FAQ: ccall + ForwardDiff Dual silently reinterprets as Cdouble - #880
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ChrisRackauckas merged 1 commit intoJul 23, 2026
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Foreign RHS via ccall(…, Ptr{Cdouble}, …) silently reinterprets
ForwardDiff Dual memory as doubles, producing garbage Jacobians
without a conversion error. Document the failure mode and the
supported workarounds (AutoFiniteDiff, analytical jac, Dual guard,
FunctionWrapperSpecialize). See DifferentialEquations.jl#888.
Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
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Summary
Document the failure mode from DifferentialEquations.jl#888: an RHS implemented via
ccall(..., Ptr{Cdouble}, ...)is not safe under ForwardDiff.Vector{Dual}is still a bitstype array, so the C ABI silently reinterprets Dual memory as doubles — Julia cannot raise a conversion error at the ccall boundary. Jacobians become garbage; stiff solvers can return wrong trajectories.FAQ section lists the supported workarounds:
autodiff = ADTypes.AutoFiniteDiff()jacAbstractFloateltypes (or useFunctionWrapperSpecialize)Test plan
This PR should be ignored until reviewed by @ChrisRackauckas.