🚀 go-pugleaf

99% of the uses of fork are followed by a few standard file i/o
re-arrangements, or by nothing, before an exec. These can all be
adequately covered by a fexec (fork + exec) type call, providing it
gives you some hooks to re-arrange the environment.

But the result is sort of like the MVS file system compared to the
Unix file system. On Unix, a file is a stream of bytes, and you are
free to lay whatever structure you want on top of it. Of course, you
have to do the work. On MVS, files come in a large set of flavors.
Large, but fixed. It does all the work for you for the structure it
provides, but if you need a structure that's not there, you're
screwed.

Likewise, fork/exec gives you the _total_ environment to play with in
any way you want. A fexec only lets you change what the designers
decided you needed to change. Note that fexec can be simulated with
fork/exec, but not vice-versa.

Calling the lack of a fexec in Unix a limitation is wrong. It was
designed to give you much more power than that. In the original
environment (64K-max processes with an MMU + hard disk), it was fast
elegant. In the environment under discussion (1/2Meg+ processes, and
no MMU, and maybe no hard disk) it's elegant, but a PAIN to implement,
and slow when it's done.

What I prefer is two system calls:

1) fexec - run another process in a fork, with a block describing how
the environment should be modified on the way. As stated above, this
does what you want for a new process most of the time.

Note that you still need a Unix exec()-like call, but that can be done
by fexec() followed by the parent exiting. If fexec can be structured
correctly, it would be nice to proivde this as a system call, thus
saveing process creation/exit overhead, and probably more.

2) newthread - roughly, vfork in 4BSD. This creates another process
that shares all resources with the parent. If either one does an
exit() (or exec()) the other one just continues. YOu probably want to
schedule the child to run first. Some technic for rendezvous will be
required.

Newthread gives you the capabilities of Unix fork/exec, you just
have to take more care about things. It should cover all the cases you
want fork for. Of course, the cases you want it for fork/exec

	Comments?
	<mike