=N)} variable is represented as an +@code{INTEGER(KIND=N)} variable, however, with only two permissible +values: @code{1} for @code{.TRUE.} and @code{0} for +@code{.FALSE.}. Any other integer value results in undefined behavior. + +Note that for mixed-language programming using the +@code{ISO_C_BINDING} feature, there is a @code{C_BOOL} kind that can +be used to create @code{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 @code{LOGICAL} variables in GNU Fortran is identical +to C99 _Bool, except for a possible difference in storage size +depending on the kind. + + @c --------------------------------------------------------------------- @c Extensions @c --------------------------------------------------------------------- |