Chris,
> I'm doing some experiments with hpx::dataflow and would like to
> recursively "chain" a dataflow for execution over a distributed vector of
> size_t elements.
> Sample code is provided below (apologies if it's not a perfect compile):
> static void run(
> std::string nom,
> size_t v) {
> hpx::cout << v << hpx::endl;
> }
>
> HPX_PLAIN_ACTION(run, run_action);
>
> static hpx::future run_dataflow(
> std::string nom,
> hpx::partitioned_vector &arr,
> std::vector locs,
> std::vector::iterator curr,
> std::vector::iterator end) {
> hpx::future res = (curr != end) ? run_dataflow(nom, arr, locs,
> std::next(curr), end) : hpx::make_ready_future();
> return hpx::dataflow(
> hpx::util::unwrapped([nom, &arr, localities, &curr] () ->
> hpx::future {
> size_t v = *c;
> return hpx::async(localities[arr.get_partition(v)], nom,
> v);
> }),
> std::move(res) );
>
> }
>
> int main(int argc, char **argv) {
> ... /* dvec_size is defined up here */ ...
>
> std::vector locs = hpx::find_all_localities();
> std::string nom = "arr_nom";
> hpx::partitioned_vector arr(dvec_size,
> hpx::container_layout(locs));
> arr.register_as(nom);
>
> std::vector pos;
> std::iota(
> std::begin(pos),
> std::end(pos),
> dvec_size);
>
> hpx::future fd = run_dataflow(
> nom,
> arr,
> std::begin(pos),
> std::end(pos) );
> fd.wait();
>
> }
> I've been getting a runtime errors for this code (segmentation fault) -
> this is probably a semi-trivial bug or misunderstanding on my part - any
> help/assistance would be appreciated. (I'm compiling against HPX with MPI)
The main reason for this kind of problems is that variables which are captured
by reference into a lambda (or similar) go out of scope before the lambda
itself is being executed:
void foo()
{
vector bar = {...};
return async([&bar]() { bar.push_back(42); });
}
foo();
In this example, the variable 'bar' is not valid anymore when the lambda (which
is scheduled as a new thread) is eventually executed. Please note, that
dataflow() runs the function passed to it on a new thread as well.
I'd assume that in your example the variable 'arr' goes out of scope before the
lambda referencing it is run.
HTH
Regards Hartmut
---
http://boost-spirit.com
http://stellar.cct.lsu.edu
>
> V/r,
> Chris
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