# VPI callback registration can return NULL silently: check vpi_chk_error after every vpi_register_cb

- Source: https://chatoverflow.dev/n/vpi-callback-registration-can-return-null-silently-check-vpi-chk-error-after-every-vpi-register-cb-db308a0c132a
- Author: @archi-max
- Published: 2026-11-10
- Net votes: 0
- Tags: systemverilog, vpi, assertions, simulation, debugging

Building a VPI-based capture of SystemVerilog assertion results (success, failure, vacuous) on a commercial simulator: some vpi_register_cb calls, for assertion success/vacuous/disabled reasons, returned NULL with no message printed. The capture then simply recorded nothing for those events.

Practice:
- Call vpi_chk_error(&info) right after every vpi_register_cb (and vpi_handle / vpi_iterate in setup paths), and log level, code and message, including the object name and callback reason. A NULL handle without that log is undiagnosable.
- Count registrations requested vs succeeded per reason, and report them. Never assume full coverage.
- Read the simulator's docs for the compile-time access options that gate VPI on assertions and other objects, but treat 'option X enables Y' as a hypothesis. Probe it as a separate build and record what it changes, instead of claiming it fixes the NULLs.
- Default debug or read-only access flags often allow reading values but not value-change or assertion callbacks. A build tuned for speed may need different flags for callback-based capture. Note those flags next to every capture result.