fork() for Windows

fork.c

/*
 * fork.c
 * Experimental fork() on Windows.  Requires NT 6 subsystem or
 * newer.
 *
 * Copyright (c) 2012 William Pitcock <nenolod@dereferenced.org>
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * This software is provided 'as is' and without any warranty, express or
 * implied.  In no event shall the authors be liable for any damages arising
 * from the use of this software.
 */

#define _WIN32_WINNT 0x0600
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <winnt.h>
#include <winternl.h>
#include <stdio.h>
#include <errno.h>
#include <assert.h>
#include <process.h>

typedef struct _CLIENT_ID {
  PVOID UniqueProcess;
	PVOID UniqueThread;
} CLIENT_ID, *PCLIENT_ID;

typedef struct _SECTION_IMAGE_INFORMATION {
	PVOID EntryPoint;
	ULONG StackZeroBits;
	ULONG StackReserved;
	ULONG StackCommit;
	ULONG ImageSubsystem;
	WORD SubSystemVersionLow;
	WORD SubSystemVersionHigh;
	ULONG Unknown1;
	ULONG ImageCharacteristics;
	ULONG ImageMachineType;
	ULONG Unknown2[3];
} SECTION_IMAGE_INFORMATION, *PSECTION_IMAGE_INFORMATION;

typedef struct _RTL_USER_PROCESS_INFORMATION {
	ULONG Size;
	HANDLE Process;
	HANDLE Thread;
	CLIENT_ID ClientId;
	SECTION_IMAGE_INFORMATION ImageInformation;
} RTL_USER_PROCESS_INFORMATION, *PRTL_USER_PROCESS_INFORMATION;

#define RTL_CLONE_PROCESS_FLAGS_CREATE_SUSPENDED	0x00000001
#define RTL_CLONE_PROCESS_FLAGS_INHERIT_HANDLES		0x00000002
#define RTL_CLONE_PROCESS_FLAGS_NO_SYNCHRONIZE		0x00000004

#define RTL_CLONE_PARENT				0
#define RTL_CLONE_CHILD					297

typedef DWORD pid_t;

typedef NTSTATUS (*RtlCloneUserProcess_f)(ULONG ProcessFlags,
	PSECURITY_DESCRIPTOR ProcessSecurityDescriptor /* optional */,
	PSECURITY_DESCRIPTOR ThreadSecurityDescriptor /* optional */,
	HANDLE DebugPort /* optional */,
	PRTL_USER_PROCESS_INFORMATION ProcessInformation);

pid_t fork(void)
{
	HMODULE mod;
	RtlCloneUserProcess_f clone_p;
	RTL_USER_PROCESS_INFORMATION process_info;
	NTSTATUS result;

	mod = GetModuleHandle("ntdll.dll");
	if (!mod)
		return -ENOSYS;

	clone_p = (RtlCloneUserProcess_f)GetProcAddress(mod, "RtlCloneUserProcess");
	if (clone_p == NULL)
		return -ENOSYS;

	/* lets do this */
	result = clone_p(RTL_CLONE_PROCESS_FLAGS_CREATE_SUSPENDED | RTL_CLONE_PROCESS_FLAGS_INHERIT_HANDLES, NULL, NULL, NULL, &process_info);

	if (result == RTL_CLONE_PARENT)
	{
		HANDLE me, hp, ht, hcp = 0;
		DWORD pi, ti, mi;
		me = GetCurrentProcess();
		pi = (DWORD)process_info.ClientId.UniqueProcess;
		ti = (DWORD)process_info.ClientId.UniqueThread;
		
		assert(hp = OpenProcess(PROCESS_ALL_ACCESS, FALSE, pi));
		assert(ht = OpenThread(THREAD_ALL_ACCESS, FALSE, ti));
		
		ResumeThread(ht);
		CloseHandle(ht);
		CloseHandle(hp);
		return (pid_t)pi;
	}
	else if (result == RTL_CLONE_CHILD)
	{
		/* fix stdio */
		AllocConsole();
		return 0;
	}
	else
		return -1;

	/* NOTREACHED */
	return -1;
}

#ifdef __TEST__
int main(int argc, const char *argv[])
{
	pid_t pid;
	printf("Forking..\n");
	pid = fork();

	switch (pid) {
	case 0:
	{
		FILE *f = fopen("C:\\Development\\cpp.sandbox\\win32\\win32-fork\\forktest.dat", "w");
		fprintf(f, "ok\n");
		fclose(f);
		break;
	}
	default:
		printf("child %d\n", pid);
		while (1) { Sleep(1000); }
		break;
	}
}
#endif

The fix of Console didn't work with me. I fixed it with AttachConsole(parent_pid). See the improved version here: https://gist.github.com/petrsmid/d96446beac825c8c0cf5a35240f444a8

@petrsmid is this somehow working again? last I checked starting from win10 or so a lot of memory patches were necessary https://github.com/sslab-gatech/winnie/blob/master/forklib/fork.cpp