Checkpoints for large jobs

[Jim Reid]

In article <3193 at syma.sussex.ac.uk> william at syma.sussex.ac.uk (William Craven) writes:

   I was wondering whether there is a system which will allow a job to
   start off from when it last killed either by means of checkpointing
   or setjmp/longjmp. If there is such a scheme I would be grateful for
   pointers.

Yes. Unix processes have variables end, etext and edata which are
respectively the addresses at the end of the uninitialized data, text
and data "segments" of its address space. All that's needed is to
write out the data space and somehow bodge a stack pointer using
setjmp/longjmp. When the process is restarted, it uses malloc to grow
the data space if needed and then reads the file containing the
dumpded data. The process then has to re-open the files it had open
before the dump and then finally do a longjmp to put the stack back to
a known state before resuming execution. See end(3).

This is more or less what sendmail does to create a frozen
configuration file. On Sequents, all bets are off if the process has used
shared/private memory with lightweight processes created by m_fork(3).
The formats of executable files and core dumps is given by the man
pages for a.out and core, though these files are not nice to poke
around in.

		Jim