Processes, Threads, and Jobs in the Windows Operating System

  • 6/17/2009
In this chapter from Windows Internals, 5th Edition, learn the data structures and algorithms that deal with processes, threads, and jobs in the Windows operating system. The first section focuses on the internal structures that make up a process. The second section outlines the steps involved in creating a process (and its initial thread). The internals of threads and thread scheduling are then described. The chapter concludes with a description of the job object.

Where relevant performance counters or kernel variables exist, they are mentioned. Although this book isn’t a Windows programming book, the pertinent process, thread, and job Windows functions are listed so that you can pursue additional information on their use.

Because processes and threads touch so many components in Windows, a number of terms and data structures (such as working sets, objects and handles, system memory heaps, and so on) are referred to in this chapter but are explained in detail elsewhere in the book. To fully understand this chapter, you need to be familiar with the terms and concepts explained in Chapter 1 and Chapter 2, such as the difference between a process and a thread, the Windows virtual address space layout, and the difference between user mode and kernel mode.

Process Internals

This section describes the key Windows process data structures. Also listed are key kernel variables, performance counters, and functions and tools that relate to processes.

Data Structures

Each Windows process is represented by an executive process (EPROCESS) block. Besides containing many attributes relating to a process, an EPROCESS block contains and points to a number of other related data structures. For example, each process has one or more threads represented by executive thread (ETHREAD) blocks. (Thread data structures are explained in the section Thread Internals later in this chapter.) The EPROCESS block and its related data structures exist in system address space, with the exception of the process environment block (PEB), which exists in the process address space (because it contains information that needs to be accessed by user-mode code).

In addition to the EPROCESS block and the PEB, the Windows subsystem process (Csrss) maintains a parallel structure for each process that is executing a Windows program. Finally, the kernel-mode part of the Windows subsystem (Win32k.sys) will also maintain a per-process data structure that is created the first time a thread calls a Windows USER or GDI function that is implemented in kernel mode.

Figure 5-1 is a simplified diagram of the process and thread data structures. Each data structure shown in the figure is described in detail in this chapter.

Figure 5-1

Figure 5-1. Data structures associated with processes and threads

First let’s focus on the process block. (We’ll get to the thread block in the section Thread Internals later in the chapter.) Figure 5-2 shows the key fields in an EPROCESS block.

Figure 5-2

Figure 5-2. Structure of an executive process block

Table 5-1 explains some of the fields in the preceding experiment in more detail and includes references to other places in the book where you can find more information about them. As we’ve said before and will no doubt say again, processes and threads are such integral parts of Windows that it’s impossible to talk about them without referring to many other parts of the system. To keep the length of this chapter manageable, however, we’ve covered those related subjects (such as memory management, security, objects, and handles) elsewhere.

Table 5-1. Contents of the EPROCESS Block



Additional Reference

Kernel process (KPROCESS) block

Common dispatcher object header, pointer to the process page directory, list of kernel thread (KTHREAD) blocks belonging to the process, default base priority, affinity mask, and total kernel and user time and CPU clock cycles for the threads in the process.

Thread scheduling (Chapter 5)

Process identification

Unique process ID, creating process ID, name of image being run, window station process is running on.

Quota block

Limits on processor usage, nonpaged pool, paged pool, and page file usage plus current and peak process nonpaged and paged pool usage. (Note: Several processes can share this structure: all the system processes in session 0 point to a single systemwide quota block; all other processes in interactive sessions share a single quota block.)

Virtual address descriptors (VADs)

Series of data structures that describes the status of the portions of the address space that exist in the process.

Virtual address descriptors (Chapter 9)

Working set information

Pointer to working set list (MMWSL structure); current, peak, minimum, and maximum working set size; last trim time; page fault count; memory priority; outswap flags; page fault history.

Working sets (Chapter 9)

Virtual memory information

Current and peak virtual size, page file usage, hardware page table entry for process page directory.

Chapter 9

Exception legacy local procedure call (LPC) port

Interprocess communication channel to which the process manager sends a message when one of the process’s threads causes an exception.

Exception dispatching (Chapter 3)

Debugging object

Executive object through which the user-mode debugging infrastructure sends notifications when one of the process’s threads causes a debug event.

User-mode debugging (Chapter 3)

Access token (TOKEN)

Executive object describing the security profile of this process.

Chapter 6

Handle table

Address of per-process handle table.

Object handles and the process handle table (Chapter 3)

Device map

Address of object directory to resolve device name references in (supports multiple users).

Object names (Chapter 3)

Process environment block (PEB)

Image information (base address, version numbers, module list), process heap information, and thread-local storage utilization. (Note: The pointers to the process heaps start at the first byte after the PEB.)

Chapter 5

Windows subsystem process block (W32PROCESS)

Process details needed by the kernel-mode component of the Windows subsystem.

The kernel process (KPROCESS) block, which is part of the EPROCESS block, and the process environment block (PEB), which is pointed to by the EPROCESS block, contain additional details about the process object. The KPROCESS block (which is sometimes called the PCB or process control block) is illustrated in Figure 5-3. It contains the basic information that the Windows kernel needs to schedule the threads inside a process. (Page directories are covered in Chapter 9, and kernel thread blocks are described in more detail later in this chapter.)

The PEB, which lives in the user process address space, contains information needed by the image loader, the heap manager, and other Windows system DLLs that need to access it from user mode. (The EPROCESS and KPROCESS blocks are accessible only from kernel mode.) The basic structure of the PEB is illustrated in Figure 5-4 and is explained in more detail later in this chapter.

Figure 5-3

Figure 5-3. Structure of the executive process block

Figure 5-4

Figure 5-4. Fields of the process environment block

Kernel Variables

A few key kernel global variables that relate to processes are listed in Table 5-2. These variables are referred to later in the chapter, when the steps in creating a process are described.

Table 5-2. Process-Related Kernel Variables



Additional Reference


Doubly linked list

List head of process blocks


Pointer to EPROCESS

Idle process block


Pointer to EPROCESS

Pointer to the process block of the initial system process that contains the system threads


Array of executive callback objects

Array of callback objects describing the routines to be called on process creation and deletion (maximum of eight)


32-bit integer

Count of registered process notification routines


32-bit integer

Count of registered extended process notification routines


Array of executive callback objects

Array of callback objects describing the routines to be called on image load (maximum of eight)


32-bit integer

Count of registered image-load notification routines


32-bit integer

Mask for quickly checking whether any extended or standard notification routines are enabled



Handle table for process and thread client IDs

Performance Counters

Windows maintains a number of counters with which you can track the processes running on your system; you can retrieve these counters programmatically or view them with the Performance tool. Table 5-3 lists the performance counters relevant to processes.

Table 5-3. Process-Related Performance Counters

Object: Counter


Process: % Privileged Time

Describes the percentage of time that the threads in the process have run in kernel mode during a specified interval.

Process: % Processor Time

Describes the percentage of CPU time that the threads in the process have used during a specified interval. This count is the sum of % Privileged Time and % User Time.

Process: % User Time

Describes the percentage of time that the threads in the process have run in user mode during a specified interval.

Process: Elapsed Time

Describes the total elapsed time in seconds since this process was created.

Process: ID Process

Returns the process ID. This ID applies only while the process exists because process IDs are reused.

Process: Creating Process ID

Returns the process ID of the creating process. This value isn’t updated if the creating process exits.

Process: Thread Count

Returns the number of threads in the process.

Process: Handle Count

Returns the number of handles open in the process.

Relevant Functions

For reference purposes, some of the Windows functions that apply to processes are described in Table 5-4. For further information, consult the Windows API documentation in the MSDN Library.

Table 5-4. Process-Related Functions




Creates a new process and thread using the caller’s security identification


Creates a new process and thread with the specified alternate security token


Creates a new process and thread to run under the credentials of the specified username and password


Creates a new process and thread with the specified alternate security token, with additional options such as allowing the user profile to be loaded


Returns a handle to the specified process object


Ends a process, and notifies all attached DLLs


Ends a process without notifying the DLLs


Empties the specified process’s instruction cache


Empties the specified process’s write queue


Obtains a process’s timing information, describing how much time the threads inside the process spent in user and kernel mode


Obtains a process’s CPU timing information, describing how many clock cycles the threads inside the process have spent in total


Defines whether the process’s affinity is automatically updated if new processors are added to the running system


Returns or sets the DEP (Data Execution Protection) policy for the process


Returns the exit code for a process, indicating how and why the process shut down


Returns a pointer to the command-line string passed to the current process


Returns the full name of the executable image associated with the process


Returns a pseudo handle for the current process


Returns the ID of the current process


Returns the major and minor versions of the Windows version on which the specified process expects to run


Returns the contents of the STARTUPINFO structure specified during CreateProcess


Returns the address of the environment block


Returns or sets a specific environment variable


Defines the shutdown priority and number of retries for the current process


Specifies whether the process is aware of dots per inch (DPI) settings


Returns a count of User and GDI handles