What is the closest thing Windows has to fork()?

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.

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.html > > This 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).

People have tried to implement fork on Windows. This is the closest thing to it I can find:

Taken from: http://doxygen.scilab.org/5.3/d0/d8f/forkWindows_8c_source.html#l00216

static BOOL haveLoadedFunctionsForFork(void);

int fork(void) 
{
    HANDLE hProcess = 0, hThread = 0;
    OBJECT_ATTRIBUTES oa = { sizeof(oa) };
    MEMORY_BASIC_INFORMATION mbi;
    CLIENT_ID cid;
    USER_STACK stack;
    PNT_TIB tib;
    THREAD_BASIC_INFORMATION tbi;

    CONTEXT context = {
        CONTEXT_FULL | 
        CONTEXT_DEBUG_REGISTERS | 
        CONTEXT_FLOATING_POINT
    };

    if (setjmp(jenv) != 0) return 0; /* return as a child */

    /* check whether the entry points are 
       initilized and get them if necessary */
    if (!ZwCreateProcess && !haveLoadedFunctionsForFork()) return -1;

    /* create forked process */
    ZwCreateProcess(&hProcess, PROCESS_ALL_ACCESS, &oa,
        NtCurrentProcess(), TRUE, 0, 0, 0);

    /* set the Eip for the child process to our child function */
    ZwGetContextThread(NtCurrentThread(), &context);

    /* In x64 the Eip and Esp are not present, 
       their x64 counterparts are Rip and Rsp respectively. */
#if _WIN64
    context.Rip = (ULONG)child_entry;
#else
    context.Eip = (ULONG)child_entry;
#endif
    
#if _WIN64
    ZwQueryVirtualMemory(NtCurrentProcess(), (PVOID)context.Rsp,
        MemoryBasicInformation, &mbi, sizeof mbi, 0);
#else
    ZwQueryVirtualMemory(NtCurrentProcess(), (PVOID)context.Esp,
        MemoryBasicInformation, &mbi, sizeof mbi, 0);
#endif

    stack.FixedStackBase = 0;
    stack.FixedStackLimit = 0;
    stack.ExpandableStackBase = (PCHAR)mbi.BaseAddress + mbi.RegionSize;
    stack.ExpandableStackLimit = mbi.BaseAddress;
    stack.ExpandableStackBottom = mbi.AllocationBase;

    /* create thread using the modified context and stack */
    ZwCreateThread(&hThread, THREAD_ALL_ACCESS, &oa, hProcess,
        &cid, &context, &stack, TRUE);

    /* copy exception table */
    ZwQueryInformationThread(NtCurrentThread(), ThreadBasicInformation,
        &tbi, sizeof tbi, 0);
    tib = (PNT_TIB)tbi.TebBaseAddress;
    ZwQueryInformationThread(hThread, ThreadBasicInformation,
        &tbi, sizeof tbi, 0);
    ZwWriteVirtualMemory(hProcess, tbi.TebBaseAddress, 
        &tib->ExceptionList, sizeof tib->ExceptionList, 0);

    /* start (resume really) the child */
    ZwResumeThread(hThread, 0);

    /* clean up */
    ZwClose(hThread);
    ZwClose(hProcess);

    /* exit with child's pid */
    return (int)cid.UniqueProcess;
}
static BOOL haveLoadedFunctionsForFork(void)
{
    HANDLE ntdll = GetModuleHandle("ntdll");
    if (ntdll == NULL) return FALSE;

    if (ZwCreateProcess && ZwQuerySystemInformation && ZwQueryVirtualMemory &&
        ZwCreateThread && ZwGetContextThread && ZwResumeThread &&
        ZwQueryInformationThread && ZwWriteVirtualMemory && ZwClose)
    {
        return TRUE;
    }

    ZwCreateProcess = (ZwCreateProcess_t) GetProcAddress(ntdll,
        "ZwCreateProcess");
    ZwQuerySystemInformation = (ZwQuerySystemInformation_t)
        GetProcAddress(ntdll, "ZwQuerySystemInformation");
    ZwQueryVirtualMemory = (ZwQueryVirtualMemory_t)
        GetProcAddress(ntdll, "ZwQueryVirtualMemory");
    ZwCreateThread = (ZwCreateThread_t)
        GetProcAddress(ntdll, "ZwCreateThread");
    ZwGetContextThread = (ZwGetContextThread_t)
        GetProcAddress(ntdll, "ZwGetContextThread");
    ZwResumeThread = (ZwResumeThread_t)
        GetProcAddress(ntdll, "ZwResumeThread");
    ZwQueryInformationThread = (ZwQueryInformationThread_t)
        GetProcAddress(ntdll, "ZwQueryInformationThread");
    ZwWriteVirtualMemory = (ZwWriteVirtualMemory_t)
        GetProcAddress(ntdll, "ZwWriteVirtualMemory");
    ZwClose = (ZwClose_t) GetProcAddress(ntdll, "ZwClose");

    if (ZwCreateProcess && ZwQuerySystemInformation && ZwQueryVirtualMemory &&
        ZwCreateThread && ZwGetContextThread && ZwResumeThread &&
        ZwQueryInformationThread && ZwWriteVirtualMemory && ZwClose)
    {
        return TRUE;
    }
    else
    {
        ZwCreateProcess = NULL;
        ZwQuerySystemInformation = NULL;
        ZwQueryVirtualMemory = NULL;
        ZwCreateThread = NULL;
        ZwGetContextThread = NULL;
        ZwResumeThread = NULL;
        ZwQueryInformationThread = NULL;
        ZwWriteVirtualMemory = NULL;
        ZwClose = NULL;
    }
    return FALSE;
}

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() :-)

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.

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.

"as soon as you want to do file access or printf then io are refused"

• You cannot have your cake and eat it too... in msvcrt.dll, printf() is based on the Console API, which in itself uses lpc to communicate with the console subsystem (csrss.exe). Connection with csrss is initiated at process start-up, which means that any process that begins its execution "in the middle" will have that step skipped. Unless you have access to the source code of the operating system, then there is no point in trying to connect to csrss manually. Instead, you should create your own subsystem, and accordingly avoid the console functions in applications that use fork().

• once you have implemented your own subsystem, don't forget to also duplicate all of the parent's handles for the child process;-)

"Also, you probably shouldn't use the Zw* functions unless you're in kernel mode, you should probably use the Nt* functions instead."

• This is incorrect. When accessed in user mode, there is absolutely no difference between Zw*** Nt***; these are merely two different (ntdll.dll) exported names that refer to the same (relative) virtual address.

ZwGetContextThread(NtCurrentThread(), &context);

• obtaining the context of the current (running) thread by calling ZwGetContextThread is wrong, is likely to crash, and (due to the extra system call) is also not the fastest way to accomplishing the task.

Your best options are CreateProcess() or CreateThread(). There is more information on porting here.

There is no easy way to emulate fork() on Windows.

I suggest you to use threads instead.

fork() semantics are necessary where the child needs access to the actual memory state of the parent as of the instant fork() is called. I have a piece of software which relies on the implicit mutex of memory copying as of the instant fork() is called, which makes threads impossible to use. (This is emulated on modern *nix platforms via copy-on-write/update-memory-table semantics.)

The closest that exists on Windows as a syscall is CreateProcess. The best that can be done is for the parent to freeze all other threads during the time that it is copying memory over to the new process's memory space, then thaw them. Neither the Cygwin frok [sic] class nor the Scilab code that Eric des Courtis posted does the thread-freezing, that I can see.

Also, you probably shouldn't use the Zw* functions unless you're in kernel mode, you should probably use the Nt* functions instead. There's an extra branch that checks whether you're in kernel mode and, if not, performs all of the bounds checking and parameter verification that Nt* always do. Thus, it's very slightly less efficient to call them from user mode.

The closest you say... Let me think... This must be fork() I guess :)

For details see Does Interix implement fork()?

If you only care about creating a subprocess and waiting for it, perhaps _spawn* API's in process.h are sufficient. Here's more information about that:

https://docs.microsoft.com/en-us/cpp/c-runtime-library/process-and-environment-control https://en.wikipedia.org/wiki/Process.h

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).

https://github.com/sslab-gatech/winnie/tree/master/forklib