Co-simulation with GHDL

Three main approaches are used to co-simulate (co-execute) VHDL sources along with software applications written in a language other than VHDL (typically C/C++/SystemC):

  • Verilog Procedural Interface (VPI), also known as Program Language Interface (PLI) 2.0.
  • VHDL Procedural Interface (VHPI), or specific implementations, such as Foreign Language Interface (FLI).
  • Generation of C/C++ models/sources through a transpiler.

VPI and VHPI are complex APIs which allow to inspect the hierarchy, set callbacks and/or assign signals. Because provided features are similar, GHDL supports VPI only. Furthermore, as an easier to use alternative, GHDL features a custom coexecution procedure named VHPIDIRECT, similar to SystemVerilog’s Direct Programming Interface (DPI). As of today, generation of C/C++ models à la Verilator is not supported. However, a ghdlator might be available in the future.

Interfacing with foreign languages through VHPIDIRECT is possible on any platform. You can define a subprogram in a foreign language (such as C or Ada) and import it into a VHDL design.


GHDL supports different backends, and not all of them generate binary artifacts. Precisely, mcode is an in-memory backend. Hence, the examples need to be built/executed with either LLVM or GCC backends. A few of them, the ones that do not require linking object files, can be used with mcode.


As a consequence of the runtime copyright, you are not allowed to distribute an executable produced by GHDL without allowing access to the VHDL sources. See Copyrights | Licenses.


See #1053 for on-going work with regard to VHPIDIRECT.


See VPI build commands.