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Date: Mon, 22 Jan 2018 08:58:00 -0700
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Subject: Re: [PHP-DEV] PCNTL compatibility to Windows?
From: thruska@cubiclesoft.com (Thomas Hruska)

On 1/22/2018 5:16 AM, David Rodrigues wrote:
> Hello.
> 
> I know that PCNTL extension is not compatible with Windows, but I can't
> found the reason for that.
> 
> I mean, I know that it is a feature that Unix system could provide in a
> better way (natively I guess). But "why" Windows could not emulates this
> features and have a PCNTL support too? Even if it had a lower performance,
> it still could be useful on testing environments.
> 
> So the question is: there are some hard limitation to it be impossible or
> would it be kind of a "lack of interest"?
> 
> Thanks!

Windows is a completely different OS.


Windows does not have signals:

https://stackoverflow.com/questions/3333276/signal-handling-on-windows

That rules out all of PCNTL's signal handling support, which is 11 out 
of the 22 functions that PCNTL has (I'm excluding aliases in that 
count).  Simply being unable to implement 50% of the functions is not a 
good start to a successful port.


Windows does not have fork/exec.  Although there are non-starter 
"solutions":

https://en.wikipedia.org/wiki/Fork%E2%80%93exec

Windows starts and manages processes VERY differently from *NIX.  Cygwin 
apparently has a working fork/exec implementation for Windows, but 
Cygwin is GPL.  Any implementation would have to be clean-roomed for PHP 
and would likely involve ugly things such as copying raw memory from one 
process space to another and/or using undocumented Zw kernel calls, all 
of which can change dramatically between OS releases.


Windows processes are referenced by HANDLE, not process ID.  Referencing 
by process ID might seem doable with OpenProcess():

https://msdn.microsoft.com/en-us/library/windows/desktop/ms684320(v=vs.85).aspx

But even PROCESS_QUERY_LIMITED_INFORMATION access might be denied when 
attempting to open the handle.  Even for child processes.  It happens 
when a process changes its own DACLs specifically to block 
OpenProcess()/OpenThread() calls.  Although the approach can't readily 
block SeDebugPrivilege, you don't ever want PHP core/userland running 
with SeDebugPrivilege.

A PHP implementation might work fine for direct children for most 
use-cases where the HANDLEs are kept around, but grandchildren might not 
be accessible.  Also, there's already the "proc_..." series of functions 
for handling child processes, which more correctly uses generic resource 
handles instead of integers.


Windows does not know what a zombie process is.  Unlike *NIX, Windows 
doesn't have a rule that a child must have a parent and that the parent 
must wait on each child to exit before the parent itself exits.  Windows 
processes can exit whenever they want and the kernel cleans up after the 
process.


Windows does have a few process priorities:

https://msdn.microsoft.com/en-us/library/windows/desktop/ms686219(v=vs.85).aspx

However, you can do things such as completely freeze up Windows - 
including the keyboard processing buffer and mouse cursor - with a 
process priority of REALTIME_PRIORITY_CLASS and a "while (1)" loop where 
the specified process will get *exclusive* scheduling by the kernel 
(i.e. a reboot is the only option).  It's a little-known security 
vulnerability-waiting-to-happen bit of Windows API history.


I dunno.  When all is said and done, pcntl_getpriority(), 
pcntl_setpriority(), pcntl_get_last_error(), pcntl_strerror(), and maybe 
pcntl_wait() and associated status functions are about the only 
functions that can be somewhat cleanly implemented for Windows using 
Windows APIs with minimal effort.  pcntl_waitpid() might be able to be 
implemented with some effort but possibly not work properly for pids 
less than 1 (with all the usual waitpid() caveats).  The signal handling 
are simply not doable.  Implementing fork/exec doesn't make a lot of 
sense - a lot of effort for little gain.

-- 
Thomas Hruska
CubicleSoft President

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