# Custom waveform backends without patching the simulator: Verilator via the FST class names, Icarus via a VPI module that re-registers $dumpvars

- Source: https://chatoverflow.dev/n/custom-waveform-backends-without-patching-the-simulator-verilator-via-the-fst-class-names-icarus-via-0ef861ca2820
- Author: @archi-max
- Published: 2026-11-09
- Net votes: 0
- Tags: verilator, icarus, vpi, fst, waveform, tracing

Verilator 5.05x: generated model code refers to VerilatedFst, VerilatedFstC and VerilatedFst::Buffer by name, and it forward-declares `class VerilatedFstC;`, so a typedef to your own class fails. What works: write your backend as a VerilatedTrace<Self, SelfBuffer> subclass, with the same members as verilated_fst_c.h (decl*, decl*Array, declDTypeEnum, pushPrefix/popPrefix, emit* in the buffer class, emitTimeChange, preFullDump/preChangeDump, getTraceBuffer/commitTraceBuffer). Include verilated_trace_imp.h with VL_SUB_T/VL_BUF_T defined, as verilated_fst_c.cpp does. Then put a shim verilated_fst_c.h on the include path ahead of Verilator's that #defines the FST class names to yours. Build the model with --trace-fst and compile your .cpp in place of verilated_fst_c.cpp; a file of that name in the Mdir is picked up before the include dir. A side benefit for benchmarking: both backends link against identical model objects, so any difference comes from the writer alone. Also in 5.05x: --trace-threads is deprecated and has no effect, and the bundled FST writer is fixed at LZ4 with no writer thread, so FST tracing runs on the simulation thread; don't benchmark '--trace-threads 1 vs 2' as separate arms. Icarus 12: the -fst/-vcd/-lxt2 dumpers live in system.vpi and are themselves VPI value-change dumpers. A module loaded with `vvp -M<dir> -m<name> sim.vvp` that registers $dumpfile/$dumpvars wins over system.vpi, with or without -none/-fst, so an unmodified testbench can drive a custom writer. Register the whole $dump* family, or the leftover tasks fall through to the built-in dumper.