Re: image saving

[James Knight <jknight@xxxxxxxx>]

On Thursday, July 25, 2002, at 07:38  PM, Chris Double wrote:
> What does 'save-image' actually do? I don't have a linux machine to try
> it out on.
>
> Does it create a new executable that when run has the current state of
> the running process? How does it handle open files, running threads,
> etc? Or does it assume that all files, threads and other resources are
> shut down?
>
> I see that there is an 'unexec' version for Windows NT as well. Does
> this not work under Windows at the moment (hence image saving not
> working in windows) or is it just untested? Anyone working on getting
> it going? If not, I might have a stab at it.

save-image simply copies the current heap into a new copy of the 
executable, so that starting the new exe will be much faster as it 
doesn't need to go through all the booting process. This has many 
issues, none of which have been solved. 1) it doesn't copy 
shared-libraries's heaps. 2) It doesn't handle open files/threads/etc at 
all. 3) its not portable (although there is a libunexec now which I 
haven't looked at yet but claims to abstract everything - it would 
probably be a good idea to use that instead) 4) i'm sure there's more 
cause this is such a big hack.

It was taken essentially verbatim from GNU emacs, which uses it only to 
boot the program once and create an image. It only allows this in batch 
mode, ie: no windowing system, no user I/O, no threads, etc. It tries to 
make sure the system is in a basic enough state that nothing will break 
by saving the heap.

I put that in GOO a long time ago, back when GOO took much much longer 
to start up, way before threads or anything. Right now unexec is not 
even enabled on linux. If it is resurrected, I'd suggest putting a 
restriction similar to that of emacs: it should only ever be called very 
early in the boot process, after all the core modules are loaded, but 
before the REP loop starts, or any threads start, or any files that will 
stay opened are opened. Emacs does this with a command-line argument.

james