About distance acquisition of optix7.0

Hello. I am always indebted.
I am transitioning from optix6.0 to optix7.0 and studying.
Ask one basic question.
I want to get the distance of the latest point by ray tracing.
In optix6.0,
rtDeclareVariable (PerRayData_radiance, prd_radiance, rtPayload,);
I remember that I was able to get it.
Is there an equivalent variable in optix7.0?

Please start by reading through the OptiX 7 Programming Guide and API Reference: https://raytracing-docs.nvidia.com/

For the hit distance in OptiX 6 and earlier you needed to declare a float variable with the semantic rtIntersectionDistance.
https://raytracing-docs.nvidia.com/optix6/guide_6_5/index.html#programs#internally-provided-semantics
Something like this: rtDeclareVariable(float, theIntersectionDistance, rtIntersectionDistance, );

In OptiX 7 there are no semantic variables. The way to get information from OptiX 7 inside device code is to call one of the provided device-side functions:
https://raytracing-docs.nvidia.com/optix7/guide/index.html#device_side_functions#device-side-functions
The online docs have a search functionality in the top right which works quite well. Try optixGet there.

For the intersection distance you query optixGetRayTmin() inside the closest hit program.
https://raytracing-docs.nvidia.com/optix7/api/html/group__optix__device__api.html#gaef57333cc09360082b9cb329541a023a
In OptiX 6 and earlier that was the float variable with semantic

Payloads in OptiX 7 are a maximum of eight unsigned int registers on the optixTrace() call.
https://raytracing-docs.nvidia.com/optix7/guide/index.html#device_side_functions#trace
https://raytracing-docs.nvidia.com/optix7/guide/index.html#basic_concepts_and_definitions#ray-payload

Note that you need to define how many payload registers your OptixPipeline uses.
https://raytracing-docs.nvidia.com/optix7/api/html/struct_optix_pipeline_compile_options.html

It’s recommended to use the registers if you do not need more data inside the payload.
Mans if you only need the intersection distance, you can put that 32-bit float value into one of these unsigned int palyoad register.

If you need more payload data you can encode a 64-bit pointer to a structure into two of these payload registers. The OptiX SDK examples show how. Just search for all optixTrace calls.

After reading the complete OptiX 7 Programming Guide, I would recommend to look through the OptiX 7 SIGGRAPH course examples, the OptiX SDK examples, and the OptiX 7 advanced examples.
Links here:
https://devtalk.nvidia.com/default/topic/1058310/optix/optix-7-0-release/
https://devtalk.nvidia.com/default/topic/998546/optix/optix-advanced-samples-on-github/post/5428301/#5428301

Thank you for your prompt reply.
I would like to continue learning optix7.0 based on your advice!