\pagebreak
\section{The \code{requires} Directive}
\label{sec:requires}

The declarative \code{requires} directive can be used to 
specify features that an implementation must provide to compile and 
execute correctly.

In the following example the \code{unified\_shared\_memory} clause 
of the \code{requires} directive ensures that the host and all 
devices accessible through OpenMP provide a \plc{unified address} space
for memory that is shared by all devices.

The example illustrates the use of the \code{requires} directive specifying
\plc{unified shared memory} in file scope, before any device 
directives or device routines. No \code{map} clause is needed for
the \plc{p} structure on the device (and its address \plc{\&p}, for the C++ code,
is the same address on the host and device).
However, scalar variables referenced within the \code{target}
construct still have a default data-sharing attribute of firstprivate.
The \plc{q} scalar is incremented on the device, and its change is
not updated on the host.
% will defaultmap(toform:scalar) make q use shared address space? 
%Or will it be ignored at this point.
% Does before device routines also mean before prototype?

%\pagebreak

\cppexample{requires}{1}

\ffreeexample{requires}{1}