Adding support for core files to DJGPP

[Eli Zaretskii <eliz at delorie dot com>]

We are trying to add support for core files to the DJGPP port of GDB.
However, the available documentation which describes what needs to be
done is, at best, scanty ;-)  So I wonder if someone here can help.

Producing a core file in a format already supported by BFD is not
easy, because under the default algorithm used by DJGPP's sbrk the
program's address space may be non-contiguous.  This is due to
peculiarities of various DPMI hosts used on specific platforms,
most notably Windows, and the fact that the DPMI spec doesn't provide
any way for the program to specify where in memory will the program's
code and data be put; you just request a chunk of memory of the size
you need and get a buffer wherever the DPMI host wants it to be.  The
same problem also makes it next to impossible to reproduce the exact
way the program's address space was layed out after reading the core
file.

(If this is not clear enough, and if someone is interested, I can tell
more details.)

We already have a prototype version of code which writes to a disk
file the program's memory image and some additional information, such
as the general-purpose registers, segment selectors, etc.  But the
format of the core file is not something BFD can understand.

For these reasons, we would try first to explore the possibility of
using a custom core file format, instead of going through BFD.
However, I cannot find any real documentation of the API used by GDB
for accessing core files.  I'd expect GDB to need a few functions to
read the core file (or, perhaps, its specific portions), find out the
values in registers at the point of crash, what was the signal that
aborted the program, etc.  Where can I look for some info about this?

If someone has experience in providing such kind of core file support,
could they please post any advice that could be useful, and/or point
to any docs and code that could be of use to us?

Thanks in advance for any help.