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50 lines
2.5 KiB
TeX
50 lines
2.5 KiB
TeX
%\newpage
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\subsection{Device and Host Memory Association}
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\label{subsec:target_associate_ptr}
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\label{sec:target_associate_ptr}
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\index{routines!omp_target_associate_ptr@\scode{omp_target_associate_ptr}}
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\index{omp_target_associate_ptr routine@\scode{omp_target_associate_ptr} routine}
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\index{routines!omp_target_alloc@\scode{omp_target_alloc}}
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\index{omp_target_alloc routine@\scode{omp_target_alloc} routine}
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The association of device memory with host memory
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can be established by calling the \scode{omp_target_associate_ptr}
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API routine as part of the mapping.
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The following example shows the use of this routine
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to associate device memory of size \splc{CS},
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allocated by the \scode{omp_target_alloc} routine and
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pointed to by the device pointer \splc{dev_ptr},
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with a chunk of the host array \splc{arr} starting at index \splc{ioff}.
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In Fortran, the intrinsic function \scode{c_loc} is called
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to obtain the corresponding C pointer (\splc{h_ptr}) of \splc{arr(ioff)}
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for use in the call to the API routine.
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\index{constructs!target update@\code{target}~\code{update}}
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\index{target update construct@\code{target}~\code{update} construct}
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\index{map clause@\code{map} clause!always modifier@\code{always} modifier}
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\index{always modifier@\code{always} modifier}
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Since the reference count of the resulting mapping is infinite,
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it is necessary to use the \scode{target}~\scode{update} directive (or
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the \scode{always} modifier in a \scode{map} clause) to accomplish a
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data transfer between host and device.
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The explicit mapping of the array section \splc{arr[ioff:CS]}
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(or \splc{arr(ioff:ioff+CS-1)} in Fortran) on the \scode{target}
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construct ensures that the allocated and associated device memory is used
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when referencing the array \splc{arr} in the \scode{target} region.
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The device pointer \splc{dev_ptr} cannot be accessed directly
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after a call to the \scode{omp_target_associate_ptr} routine.
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\index{routines!omp_target_disassociate_ptr@\scode{omp_target_disassociate_ptr}}
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\index{omp_target_disassociate_ptr routine@\scode{omp_target_disassociate_ptr} routine}
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\index{routines!omp_target_free@\scode{omp_target_free}}
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\index{omp_target_free routine@\scode{omp_target_free} routine}
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After the \scode{target} region, the device pointer is disassociated from
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the current chunk of the host memory by calling the \scode{omp_target_disassociate_ptr} routine before working on the next chunk.
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The device memory is freed by calling the \scode{omp_target_free}
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routine at the end.
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\cexample[4.5]{target_associate_ptr}{1}
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\ffreeexample[5.1]{target_associate_ptr}{1}
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