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A process is a software program that is currently running in Windows. Every process has an ID, a number that identifies it. A thread is an object that identifies which part of the program is running. Each thread has an ID, a number that identifies it.

A process may have more than one thread. The purpose of a thread is to allocate processor time. On a machine with one processor, more than one thread can be allocated, but only one thread can run at a time. Each thread only runs a short time and then the execution is passed on to the next thread, giving the user the illusion that more than one thing is happening at once. On a machine with more than one processor, true multi-threading can take place. If an application has multiple threads, the threads can run simultaneously on different processors.

The Windows kernel-mode process and thread manager handles the execution of all threads in a process. Whether you have one processor or more, great care must be taken in driver programming to make sure that all threads of your process are designed so that no matter what order the threads are handled, your driver will operate properly.

If threads from different processes attempt to use the same resource at the same time, problems can occur. Windows provides several techniques to avoid this problem. The technique of making sure that threads from different processes do not touch the same resource is called synchronization . For more information about synchronization, see Synchronization Techniques .

Routines that provide a direct interface to the process and thread manager are usually prefixed with the letters " Ps "; for example, PsCreateSystemThread . For a list of kernel DDIs, see Windows kernel .

Best practices for implementing process and thread-related callback functions

This set of guidelines applies to these callback routines:

PCREATE_PROCESS_NOTIFY_ROUTINE

PCREATE_PROCESS_NOTIFY_ROUTINE_EX

PCREATE_THREAD_NOTIFY_ROUTINE

PLOAD_IMAGE_NOTIFY_ROUTINE

POB_PRE_OPERATION_CALLBACK

POB_POST_OPERATION_CALLBACK

Keep routines short and simple.

Do not make calls into a user mode service to validate the process, thread, or image.

Do not make registry calls.

Do not make blocking and/or Interprocess Communication (IPC) function calls.

Do not synchronize with other threads because it can lead to reentrancy deadlocks.

Use System Worker Threads to queue work especially work involving:

Slow API’s or API’s that call into other process.

Any blocking behavior which could interrupt threads in core services.

If you use System Worker Threads do not wait on the work to complete. Doing so defeats the purpose of queuing the work to be completed asynchronously.

Be considerate of best practices for kernel mode stack usage. For examples, see How do I keep my driver from running out of kernel-mode stack? and Key Driver Concepts and Tips .

Subsystem Processes

Starting in Windows 10, the Windows Subsystem for Linux (WSL) enables a user to run native Linux ELF64 binaries on Windows, alongside other Windows applications. For information about WSL architecture and the user-mode and kernel-mode components that are required to run the binaries, see the posts on the Windows Subsystem for Linux blog.

One of the components is a subsystem process that hosts the unmodified user-mode Linux binary, such as /bin/bash. Subsystem processes do not contain data structures associated with Win32 processes, such as Process Environment Block (PEB) and Thread Environment Block (TEB). For a subsystem process, system calls and user mode exceptions are dispatched to a paired driver.

Here are the changes to the Process and Thread Manager Routines in order to support subsystem processes:

The WSL type is indicated by the SubsystemInformationTypeWSL value in the SUBSYSTEM_INFORMATION_TYPE enumeration. Drivers can call NtQueryInformationProcess and NtQueryInformationThread to determine the underlying subsystem. Those calls return SubsystemInformationTypeWSL for WSL.

Other kernel mode drivers can get notified about subsystem process creation/deletion by registering their callback routine through the PsSetCreateProcessNotifyRoutineEx2 call. To get notifications about thread creation/deletion, drivers can call PsSetCreateThreadNotifyRoutineEx , and specify PsCreateThreadNotifySubsystems as the type of notification.

The PS_CREATE_NOTIFY_INFO structure has been extended to include a IsSubsystemProcess member that indicates a subsystem other than Win32. Other members such as FileObject , ImageFileName , CommandLine indicate additional information about the subsystem process. For information about the behavior of those members, see SUBSYSTEM_INFORMATION_TYPE .