winpty/misc/console-handles.md

Console Handles and Standard Handles

This document will attempt to explain how console handles work and how they interact with process creation and console attachment and detachment. It is based on experiments that I ran against various versions of Windows from Windows XP to Windows 10.

Common semantics

There are three flags to CreateProcess that affect what console a new console process is attached to:

• CREATE_NEW_CONSOLE
• CREATE_NO_WINDOW
• DETACHED_PROCESS

These flags are interpreted to produce what I will call the CreationConsoleMode. CREATE_NO_WINDOW is ignored if combined with either other flag, and the combination of CREATE_NEW_CONSOLE and DETACHED_PROCESS is an error:

Criteria	Resulting CreationConsoleMode
None of the flags (parent has a console)	Inherit
None of the flags (parent has no console)	NewConsole
CREATE_NEW_CONSOLE	NewConsole
`CREATE_NEW_CONSOLE	CREATE_NO_WINDOW`
CREATE_NO_WINDOW	NewConsoleNoWindow
DETACHED_PROCESS	Detach
`DETACHED_PROCESS	CREATE_NO_WINDOW`
`CREATE_NEW_CONSOLE	DETACHED_PROCESS`
All three flags	none - the CreateProcess call fails

Windows' behavior depends on the CreationConsoleMode:

• NewConsole or NewConsoleNoWindow: Windows attaches the new process to a new console. NewConsoleNoWindow is special--it creates an invisible console. (Prior to Windows 7, GetConsoleWindow returned a handle to an invisible window. Starting with Windows 7, GetConsoleWindow returns NULL.)

• Inherit: The child attaches to its parent's console.

• Detach: The child has no attached console, even if its parent had one.

I have not tested whether or how these flags affect non-console programs (i.e. programs whose PE header subsystem is WINDOWS rather than CONSOLE).

There is one other CreateProcess flag that plays an important role in understanding console handles -- STARTF_USESTDHANDLES. This flag influences whether the AllocConsole and AttachConsole APIs change the "standard handles" (STDIN/STDOUT/STDERR) during the lifetime of the new process, as well as the new process' initial standard handles, of course. The standard handles are accessed with GetStdHandle and SetStdHandle, which are effectively wrappers around a global HANDLE[3] variable -- these APIs do not use DuplicateHandle or CloseHandle internally, and while NT kernels objects are reference counted, HANDLEs are not.

The FreeConsole API detaches a process from its console, but it never alters the standard handles.

(Note that by "standard handles", I am strictly referring to HANDLE values and not int file descriptors or FILE* file streams provided by the C language. C and C++ standard I/O is implemented on top of Windows HANDLEs.)

Traditional semantics

Console handles and handle sets (traditional)

In releases prior to Windows 8, console handles are not true NT handles. Instead, the values are always multiples of four minus one (i.e. 0x3, 0x7, 0xb, 0xf, ...), and the functions in kernel32.dll detect the special handles and perform LPCs to csrss.exe and/or conhost.exe.

A new console's initial console handles are always inheritable, but non-inheritable handles can also be created. The inheritability can usually be changed, except on Windows 7 (see [win7inh]).

Traditional console handles cannot be duplicated to other processes. If such a handle is used with DuplicateHandle, the source and target process handles must be the GetCurrentProcess() pseudo-handle, not a real handle to the current process.

Whenever a process creates a new console (either during startup or when it calls AllocConsole), Windows replaces that process' set of open console handles (its ConsoleHandleSet) with three inheritable handles (0x3, 0x7, 0xb). Whenever a process attaches to an existing console (either during startup or when it calls AttachConsole), Windows completely replaces that process' ConsoleHandleSet with the set of inheritable open handles from the originating process. These "imported" handles are also inheritable.

CreateProcess (traditional)

The manner in which Windows sets standard handles is influenced by two flags:

• Whether STARTF_USESTDHANDLES was set in STARTUPINFO when the process started (UseStdHandles)
• Whether the CreateProcess parameter, bInheritHandles, was TRUE (InheritHandles)

From Window XP up until Windows 8, CreateProcess sets standard handles using the first matching rule:

1. If UseStdHandles, then the child uses the STARTUPINFO fields. Windows makes no attempt to validate the handles, nor will it treat a non-inheritable handle as inheritable simply because it is listed in STARTUPINFO.

2. If ConsoleCreationMode is NewConsole or NewConsoleNoWindow, then Windows sets the handles to (0x3, 0x7, 0xb).

3. If ConsoleCreationMode is Detach, then Windows sets the handles to (NULL, NULL, NULL).

4. If InheritHandles, then the parent's standard handles are copied as-is to the child, without exception.

5. Windows duplicates each of the parent's non-console standard handles into the child. Any standard handle that looks like a traditional console handle, up to 0x0FFFFFFF, is copied as-is, whether or not the handle is open. ^[1]

   If Windows fails to duplicate a handle for any reason (e.g. because it is NULL or not open), then the child's new handle is NULL. The child handles have the same inheritability as the parent handles. These handles are not closed by FreeConsole. (Bugs: [dupproc] [xppipe])

The bInheritHandles parameter to CreateProcess does not affect whether console handles are inherited. Console handles are inherited if and only if they are marked inheritable. The PROC_THREAD_ATTRIBUTE_HANDLE_LIST attribute added in Vista does not restrict console handle inheritance, and erratic behavior may result from specifying a traditional console handle in PROC_THREAD_ATTRIBUTE_HANDLE_LIST's HANDLE list. (See the Test_CreateProcess_STARTUPINFOEX test in misc/buffer-tests.)

AllocConsole, AttachConsole (traditional)

AllocConsole and AttachConsole set the standard handles as follows:

• If UseStdHandles, then Windows does not modify the standard handles.
• If !UseStdHandles, then Windows changes the standard handles to (0x3, 0x7, 0xb), even if those handles are not open.

FreeConsole (traditional)

After calling FreeConsole, no console APIs work, and all previous console handles are apparently closed -- even GetHandleInformation fails on the handles. FreeConsole has no effect on the STDIN/STDOUT/STDERR values.

Modern semantics

Console handles (modern)

Starting with Windows 8, console handles are true NT kernel handles that reference NT kernel objects.

If a process is attached to a console, then it will have two handles open to \Device\ConDrv that Windows uses internally. These handles are never observable by the user program. (To view them, use handle.exe from sysinternals, i.e. handle.exe -a -p <pid>.) A process with no attached console never has these two handles open.

Ordinary I/O console handles are also associated with \Device\ConDrv. The underlying console objects can be classified in two ways:

• Input vs Output
• Bound vs Unbound

A Bound Input object is tied to a particular console, and a Bound Output object is tied to a particular console screen buffer. These objects are usable only if the process is attached to the correct console. Bound objects are created through these methods only:

• CreateConsoleScreenBuffer
• opening CONIN$ or CONOUT$

Most console objects are Unbound, which are created during console initialization. For any given console API call, an Unbound Input object refers to the currently attached console's input queue, and an Unbound Output object refers to the screen buffer that was active during the calling process' console initialization. These objects are usable as long as the calling process has any console attached.

Unlike traditional console handles, modern console handles can be duplicated to other processes.

CreateProcess (modern)

Whenever a process is attached to a console (during startup, AttachConsole, or AllocConsole), Windows will sometimes create new Unbound console objects and assign them to one or more standard handles. If it assigns to both STDOUT and STDERR, it reuses the same new Unbound Output object for both.

As with previous releases, standard handle determination is affected by the UseStdHandles and InheritHandles flags.

Each of the child's standard handles is set using the first match:

1. If InheritHandles, UseStdHandles, and the relevant STARTUPINFO field is non-NULL, then Windows uses the STARTUPINFO field. As with previous releases, Windows makes no effort to validate the handle, nor will it treat a non-inheritable handle as inheritable simply because it is listed in STARTUPINFO. ^[2]

2. If CreationConsoleMode is NewConsole or NewConsoleNoWindow, then Windows opens a handle to a new Unbound console object. This handle will be closed if FreeConsole is later called. (N.B.: Windows reuses the same Unbound output object if it creates handles for both STDOUT and STDERR. The handles themselves are still different, though.)

3. If ConsoleCreationMode is Detach, then Windows sets the handles to (NULL, NULL, NULL).

4. If UseStdHandles, the child's standard handle becomes NULL.

5. If InheritHandles, and there is no PROC_THREAD_ATTRIBUTE_HANDLE_LIST specified, then the parent's standard handle is copied as-is.

6. The parent's standard handle is duplicated. As with previous releases, if the handle cannot be duplicated, then the child's handle becomes NULL. The child handle has the same inheritability as the parent handle. FreeConsole does not close this handle, even if it happens to be a console handle (which is not unlikely). (Bugs: [dupproc])

AllocConsole, AttachConsole (modern)

AllocConsole and AttachConsole set the standard handles as follows:

• If UseStdHandles, then Windows opens a console handle for each standard handle that is currently NULL.
• If !UseStdHandles, then Windows opens three new console handles.

Implicit screen buffer refcount

When a process' console state is initialized (at startup, AllocConsole or AttachConsole), Windows increments a refcount on the console's currently active screen buffer, which decrements only when the process detaches from the console. All Unbound Output console objects reference this screen buffer.

FreeConsole (modern)

As in previous Windows releases, FreeConsole in Windows 8 does not change the STDIN/STDOUT/STDERR values. If Windows opened new console handles for STDIN/STDOUT/STDERR when it initialized the process' console state, then FreeConsole will close those handles. Otherwise, FreeConsole will only close the two internal handles.

Interesting properties

• FreeConsole can close a non-console handle. This happens if:

  1. Windows had opened handles during console initialization.
  2. The program closes its standard handles and opens new non-console handles with the same values.
  3. The program calls FreeConsole.

  (Perhaps programs are not expected to close their standard handles.)

• Console handles--Bound or Unbound--can be duplicated to other processes. The duplicated handles are sometimes usable, especially if Unbound. The same Unbound Output object can be open in two different processes and refer to different screen buffers in the same console or in different consoles.

• Even without duplicating console handles, it is possible to have open console handles that are not usable, even with a console attached.

• Dangling Bound handles are not allowed, so it is possible to have consoles with no attached processes. The console cannot be directly modified (or attached to), but its visible content can be changed by closing Bound Output handles to activate other screen buffers.

• A program that repeatedly reinvoked itself with CREATE_NEW_CONSOLE and bInheritHandles=TRUE would accumulate console handles. Each child would inherit all of the previous child's console handles, then allocate three more for itself. All of the handles would be usable (if the program kept track of them somehow).

Other notes

SetActiveConsoleScreenBuffer

Screen buffers are referenced counted. Changing the active screen buffer with SetActiveConsoleScreenBuffer does not increment a refcount on the buffer. If the active buffer's refcount hits zero, then Windows chooses another buffer and activates it.

CREATE_NO_WINDOW process creation flag

The documentation for CREATE_NO_WINDOW is confusing:

The process is a console application that is being run without a console window. Therefore, the console handle for the application is not set.

This flag is ignored if the application is not a console application, or if it is used with either CREATE_NEW_CONSOLE or DETACHED_PROCESS.

Here's what's evident from examining the OS behavior:

• Specifying both CREATE_NEW_CONSOLE and DETACHED_PROCESS causes the CreateProcess call to fail.

• If CREATE_NO_WINDOW is specified together with CREATE_NEW_CONSOLE or DETACHED_PROCESS, it is quietly ignored, just as documented.

• Otherwise, CreateProcess behaves the same way with CREATE_NO_WINDOW as it does with CREATE_NEW_CONSOLE, except that the new console either has a hidden window (before Windows 7) or has no window at all (Windows 7 and later). These situations can be distinguished using the GetConsoleWindow and IsWindowVisible calls. GetConsoleWindow returns NULL starting with Windows 7.

PROC_THREAD_ATTRIBUTE_HANDLE_LIST

The PROC_THREAD_ATTRIBUTE_HANDLE_LIST list cannot be empty; the UpdateProcThreadAttribute call fails if cbSize is 0. However, a list containing a NULL is apparently OK and equivalent to an empty list. Curiously, if the inherit list has both a non-NULL handle and a NULL handle, the list is still treated as empty (i.e. the non-NULL handle is not inherited).

Starting with Windows 8, CreateProcess duplicates the parent's handles into the child when PROC_THREAD_ATTRIBUTE_HANDLE_LIST and these other parameters are specified:

• InheritHandles is true
• UseStdHandles is false
• CreationConsoleMode is Inherit

Windows XP pipe read handle inheritance anomaly [xppipe]

On Windows XP, CreateProcess fails to propagate a handle in this situation:

• bInheritHandles is FALSE.
• STARTF_USESTDHANDLES is not specified in STARTUPINFO.dwFlags.
• One of the STDIN/STDOUT/STDERR handles is set to the read end of an anonymous pipe.

In this situation, Windows XP will set the child process's standard handle to NULL. The write end of the pipe works fine. Passing a bInheritHandles of TRUE (and an inheritable pipe handle) works fine. Using STARTF_USESTDHANDLES also works. See Test_CreateProcess_Duplicate_XPPipeBug in misc/buffer-tests for a test case.

CreateProcess duplicates INVALID_HANDLE_VALUE until Windows 8.1 [dupproc]

From Windows XP to Windows 8, when CreateProcess duplicates parent standard handles into the child, it duplicates INVALID_HANDLE_VALUE (aka the GetCurrentProcess() pseudo-handle) to a true handle to the parent process. This bug was fixed in Windows 8.1.

Windows Vista BSOD

It is easy to cause a BSOD on Vista and Server 2008 by (1) closing all handles to the last screen buffer, then (2) creating a new screen buffer:

#include <windows.h>
int main() {
    FreeConsole();
    AllocConsole();
    CloseHandle((HANDLE)0x7);
    CloseHandle((HANDLE)0xb);
    CreateConsoleScreenBuffer(
        GENERIC_READ | GENERIC_WRITE,
        FILE_SHARE_READ | FILE_SHARE_WRITE,
        NULL,
        CONSOLE_TEXTMODE_BUFFER,
        NULL);
    return 0;
}

Windows 7 inheritability [win7inh]

• Calling DuplicateHandle(bInheritHandle=FALSE) on an inheritable console handle produces an inheritable handle. According to documentation and previous releases, it should be non-inheritable.

• Calling SetHandleInformation fails on console handles.

Windows 7 conhost.exe crash with CONOUT$ [win7_conout_crash]

There is a bug in Windows 7 involving CONOUT$ and CloseHandle that can easily crash conhost.exe and/or activate the wrong screen buffer. The bug is triggered when a process without a handle to the active screen buffer opens CONOUT$ and then closes it using CloseHandle.

Here's what seems to be going on:

Each process may have at most one "console object" referencing a particular buffer. A single console object can be shared between multiple processes, and whenever console handles are imported (CreateProcess and AttachConsole), the objects are reused.

If a process opens CONOUT$, however, and does not already have a reference to the active screen buffer, then Windows creates a new console object. The bug in Windows 7 is this: if a process calls CloseHandle on the last handle for a console object, then the screen buffer is freed, even if there are other handles/objects still referencing it. At that point, the console might display the wrong screen buffer, but using the other handles to the buffer can return garbage and/or crash conhost.exe. Closing a dangling handle is especially likely to trigger a crash.

Rather than using CloseHandle, letting Windows automatically clean up a console handle via DetachConsole or exiting somehow avoids the problem.

The bug affects Windows 7 SP1, but does not affect Windows Server 2008 R2 SP1, the server version of the OS.

See misc/buffer-tests/HandleTests/Win7_Conout_Crash.cc.

Footnotes

1: From the previous discussion, it follows that if a standard handle is a non-inheritable console handle, then the child's standard handle will be invalid:

• Traditional console standard handles are copied as-is to the child.
• The child has the same ConsoleHandleSet as the parent, excluding non-inheritable handles.

It's an interesting edge case, though, so I test for it specifically. As of Windows 8, the non-inheritable console handle would be successfully duplicated.

2: Suppose a console program invokes CreateProcess with these parameters:

• bInheritHandles is FALSE.
• STARTF_USESTDHANDLES is set.
• STARTUPINFO refers to inheritable console handles (e.g. the default standard handles)

Prior to Windows 8, the child would have received valid standard handles. As of Windows 8, the child's standard handles will be NULL instead.