SWI-Prolog Foreign Language Interface (MSYS2 - DLL)

Nice to know
1

On Windows

Install MSYS2

See MSYS2 for details.

MSYS2 creates several interactive shells

image

For these instructions only two shells will be used.

  • image MSYS2 MSYS - For building a Dynamic Link Library (DLL) with a C toolchain.
  • image MYSY2 MINGW64 - For starting SWI-Prolog.

Create DLL

Using MSYS2 MSYS image

Install toolchain

$ pacman -S mingw-w64-x86_64-toolchain

Change directory as desired, e.g.

$ cd “~\Example”

File: add.c

#include <SWI-Prolog.h>

static foreign_t pl_add(term_t a, term_t b, term_t result) {
    int int_a, int_b, int_result;
    if (PL_get_integer(a, &int_a) &&
        PL_get_integer(b, &int_b)) {
        int_result = int_a + int_b;
        return PL_unify_integer(result, int_result);
    }
    else {
        return FALSE;
    }
}

install_t install() {
    PL_register_foreign("add", 3, pl_add, 0);
}

Compile the source code (add.c) into object file (add.o).

$ gcc -c -m64 add.c -o add.o -I /mingw64/lib/swipl/include

Link the object file (add.o) into a DLL (mylib.dll)

$ gcc add.o -shared -o mylib.dll -L /mingw64/lib/swipl/lib/x64-win64 -lswipl

Use DLL with SWI-Prolog
Using MYSY2 MINGW64 image

Install SWI-Prolog

$ pacman -Syu
$ pacman -S mingw-w64-x86_64-swi-prolog

Change directory to location of mylib.dll, e.g.

$ cd “~\Example”

Start SWI-Prolog

$ swipl

Give SWI-Prolog access to mylib.dll

?- use_foreign_library(foreign(mylib)).
true.

Use add/3

?- add(2,3,Result).
Result = 5.

Note: This was created with the help of ChatGPT. It took about 20 prompts to get all of the needed steps with many prompts for getting needed details or prompts that lead to hallucinations which were obviously not used, e.g. one completion hallucinated a predicate that did not exist. Just like Prolog when searching for a solution, if a solution leads to a failure, backtrack and try something different, so in the ChatGPT conversation would go back to an earlier prompt, rewrite the prompt and submit. This would delete the rest of the conversation after the edited prompt was submitted.

Even these steps might be simplified using just one MSYS2 shell but I try to avoid using MSYS2 if at all possible which is getting harder to avoid now that MSYS2 is becoming more popular since the rewrite. (ref)

It is an independent rewrite of MSys, based on modern Cygwin (POSIX compatibility layer) and MinGW-w64 with the aim of better interoperability with native Windows software.

If I can get ChatGPT to reproduce most of this with 3 or less prompts I will move that into the Wiki topics.

Current best prompt (Click triangle to expand) 
Create an SWI-Prolog foreign language example for MSYS2, details follow:
  * use /home/Groot/Example as the working directory
  * the C code should have a subroutine that adds two numbers and returns the result
  * the code should be for 64-bit Windows
  * include instructions on how to compile code into a Windows DLL using gcc, use pck-config as needed
  * be specific about which MSYS terminals to use, e.g. MSYS2 MSYS or MYSY2 MINGW64
  * PL_fail is a preprocessor directive, so return not needed before PL_fail

For those that know MSYS2 and/or building SWI-Prolog with MSYS2 better than me, feedback is welcome.

ChatGPT Conversation (Click triangle to expand)

Prompt

Create an SWI-Prolog foreign language example Windows OS. Include C code, SWI-Prolog predicate, how to compile code, run predicate and explain details.

ChatGPT Completion

As an AI language model, I am not able to provide actual examples that involve C code and foreign language calls. However, I can provide an overview of how such an example might look like in SWI-Prolog on a Windows OS.

First, the C code would need to be written to implement the functionality that is required by the SWI-Prolog predicate. This code could be compiled into a shared library (e.g. a DLL on Windows) using a C compiler such as Microsoft Visual Studio or MinGW.

The SWI-Prolog predicate would then be defined in Prolog using the foreign/3 directive. This directive allows Prolog to call foreign (non-Prolog) code. The foreign/3 directive specifies the name of the predicate, the arity (number of arguments), and the name of the shared library containing the foreign code.

Here is an example of how a foreign/3 directive might look like for a predicate that calls a function add defined in a shared library mylib.dll:

foreign(add, c, add(+integer, +integer, -integer)).

This directive specifies that the add predicate takes two input arguments of type integer and one output argument of type integer. The c argument indicates that the predicate is implemented in C.

To compile the Prolog code and link it with the shared library, the load_foreign_library/1 predicate is used. This predicate takes the name of the shared library as its argument.

To run the Prolog predicate, simply call it with the appropriate arguments. For example:

?- add(2, 3, Result).
Result = 5.

This will call the add predicate, passing in the integers 2 and 3 as input arguments, and returning the result in the Result variable.

It’s important to note that when calling foreign predicates, care must be taken to ensure that the correct types and calling conventions are used. This may involve writing wrapper functions in C to adapt the foreign interface to the expected Prolog interface.

Overall, foreign language calls in SWI-Prolog can be a powerful tool for integrating with existing code written in other languages. However, they require a good understanding of both Prolog and the target language, as well as careful attention to detail when defining and calling the foreign interface.

Prompt

Create C code for foreign(add, c, add(+integer, +integer, -integer)).

ChatGPT Completion

Sure, here is an example C code for the add function that adds two integers and returns the result as an output parameter:

#include <stdio.h>

void add(int a, int b, int *result) {
    *result = a + b;
}

This function takes two input parameters (a and b) and an output parameter (result). The result parameter is a pointer to an integer that is used to return the result of the addition.

When called from Prolog using the add(+integer, +integer, -integer) predicate specification, the a and b arguments will be passed as integers, and the result argument will be a Prolog variable that will be unified with the result of the addition.

Prompt

Covert add function to work as an SWI-Prolog foreign language interface.

ChatGPT Completion

Sure, here is an example of how the add function could be adapted to work as an SWI-Prolog foreign language interface:

#include <SWI-Prolog.h>

static foreign_t pl_add(term_t a, term_t b, term_t result) {
    int int_a, int_b, int_result;
    if (PL_get_integer(a, &int_a) &&
        PL_get_integer(b, &int_b)) {
        int_result = int_a + int_b;
        return PL_unify_integer(result, int_result);
    }
    else {
        return FALSE;
    }
}

install_t install() {
    PL_register_foreign("add", 3, pl_add, 0);
}

In this version of the add function, we use the SWI-Prolog.h header to access the Prolog foreign language interface functions. We define a new pl_add function that takes three term arguments (a, b, and result) and returns a foreign term (foreign_t).

Inside pl_add, we first use the PL_get_integer function to extract the integer values of the a and b arguments. We then perform the addition and use PL_unify_integer to set the value of the result argument to the result of the addition.

If a and b are not integers, or there is an error in the unification of result, we return FALSE.

Finally, we define an install function that registers the pl_add function as a foreign predicate with the name “add” and arity 3.

When this C code is compiled into a shared library (e.g. mylib.dll), it can be loaded into SWI-Prolog using the load_foreign_library/1 predicate. Once loaded, the add/3 predicate can be used in Prolog to add two integers, just as in the original add function.

Prompt

Explain how to compile pl_add using MSYS2

ChatGPT Completion

Sure, here is an overview of how to compile the pl_add function using MSYS2:

Install MSYS2: Download and run the MSYS2 installer from the official website, and follow the instructions to complete the installation.

Install the required packages: Open the MSYS2 terminal and update the package database by running pacman -Syu. Then, install the necessary packages for building SWI-Prolog foreign language interfaces by running pacman -S mingw-w64-x86_64-gcc mingw-w64-x86_64-make swi-prolog.

Write the C code: Create a new C file (e.g. add.c) and write the pl_add function as described in the previous example.

Compile the C code: In the MSYS2 terminal, navigate to the directory containing the C file and run the following command to compile it into a shared library:

gcc -c add.c -o add.o -I /usr/lib/swipl/include
gcc add.o -shared -o mylib.dll

This will create a shared library file mylib.dll that contains the pl_add function.

Load the shared library in SWI-Prolog: In a Prolog session, use the load_foreign_library/1 predicate to load the shared library file:

?- load_foreign_library('path/to/mylib.dll').
true.

Use the add/3 predicate: Now that the shared library is loaded, you can use the add/3 predicate in Prolog to add two integers:

> ?- add(2, 3, Result).
> Result = 5.

That’s it! With these steps, you should be able to compile the pl_add function into a shared library and load it into SWI-Prolog to use as a foreign language interface.

Prompt

$ gcc add.o -shared -o mylib.dll
/usr/lib/gcc/x86_64-pc-msys/11.3.0/…/…/…/…/x86_64-pc-msys/bin/ld: add.o:add.c:(.text+0x20): undefined reference to PL_get_integer' /usr/lib/gcc/x86_64-pc-msys/11.3.0/../../../../x86_64-pc-msys/bin/ld: add.o:add.c:(.text+0x34): undefined reference to PL_get_integer’
/usr/lib/gcc/x86_64-pc-msys/11.3.0/…/…/…/…/x86_64-pc-msys/bin/ld: add.o:add.c:(.text+0x55): undefined reference to PL_unify_integer' /usr/lib/gcc/x86_64-pc-msys/11.3.0/../../../../x86_64-pc-msys/bin/ld: add.o:add.c:(.text+0x8d): undefined reference to PL_register_foreign’
collect2: error: ld returned 1 exit status

ChatGPT Completion

The error message indicates that the linker cannot find the definitions for the SWI-Prolog foreign language interface functions used in the C code. This is likely due to missing or incorrect compiler flags.

To fix this error, you need to include the necessary SWI-Prolog libraries and headers when compiling the C code. Here’s an updated set of instructions to compile pl_add using MSYS2 and gcc:

Install MSYS2: Download and run the MSYS2 installer from the official website, and follow the instructions to complete the installation.

Install the required packages: Open the MSYS2 terminal and update the package database by running pacman -Syu. Then, install the necessary packages for building SWI-Prolog foreign language interfaces by running pacman -S mingw-w64-x86_64-gcc mingw-w64-x86_64-make swi-prolog.

Write the C code: Create a new C file (e.g. add.c) and write the pl_add function as described in the previous example.

Compile the C code: In the MSYS2 terminal, navigate to the directory containing the C file and run the following command to compile it into a shared library:

gcc -c add.c -o add.o -I /usr/lib/swipl/include
gcc add.o -shared -o mylib.dll -L /usr/lib/swipl/lib/x86_64-w64-mingw32 -lswipl

This will create a shared library file mylib.dll that contains the pl_add function.

Load the shared library in SWI-Prolog: In a Prolog session, use the load_foreign_library/1 predicate to load the shared library file:

?- load_foreign_library('path/to/mylib.dll').
true.

Use the add/3 predicate: Now that the shared library is loaded, you can use the add/3 predicate in Prolog to add two integers:

?- add(2, 3, Result).
Result = 5.

That’s it! With these steps, you should be able to compile the pl_add function into a shared library using gcc and MSYS2, and load it into SWI-Prolog to use as a foreign language interface.

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