Specifically I cannot get the line: gcc -shared -o lowercase.so lowercase.o
to work properly. I get the errors:
undefined reference to `PL_get_atom_chars'
undefined reference to `PL_warning'
undefined reference to `PL_resource_error'
undefined reference to `PL_unify_atom_chars'
undefined reference to `PL_register_foreign'
I have tried compiling with swipl-ld instead of gcc, but then on the next step of “load_foreign_library(lowercase).”, SWI-Prolog gives the error: No install function in 'c:/users/.../lowercase.dll'
Surely, the example will not work on Windows as is. Using swipl-ld does seem to create a reasonable .dll. I think the install function needs to be renamed to install_lowercase().
Defining this in a way that works on any OS is a challenge
Odd. The include file is SWI-Prolog.h and it is located in include inside the SWI-Prolog installation. You can find the installation directory using
swipl --home
On Windows you need to link your dll against libswipl.dll which is in the bin directory of the installation. Its import lib is in the lib directory. swipl-ld should deal with these issues. Use -v to see which commands it executes.
Normaly, using install_t should ensure the install symbol is exported from the dll. You can check that using nm (which I think also comes with gcc on Windows).
load_foreign_library/1,2 loads the dll, finds the install function and calls it to register the predicates with Prolog.
Forgive me, I’m a Windows user and typically have the help of IDEs on this sort of thing…
What I can say is that I have included SWI-Prolog.h in the C file, but the IDE couldn’t see it until I added the header file itself to the same local folder as the .C file. Once I did, though, all the types were recognized by the IDE.
GCC however does not recognize the references to PL_ commands as mentioned above.
Since the syntax reminds me of linking a .PL file with a .C file (which I have gotten to work correctly), I decided to replace gcc with swipl-ld, and I do successfully get a .DLL with no errors. But I try to load it into Prolog and I get the error “no install function in …”.
I can run:
open_shared_object(lowercase, B).
and I get a handle returned, but I haven’t found anything else that seems to work.
Any more detail than that is beyond my depth… I don’t know where I should put nm to check install_t, and I don’t know how to link the dll against anything in particular.
Update, StackOverflow showed me a couple things which weren’t helpful, but then the syntax for gcc’s nm came up, so I did get that part figured out. This is the output of that:
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I am hoping this means something to you, because it’s a lot of nothing to me…
Type: swipl-ld -shared -o <DLL NAME> <C FILE NAME.c>
(Of note, I did not need to put DLL.dll, just DLL is fine).
(Also noteworthy, using swipl-ld circumvents copying the SWI-Prolog header file, which is required for gcc).
Open Prolog.
Type: load_foreign_library(<DLL NAME>).
(Make sure the DLL file is either in Prolog’s home directory, mine being Documents/Prolog, or provide it with an absolute path. The absolute path requires you use double forward slashes everywhere, and surround it in quotes).
Now you can call RULE(A, B).
So glad it was this simple, it just took a while for me to realize I could make Prolog do all the linking/compiling instead of gcc. The inspiration to try came from this helpful little page: Where are SWI-Prolog.h and the library files?
