# SWI-Prolog Interface to C++/C **URL:** \ **Category:** General\ **Created:** [August 8, 2024, 3:05am UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671 "2024-08-08T03:05:08Z")\ **Posts on this page:** 16\ **Page:** 1
**Author:** ![konokaiden](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/konokaiden/32/6708_2.png) [@konokaiden](https://swi-prolog.discourse.group/u/konokaiden)\ **Post date:** [August 8, 2024, 3:05am UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/1 "2024-08-08T03:05:09Z")
I have been all over the internet trying to make this work, so I will keep things brief via bullet points. - I am pretty new to Prolog and logic programming, but I have programmed with other languages before. - I want to use Prolog with other languages (ideally I’d like to get it working with C, C++, and C#) - From what I’ve found, I can’t get swipl to work with C# on even the most basic script. Could it be user error? Absolutely, but also the interfaces I found were very old, and possibly deprecated. I can’t tell. - I’ve been trying to get things working with either C or C++ recently, but I can’t get that to work either. - I have been on the github page for swipl-cpp interface ([GitHub - SWI-Prolog/packages-cpp: The SWI-Prolog C++ interface](https://github.com/SWI-Prolog/packages-cpp)) and we can’t get it working either for their test script. - My goal would be to learn both how to use foreign language scripts within swipl as a parent, AND using swipl as a logic engine within a foreign program. I already have a program where the latter would be helpful, but the former is probably easier. - I am on Windows 64-bit, and since my swipl folder is in Program Files, not Program Files (x86), I think I have the 64-bit SWI-Prolog as well. - I have no idea how to make stuff work, but if someone could provide a super basic walkthrough, or just a test.cpp, test.pl, \ so that I can get the gist of how to make it work together, that would be superb. Plus any needed installations and setup procedures. That would be fantastic… So yeah, I know this is super vague, but the manual hasn’t been very helpful, I have gotten far enough to compile a cpp and a pl program into an exe with swipl commands from the terminal (something like \), but it doesn’t run because it seems like it’s missing a dll somewhere, and the cpp interface people only knew how to make it work on Linux, so any help you can give would be appreciated! ---
**Author:** ![maren](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/maren/32/6605_2.png) [@maren](https://swi-prolog.discourse.group/u/maren)\ **Post date:** [August 8, 2024, 3:22am UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/2 "2024-08-08T03:22:37Z")
The manual is extensive on the foreign language interface: [SWI-Prolog -- Foreign Language Interface](https://www.swi-prolog.org/pldoc/man?section=foreign) Did you find this particular set of pages? ---
**Author:** ![peter.ludemann](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/peter.ludemann/32/48_2.png) [@peter.ludemann](https://swi-prolog.discourse.group/u/peter.ludemann)\ **Post date:** [August 8, 2024, 3:27am UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/3 "2024-08-08T03:27:27Z")
Details of the problem are here: [Can't find entry point...? · Issue #88 · SWI-Prolog/packages-cpp · GitHub](https://github.com/SWI-Prolog/packages-cpp/issues/88) ---
**Author:** ![maren](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/maren/32/6605_2.png) [@maren](https://swi-prolog.discourse.group/u/maren)\ **Post date:** [August 8, 2024, 3:48am UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/4 "2024-08-08T03:48:06Z")
Interesting. I wonder if this is related to the -Bstatic flag you’re using in the swipl-ld invocation there (in the screenshot in the github thread). With -Bstatic, libstdc++ will static link, but it can’t work on its own. it also needs pthreads. maybe adding `-lpthread` (and probably also `-lm`) will do something for you. Or removing -Bstatic. ---
**Author:** ![peter.ludemann](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/peter.ludemann/32/48_2.png) [@peter.ludemann](https://swi-prolog.discourse.group/u/peter.ludemann)\ **Post date:** [August 8, 2024, 4:20am UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/5 "2024-08-08T04:20:50Z")
> [@maren](#): > > I wonder if this is related to the -Bstatic flag `-Bstatic` was my suggestion; but it doesn’t seem to do anything. (The original problem was reported without `-Bstatic.) In case it isn’t clear – the problem occurs on Windows, not on Linux. My theory is that the needed DLLs aren’t on the `%PATH%` (or whatever Windows uses) but I don’t know how to either diagnose or fix this (on Linux, I’d use `ldd` and `LIBRARY_PATH`; possibly some “path” options to the link step). ---
**Author:** ![konokaiden](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/konokaiden/32/6708_2.png) [@konokaiden](https://swi-prolog.discourse.group/u/konokaiden)\ **Post date:** [August 8, 2024, 1:19pm UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/6 "2024-08-08T13:19:41Z")
I went throught the executable with Dependencies (updated Dependency Walker, link here: [GitHub - lucasg/Dependencies: A rewrite of the old legacy software “depends.exe” in C# for Windows devs to troubleshoot dll load dependencies issues.](https://github.com/lucasg/Dependencies?tab=readme-ov-file) Copied all the dependent libraries to the folder with the EXE, double-checked that my Path does include swipl/bin (it does), and still nothing worked. Also re-compiled with the slightly longer code from likes.cpp: ```prolog swipl-ld -o likes -ld g++ -goal true likes.cpp likes.pl ``` This changed nothing. I also confirmed my SWI-Prolog is the 64-bit version. It is, version 9.2.5. ---
**Author:** ![konokaiden](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/konokaiden/32/6708_2.png) [@konokaiden](https://swi-prolog.discourse.group/u/konokaiden)\ **Post date:** [August 8, 2024, 1:51pm UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/7 "2024-08-08T13:51:50Z")
Okay, we have a starting point. The code from here does in fact work: [https://www.swi-prolog.org/pldoc/man?section=foreign-example](https://www.swi-prolog.org/pldoc/man?section=foreign-example) I simply copied the Prolog to a new prolog script (pl.pl, how imaginitive!), and copied the C file info into a new calc.c file (Visual Studio 2022 if anyone cares). Then ran `swipl-ld -goal true -o calc calc.c pl.pl` After that, ran `./calc X` using various values instead of X (52/3, pi, etc). At the very least, this does work. ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [August 8, 2024, 2:20pm UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/8 "2024-08-08T14:20:29Z")
Ok, so the problem is C++ ☹ I do not have access to Windows right now. Can you add `-v` to `swipl-ld` to see what commands it executes? ---
**Author:** ![konokaiden](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/konokaiden/32/6708_2.png) [@konokaiden](https://swi-prolog.discourse.group/u/konokaiden)\ **Post date:** [August 8, 2024, 2:21pm UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/9 "2024-08-08T14:21:26Z")
Hey, I ran that before and here’s what the output was: ![image](https://global.discourse-cdn.com/free1/uploads/swiprolog/original/2X/5/521e27a5163efdf31cd8ac807725b48f2bd7aa70.png) I’m also trying to brute force troubleshoot things myself. I added these lines to likes.cpp: ![image](https://global.discourse-cdn.com/free1/uploads/swiprolog/original/2X/b/ba78900dbd72f1e916b80b0475ffe3d6d9a1651d.png) (I also got rid of the using statement and did some format cleanup, but the same issue happened before I touched anything) Anyway, the point in question is that it should print a message just after as PlEngine is called. As you can see, it didn’t. Could it be running and std::cout doesn’t print to the terminal? Or is PlEngine failing more likely? ![image](https://global.discourse-cdn.com/free1/uploads/swiprolog/original/2X/7/725ccdb39b7f4a45dcd8e79aa0f441a62ba9c0a7.png) ---
**Author:** ![konokaiden](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/konokaiden/32/6708_2.png) [@konokaiden](https://swi-prolog.discourse.group/u/konokaiden)\ **Post date:** [August 8, 2024, 2:40pm UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/10 "2024-08-08T14:40:12Z")
Not sure if this means much, but printf() doesn’t seem to affect the lack of behavior. So it probably isn’t the functions/namespaces themselves. I don’t know if there is a problem linking, or with Prolog being initialized, or what… ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [August 8, 2024, 2:48pm UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/11 "2024-08-08T14:48:48Z")
PlEngine() may throw an exception. That may tell us what is bothering? Does a print before PlEngine() work, so at least we know that loading all DLLs worked? ---
**Author:** ![konokaiden](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/konokaiden/32/6708_2.png) [@konokaiden](https://swi-prolog.discourse.group/u/konokaiden)\ **Post date:** [August 8, 2024, 2:53pm UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/12 "2024-08-08T14:53:20Z")
That would make sense, as I tried to see if Prolog would print to the console, and it didn’t. Putting the print statement before the PlEngine did not seem to change anything, so I assume it’s the DLLs? ---
**Author:** ![konokaiden](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/konokaiden/32/6708_2.png) [@konokaiden](https://swi-prolog.discourse.group/u/konokaiden)\ **Post date:** [August 11, 2024, 4:08am UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/13 "2024-08-11T04:08:47Z")
So, a few things. First, I went ahead and tried again on a different Windows computer to see if something I had done was causing problems. This is also Windows 11, x64, and the C example still works. I did have trouble compiling this time, so I wonder if I wasn’t including something I should have been (though I’m not sure why I didn’t still have at least one of these errors). First up was this one: ![Screenshot 2024-08-10 225731](https://global.discourse-cdn.com/free1/uploads/swiprolog/original/2X/f/ff24b22f45ecaaad5fed941838b8fcf7adccf9d9.png) Fixed with double quotes, telling it that this file is mine, not something native to C++. After that, there was a whole host of issues. I stopped for a moment on each section of error code, but you’ll need to pause to read it all. > **[Windows PowerShell 2024-08-10 22-58-42.mp4](https://drive.google.com/file/d/1ZMmbdEB7dRom3q116XYUQ3jGAds7Fi66/view?usp=sharing)** > > Google Drive file. ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [August 11, 2024, 9:46am UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/14 "2024-08-11T09:46:43Z")
Something strange is happening. PL\_free\_term\_ref() is introduced in 9.3.7 and backported (with a partial implementation) to 9.2.6. It not being defined hints you are running some combination of versions? ---
**Author:** ![konokaiden](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/konokaiden/32/6708_2.png) [@konokaiden](https://swi-prolog.discourse.group/u/konokaiden)\ **Post date:** [August 11, 2024, 8:32pm UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/15 "2024-08-11T20:32:15Z")
Okay, it definitely seems to be a version issue. I was on SWI-Prolog 9.0.something on the second computer (I already had Prolog on it, so I didn’t think to check the version). Everything compiled perfectly and is working now that I updated to 9.2.6. This makes me think that the other computer might also be a different version, though I thought it was also 9.2, so I’ll need to double check that. Anyway, thank you for helping to get this sorted out! ---
**Author:** ![peter.ludemann](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/peter.ludemann/32/48_2.png) [@peter.ludemann](https://swi-prolog.discourse.group/u/peter.ludemann)\ **Post date:** [August 12, 2024, 5:57pm UTC](https://swi-prolog.discourse.group/t/swi-prolog-interface-to-c-c/7671/16 "2024-08-12T17:57:39Z")
I would recommend using the “development” version (currently 9.3.8) if you’re using C++. Any bug fixes will happen there first; and @jan does a great job of ensuring that the core functionality doesn’t break.