Below are some notes meant for those who decide to modify or port
SWI-Prolog. It is by no means complete.
SAVE/1 AND FRIENDS
If autoconf fails to detect a working save/1, read the comments in
pl-save.c and work on a stand-alone test version of this system using
% gcc -DTEST pl-save.c
Two things generally need attention. First is to get the start of the
data segment right. Compile and link a very small program and run
% nm -n a.out
on the executable. Find the start of the data-segment and decide on the
The initialised data, uninitialised data and malloc() heap are supposed
to be between DATA_START and sbrk(0). If not (like OS2), rewrite the
code that defines the savable segments. See the #ifdef OS2's for a
Second problems is the type of linking used. See INSTALL.notes.
LOAD_FOREIGN AND FRIENDS
If there is some supported version of foreign loading on your system and
it is not detected, add it.
ld -A/a.out loading may be detected, but may fail to compile, link or
work. Add the proper definitions to pl-load.c.
VIRTUAL MEMORY BASED DYNAMIC STACKS
Based on mmap()/munmap() of /dev/zero to allocate memory at arbitrary
places in the address space. This doesn't work with many
implementations of mmap() due to various restrictions: cannot map at
arbitrary page boundary, can only map limited pages, cannot map private,
etc. Work on test/mmap() if the test fails, but you think your mmap()
is good enough. If there are other ways to achieve this goal, please
let me know.
MEMORY ERRORS (BUS ERROR; SEGMENTATION FAULT)
It is very likely that the system crashes during the boot compilation
called from make (./pl -O -o ... -b ... -c ...). Below is a
description of various such problems:
WORKING ON A SMALL PROGRAM
[skip this, it is out of date, and doesn't work anymore. Debug using
./pl -d <n> -b ../boot/init.pl
where <n> is the debugging level to be used.
If you are very lucky the system will compile and perform the
bootstrap compilation in one go. Normally you are not. On how to get
it through the C compiler, you are on your own. If the system does
not want to do the bootstrap compilation, write a small prolog program
that can be handled by the bootstrap compiler and work on that.
Normally, I use
:- write('Hello World!').
You can compile this using `pl -d level -b file'. If this finally
writes `Hello World' on your terminal you are getting close. Next
test program can be:
If you can compile this and you can run the result by typing `a.out'
or `pl -x a.out -d level' you can try to do the entire boot
compilation. If you decide to write more elaborate test programs you
should realise many things are not yet defined. Initially you can
only call the foreign language predicates, defined in pl-ext.c.
Constructs handles by the compiler (,/2, !, ->/2, ;/2, etc.) cannot
be called via metacall or directives (but can be used in program
CODE USED ATOMS OR FUNCTORS
Atoms needed by the C sources are addressed using macros of the form
ATOM_<name>. Functors have the form FUNCTOR_<name><arity>. See the
file ATOMS. The awk script defatoms is invoked by make to create the
necessary C source and header files. These files are named *.i[ch].