Wrapping a simulation

By default, GHDL generates an internal main function, which is the entrypoint to executable binaries/simulations. However, simulation can be optionally executed from external programs, by declaring given functions as externally defined resources. Should an alternative main function be defined, that will be used by the executable, instead of the one from GHDL.

Wrapping the simulation allows dynamically generating test data, and/or manipulating CLI arguments.


Shared libraries should not contain a main symbol, but explicitly named entrypoints. When Generating shared libraries or Loading a simulation, wrapping ghdl_main is required.

Single function exection (ghdl_main)

Function ghdl_main allows executing the simulation until termination. When called, GRT takes control and drives the simulation, without intervention from the wrapper. However, if VHPIDIRECT or VPI functions/callbacks were registered, those are triggered at the corresponding simulation time.

In your foreign sources, define the external prototype of ghdl_main as follows:

in C:

extern int ghdl_main (int argc, char **argv);

in Ada:

with System;
function Ghdl_Main (Argc : Integer; Argv : System.Address)
  return Integer;
pragma import (C, Ghdl_Main, "ghdl_main");


Don’t forget to list the object file(s) of this entry point and other foreign sources, as per Linking foreign object files to GHDL.


The ghdl_main function must be called once, since reseting/restarting the simulation runtime is not supported yet. A workaround is to build the simulation as a shared object and load the ghdl_main symbol from it (see shghdl).


Immitating the run time flags, such as -gDEPTH=12 from -gGENERIC, requires the argv to have the executable’s path at index 0, effectively shifting all other indicies along by 1. This can be taken from the 0 index of the argv passed to main(), or (not suggested, despite a lack of consequences) left empty/null.

Since ghdl_main is the entrypoint to the design (the simulation kernel/runtime named GRT), the supported CLI options are the ones shown in Simulation (runtime). Options for analysis/elaboration are not required and will NOT work. See -r.

Step by step execution

For finer grained control of the simulation steps, a set of lower level functions can be used instead of ghdl_main. Those are provided in grt.h. After initializing GRT, providing arguments and initializing the simulation, a loop allows checking the return value of each step:

// 0: delta cycle
// 1: non-delta cycle
// 2: stop
// 3: finished
// 4: stop-time reached
// 5: stop-delta reached
int ecode;
do {
  ecode = __ghdl_simulation_step ();
  printf("ecode: %d\n", ecode);
} while (ecode<3);