Interfacing to other languages

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


VHPIDIRECT is the simplest way to call C code from VHDL. VHPI is a complex API to interface C and VHDL, which allows to inspect the hierarchy, set callbacks and/or assign signals. GHDL does not support VHPI. For these kind of features, it is suggested to use VPI instead (see VPI build commands).

Foreign declarations

Only subprograms (functions or procedures) can be imported, using the foreign attribute. In this example, the sin function is imported:

package math is
  function sin (v : real) return real;
  attribute foreign of sin : function is "VHPIDIRECT sin";
end math;

package body math is
  function sin (v : real) return real is
    assert false severity failure;
  end sin;
end math;

A subprogram is made foreign if the foreign attribute decorates it. This attribute is declared in the 1993 revision of the std.standard package. Therefore, you cannot use this feature in VHDL 1987.

The decoration is achieved through an attribute specification. The attribute specification must be in the same declarative part as the subprogram and must be after it. This is a general rule for specifications. The value of the specification must be a locally static string.

Even when a subprogram is foreign, its body must be present. However, since it won’t be called, you can make it empty or simply put an assertion.

The value of the attribute must start with VHPIDIRECT (an upper-case keyword followed by one or more blanks). The linkage name of the subprogram follows.

The object file with the source code for the foreign subprogram must then be linked to GHDL, expanded upon in Wrapping and starting a GHDL simulation from a foreign program.

Restrictions on foreign declarations

Any subprogram can be imported. GHDL puts no restrictions on foreign subprograms. However, the representation of a type or of an interface in a foreign language may be obscure. Most non-composite types are easily imported:

integer types
They are represented by a 32 bit word. This generally corresponds to int for C or Integer for Ada.
physical types
They are represented by a 64 bit word. This generally corresponds to the long long for C or Long_Long_Integer for Ada.
floating point types
They are represented by a 64 bit floating point word. This generally corresponds to double for C or Long_Float for Ada.
enumeration types
They are represented by an 8 bit word, or, if the number of literals is greater than 256, by a 32 bit word. There is no corresponding C type, since arguments are not promoted.

Non-composite types are passed by value. For the in mode, this corresponds to the C or Ada mechanism. The out and inout interfaces of non-composite types are gathered in a record and this record is passed by reference as the first argument to the subprogram. As a consequence, you shouldn’t use in and inout modes in foreign subprograms, since they are not portable.

Records are represented like a C structure and are passed by reference to subprograms.

Arrays with static bounds are represented like a C array, whose length is the number of elements, and are passed by reference to subprograms.

Unconstrained arrays are represented by a fat pointer. Do not use unconstrained arrays in foreign subprograms.

Accesses to an unconstrained array are fat pointers. Other accesses correspond to an address and are passed to a subprogram like other non-composite types.

Files are represented by a 32 bit word, which corresponds to an index in a table.

Linking foreign object files to GHDL

You may add additional files or options during the link of GHDL using -Wl, as described in Passing options to other programs. For example:

ghdl -e -Wl,-lm math_tb

will create the math_tb executable with the lm (mathematical) library.

Note the c library is always linked with an executable.


The process for personal code is the same, provided the code is compiled to an object file. Analysis must be made of the HDL files, then elaboration with -Wl,personal.o toplevelEntityName as arguments. Additional object files are flagged separate -Wl,* arguments. The elaboration step will compile the executable with the custom resources. Further reading (particularly about the backend restrictions) is at Elaboration [-e] and Run [-r].

Wrapping and starting a GHDL simulation from a foreign program

You may run your design from an external program. You just have to call the ghdl_main function which can be defined:

in C:

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


Don’t forget to list the object file of this entry point as per Linking foreign object files to GHDL.

in Ada:

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

This function must be called once, and returns 0 at the end of the simulation.

Linking GHDL to Ada/C

As explained previously in Wrapping and starting a GHDL simulation from a foreign program, you can start a simulation from an Ada or C program. However the build process is not trivial: you have to elaborate your program and your VHDL design.


If the foreign language is C, this procedure is equivalent to the one described in Linking foreign object files to GHDL, which is easier. Thus, this procedure is explained for didactic purposes. When suitable, we suggest to use -e instead of --bind and --list-link.

First, you have to analyze all your design files. In this example, we suppose there is only one design file, design.vhdl.

$ ghdl -a design.vhdl

Then, bind your design. In this example, we suppose the entity at the design apex is design.

$ ghdl --bind design

Finally, compile/bind your program and link it with your VHDL design:

in C:

gcc my_prog.c -Wl,`ghdl --list-link design`

in Ada:

$ gnatmake my_prog -largs `ghdl --list-link design`

See GCC/LLVM only commands for further details about --bind and --list-link.

Dynamically loading foreign objects from GHDL

Instead of linking and building foreign objects along with GHDL, it is also possible to load foreign resources dynamically. In order to do so, provide the path and name of the shared library where the resource is to be loaded from. For example:

attribute foreign of get_rand: function is "VHPIDIRECT ./ get_rand";

Dynamically loading GHDL

In order to generate a position independent executable (PIE), be it an executable binary or a shared library, GHDL must be built with config option --default-pic. This will ensure that all the libraries and sources analyzed by GHDL generate position independent code (PIC). Furthermore, when the binary is built, argument -Wl,-pie needs to be provided.

PIE binaries can be loaded and executed from any language that supports C-alike signatures and types (C, C++, golang, Python, Rust, etc.). For example, in Python:

import ctypes
gbin = ctypes.CDLL(bin_path)

args = ['-gGENA="value"', 'gGENB="value"']

xargs = (ctypes.POINTER(ctypes.c_char) * (len(args) + 1))()
for i, arg in enumerate(args):
    xargs[i] = ctypes.create_string_buffer(arg.encode('utf-8'))
return args[0], xargs

gbin.main(len(xargv)-1, xargv)

import _ctypes
# On GNU/Linux
# On Windows

This allows seamless co-simulation using concurrent/parallel execution features available in each language: pthreads, goroutines/gochannels, multiprocessing/queues, etc. Moreover, it provides a mechanism to execute multiple GHDL simulations in parallel.

Using GRT from Ada


This topic is only for advanced users who know how to use Ada and GNAT. This is provided only for reference; we have tested this once before releasing GHDL 0.19, but this is not checked at each release.

The simulator kernel of GHDL named GRT is written in Ada95 and contains a very light and slightly adapted version of VHPI. Since it is an Ada implementation it is called AVHPI. Although being tough, you may interface to AVHPI.

For using AVHPI, you need the sources of GHDL and to recompile them (at least the GRT library). This library is usually compiled with a No_Run_Time pragma, so that the user does not need to install the GNAT runtime library. However, you certainly want to use the usual runtime library and want to avoid this pragma. For this, reset the GRT_PRAGMA_FLAG variable.

$ make GRT_PRAGMA_FLAG= grt-all

Since GRT is a self-contained library, you don’t want gnatlink to fetch individual object files (furthermore this doesn’t always work due to tricks used in GRT). For this, remove all the object files and make the .ali files read-only.

$ rm *.o
$ chmod -w *.ali

You may then install the sources files and the .ali files. I have never tested this step.

You are now ready to use it.

Here is an example, test_grt.adb which displays the top level design name.

with System; use System;
with Grt.Avhpi; use Grt.Avhpi;
with Ada.Text_IO; use Ada.Text_IO;
with Ghdl_Main;

procedure Test_Grt is
  --  VHPI handle.
  H : VhpiHandleT;
  Status : Integer;

  --  Name.
  Name : String (1 .. 64);
  Name_Len : Integer;
  --  Elaborate and run the design.
  Status := Ghdl_Main (0, Null_Address);

  --  Display the status of the simulation.
  Put_Line ("Status is " & Integer'Image (Status));

  --  Get the root instance.

  --  Disp its name using vhpi API.
  Vhpi_Get_Str (VhpiNameP, H, Name, Name_Len);
  Put_Line ("Root instance name: " & Name (1 .. Name_Len));
end Test_Grt;

First, analyze and bind your design:

$ ghdl -a counter.vhdl
$ ghdl --bind counter

Then build the whole:

$ gnatmake test_grt -aL`grt_ali_path` -aI`grt_src_path` -largs
 `ghdl --list-link counter`

Finally, run your design:

$ ./test_grt
Status is  0
Root instance name: counter