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<H3><A NAME="SECTION000152200000000000000">
LOGICAL Values</A>
</H3>
One place where the DEC system may differ from the Sun is in how logical
values are handled. The original FORTRAN compiler for the DECstation (FORTRAN
for RISC) used the Sun interpretation of logical values, <I>i.e.</I> zero is
false, non-zero is true. The more recent DEC FORTRAN compiler uses the VMS
convention that only checks the lowest bit of a value, so 0 is false, 1 is
true, 2 is false, 3 is true, <I>etc</I>.
When DEC FORTRAN sets a <TT>LOGICAL</TT> variable to <TT>TRUE</TT>, all the
bits in the data are set to 1, resulting in a numerical equivalent value of -1.
Unfortunately this means that the correct value of the
macros
<A HREF="node129.html#F77_ISFALSE"><TT>F77_ISFALSE</TT></A>
and
<A HREF="node130.html#F77_ISTRUE"><TT>F77_ISTRUE</TT></A>
used in a C function, depend on which FORTRAN compiler you are using.
It is not possible to handle this
automatically, so you must be sure to use the right values for the macros. The
default assumption is that you are using the newer DEC FORTRAN compiler.
Fortunately this is unlikely to be a problem in practice,
since a <TT>TRUE</TT> value will normally be 1 or -1, and these values will be
handled correctly by either compiler.
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<ADDRESS>
<I>CNF and F77 Mixed Language Programming -- FORTRAN and C<BR>Starlink User Note 209<BR>P.M. Allan<BR>A.J. Chipperfield<BR>R.F. Warren-Smith<BR>19 January 2000<BR>E-mail:<A HREF="mailto:ussc@star.rl.ac.uk">ussc@star.rl.ac.uk</A></I>
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