I guess the question says it all.
I want to fork on Windows. What is the most similar operation and how do I use it.
14 Answers
Cygwin has fully featured fork() on Windows. Thus if using Cygwin is acceptable for you, then the problem is solved in the case performance is not an issue.
Otherwise you can take a look at how Cygwin implements fork(). From a quite old Cygwin's architecture doc:
5.6. Process Creation The fork call in Cygwin is particularly interesting because it does not map well on top of the Win32 API. This makes it very difficult to implement correctly. Currently, the Cygwin fork is a non-copy-on-write implementation similar to what was present in early flavors of UNIX.
The first thing that happens when a parent process forks a child process is that the parent initializes a space in the Cygwin process table for the child. It then creates a suspended child process using the Win32 CreateProcess call. Next, the parent process calls setjmp to save its own context and sets a pointer to this in a Cygwin shared memory area (shared among all Cygwin tasks). It then fills in the child's .data and .bss sections by copying from its own address space into the suspended child's address space. After the child's address space is initialized, the child is run while the parent waits on a mutex. The child discovers it has been forked and longjumps using the saved jump buffer. The child then sets the mutex the parent is waiting on and blocks on another mutex. This is the signal for the parent to copy its stack and heap into the child, after which it releases the mutex the child is waiting on and returns from the fork call. Finally, the child wakes from blocking on the last mutex, recreates any memory-mapped areas passed to it via the shared area, and returns from fork itself.
While we have some ideas as to how to speed up our fork implementation by reducing the number of context switches between the parent and child process, fork will almost certainly always be inefficient under Win32. Fortunately, in most circumstances the spawn family of calls provided by Cygwin can be substituted for a fork/exec pair with only a little effort. These calls map cleanly on top of the Win32 API. As a result, they are much more efficient. Changing the compiler's driver program to call spawn instead of fork was a trivial change and increased compilation speeds by twenty to thirty percent in our tests.
However, spawn and exec present their own set of difficulties. Because there is no way to do an actual exec under Win32, Cygwin has to invent its own Process IDs (PIDs). As a result, when a process performs multiple exec calls, there will be multiple Windows PIDs associated with a single Cygwin PID. In some cases, stubs of each of these Win32 processes may linger, waiting for their exec'd Cygwin process to exit.
Sounds like a lot of work, doesn't it? And yes, it is slooooow.
EDIT: the doc is outdated, please see this excellent answer for an update
I certainly don't know the details on this because I've never done it it, but the native NT API has a capability to fork a process (the POSIX subsystem on Windows needs this capability - I'm not sure if the POSIX subsystem is even supported anymore).
A search for ZwCreateProcess() should get you some more details - for example this bit of information from Maxim Shatskih:
The most important parameter here is SectionHandle. If this parameter is NULL, the kernel will fork the current process. Otherwise, this parameter must be a handle of the SEC_IMAGE section object created on the EXE file before calling ZwCreateProcess().
Though note that Corinna Vinschen indicates that Cygwin found using ZwCreateProcess() still unreliable:
Iker Arizmendi wrote:
> Because the Cygwin project relied solely on Win32 APIs its fork > implementation is non-COW and inefficient in those cases where a fork > is not followed by exec. It's also rather complex. See here (section > 5.6) for details: > > http://www.redhat.com/support/wpapers/cygnus/cygnus_cygwin/architecture.htmlThis document is rather old, 10 years or so. While we're still using Win32 calls to emulate fork, the method has changed noticably. Especially, we don't create the child process in the suspended state anymore, unless specific datastructes need a special handling in the parent before they get copied to the child. In the current 1.5.25 release the only case for a suspended child are open sockets in the parent. The upcoming 1.7.0 release will not suspend at all.
One reason not to use ZwCreateProcess was that up to the 1.5.25 release we're still supporting Windows 9x users. However, two attempts to use ZwCreateProcess on NT-based systems failed for one reason or another.
It would be really nice if this stuff would be better or at all documented, especially a couple of datastructures and how to connect a process to a subsystem. While fork is not a Win32 concept, I don't see that it would be a bad thing to make fork easier to implement.
Well, windows doesn't really have anything quite like it. Especially since fork can be used to conceptually create a thread or a process in *nix.
So, I'd have to say:
CreateProcess()/CreateProcessEx()
and
CreateThread() (I've heard that for C applications, _beginthreadex() is better).
Prior to Microsoft introducing their new "Linux subsystem for Windows" option, CreateProcess() was the closest thing Windows has to fork(), but Windows requires you to specify an executable to run in that process.
The UNIX process creation is quite different to Windows. Its fork() call basically duplicates the current process almost in total, each in their own address space, and continues running them separately. While the processes themselves are different, they are still running the same program. See here for a good overview of the fork/exec model.
Going back the other way, the equivalent of the Windows CreateProcess() is the fork()/exec() pair of functions in UNIX.
If you were porting software to Windows and you don't mind a translation layer, Cygwin provided the capability that you want but it was rather kludgey.
Of course, with the new Linux subsystem, the closest thing Windows has to fork() is actually fork() :-)
As other answers have mentioned, NT (the kernel underlying modern versions of Windows) has an equivalent of Unix fork(). That's not the problem.
The problem is that cloning a process's entire state is not generally a sane thing to do. This is as true in the Unix world as it is in Windows, but in the Unix world, fork() is used all the time, and libraries are designed to deal with it. Windows libraries aren't.
For example, the system DLLs kernel32.dll and user32.dll maintain a private connection to the Win32 server process csrss.exe. After a fork, there are two processes on the client end of that connection, which is going to cause problems. The child process should inform csrss.exe of its existence and make a new connection – but there's no interface to do that, because these libraries weren't designed with fork() in mind.
So you have two choices. One is to forbid the use of kernel32 and user32 and other libraries that aren't designed to be forked – including any libraries that link directly or indirectly to kernel32 or user32, which is virtually all of them. This means that you can't interact with the Windows desktop at all, and are stuck in your own separate Unixy world. This is the approach taken by the various Unix subsystems for NT.
The other option is to resort to some sort of horrible hack to try to get unaware libraries to work with fork(). That's what Cygwin does. It creates a new process, lets it initialize (including registering itself with csrss.exe), then copies most of the dynamic state over from the old process and hopes for the best. It amazes me that this ever works. It certainly doesn't work reliably – even if it doesn't randomly fail due to an address space conflict, any library you're using may be silently left in a broken state. The claim of the current accepted answer that Cygwin has a "fully-featured fork()" is... dubious.
Summary: In an Interix-like environment, you can fork by calling fork(). Otherwise, please try to wean yourself from the desire to do it. Even if you're targeting Cygwin, don't use fork() unless you absolutely have to.
The following document provides some information on porting code from UNIX to Win32: https://msdn.microsoft.com/en-us/library/y23kc048.aspx
Among other things, it indicates that the process model is quite different between the two systems and recommends consideration of CreateProcess and CreateThread where fork()-like behavior is required.
There is no easy way to emulate fork() on Windows.
I suggest you to use threads instead.
Most of the hacky solutions are outdated. Winnie the fuzzer has a version of fork that works on current versions of Windows 10 (tho this requires system specific offsets and can break easily too).
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