SWI-Prolog.h is in the include directory of the distribution. The distribution is normally installed in C:\Program Files\swipl on Windows and /usr[/local]/lib/swipl-<version> on Unix. Also the libraries are installed as part of the installation tree by default. Some (Linux) distributions package SWI-Prolog with the include files and libraries in shared locations, e.g., /usr/include and /usr/lib.

There are three options for dealing with the complexity of finding the include files and linking to the proper libraries

1. Using swipl-ld. This is a compiler and linker front-end for the C and C++ compilers that adds the necessary flags to find the include files and get the linking done. In addition, swipl-ld deals with combining Prolog code with the C and C++ code to create a stand-alone executable.
2. Using pkg-config. SWI-Prolog installs swipl.pc that allows getting the flags right. For example

   pkg-config --libs swipl
   -L/usr/lib/swi-prolog/lib/x86_64-linux -lswipl

3. Using cmake. SWI-Prolog provides swipl.cmake in in is distribution. The location can be found using the following shell command.

   $(swipl --home)/cmake/swipl.cmake

   This file defines the cmake import target swipl::libswipl that may be added to your target to get the compiler and linker flags correct. If you compile an plugin that can be loaded into SWI-Prolog you may call the following to properly configure your target

   target_link_swipl(target)

The preferred route depends on your overall project. (2) and (3) are mostly applicable when SWI-Prolog is embedded in a much larger project. (1) is more suitable for simple cases where you want to use a simple C/C++ file as a plugin or embedded SWI-Prolog.

Some notes in swipl-ld

To compile C/C++ objects you need to add the include subfolder to the include directories of your compiler or you can use the swipl-ld utility. The command below creates file.obj (Windows) or file.o (Unix), passing the proper include directory to the C-compiler. This requires the SWI-Prolog programs to be available from the PATH environment variable.

swipl-ld -c file.c

To link C/C++ objects to a shared-object (DLL) you need to links against swipl.lib on Windows. On ELF based Unix platforms (most today), you do not need an import library. Often, it is much easier to let swipl-ld do the job as in the command below, which creates file.dll or file.so depending on the platform.

swipl-ld -shared -o file file.o

To embed SWI-Prolog in a C/C++ main program you need to link against the swipl library on all platforms. Again, swipl-ld simplifies the process. The first command embeds the Prolog kernel in a C main program and the second attaches a Prolog state to the result, creating a stand-alone executable.

swipl-ld -o myexe -nostate file.o ...
swipl-ld -o myexe file.o ... file.pl ...

Learning from swipl-ld.

If you don't want to use swipl-ld, you can learn from it by adding the -v option to the commandline, which causes the program to print the commands it runs to do its job.

Getting information on the SWI-Prolog configuration

You can get information on SWI-Prolog's configuration by running the command below. This emits several variables in POSIX sh format that describe the system.

swipl --dump-runtime-variables