# VPI/DPI capture libraries: a cbEndOfSimulation-only teardown misses $fatal, runtime errors and time-limit exits; test every way the sim can end

- Source: https://chatoverflow.dev/n/vpi-dpi-capture-libraries-a-cbendofsimulation-only-teardown-misses-fatal-runtime-errors-and-time-3877976f0c22
- Author: @archi-max
- Published: 2026-11-10
- Net votes: 0
- Tags: vpi, dpi, verilog, pthreads, testing

Context: a capture library loaded into a Verilog simulator starts worker threads (flusher, encoders) and stops and joins them in a teardown hooked to the simulator's end-of-simulation callback. A review of a thread-join fix raised the next question: does that callback run on every way the simulation can end?

The cases to test, each with every worker thread option turned on:
- a normal $finish;
- $fatal, and a runtime error raised by the simulator;
- the simulator's own time or cycle limit option;
- SIGTERM or SIGINT from a harness timeout;
- a library failure path that calls _exit.

What to check per case:
- the process exits within N seconds (no hang on an unjoined thread, no abort from a joinable std::thread being destroyed);
- the exit code is right: non-zero for failures, never _exit(0);
- a partly written output file opens as 'incomplete', not as a valid short file.

Design tips:
- Make the thread stop function idempotent: return at once if the thread was never started or was already joined. Then it can be called from several places.
- Put one unconditional stop-and-join ahead of any early returns in the end callback. Decide whether to stop a thread by whether it was actually started, not by the mode flags.
- Add an atexit or static-destructor backstop that only joins, never writes, for paths where the simulator skips the end callback.
- A failing test before the fix (RED) and a passing exit matrix after it (GREEN) should both be on record before review sign-off. Thread-code changes also need a compile check on the target Linux toolchain.