multi-thread VS multi-process

hi all

why most of Unix-based software is multi-process but not multi-thread?
like Qmail,Sendmail,APACHE etc. especially APACHE, its windows version is multi-thread, but it's unix version is multi-process???

Apache v2 is multi-threaded.
Linux threading implementation has for a long time been not very stable and/or efficient (and perhaps is it still the case), so it may have been a reason why developers have often chosen the simpler and more error forgiving multi-processing way.

iirc, processes are more expensive in windows than in linux. Thus, if a linux app doesn't actually *need* to share memory, it just uses processes. Even if it does need to share memory, there's always SysV shared memory which lets you get around some thread gotchas.
On windows, threads are substantially less expensive than processes.

I just shifted from Windows programming in these 3 months.
I know processes is more expensive than thread in Windows, but I don't know about Linux, so... do you mean thread in Linux is not more efficient than process?

another question: could I use pthread in Windows?
I think most of multi-process program in Linux use a lot Linux feature to run their program. If I want my multi-thread program runs in both Windows and Linux, it seems use multi-thread better than multi-process, isn't it?

I found gnu/pth is support Windows http://www.gnu.org/software/pth/
does it really works well in Windows