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Processes, Threads, and Jobs in the Windows Operating System

Job Objects

A job object is a nameable, securable, shareable kernel object that allows control of one or more processes as a group. A job object’s basic function is to allow groups of processes to be managed and manipulated as a unit. A process can be a member of only one job object. By default, its association with the job object can’t be broken and all processes created by the process and its descendents are associated with the same job object as well. The job object also records basic accounting information for all processes associated with the job and for all processes that were associated with the job but have since terminated. Table 5-22 lists the Windows functions to create and manipulate job objects.

Table 5-22. Windows API Functions for Jobs




Creates a job object (with an optional name)


Opens an existing job object by name


Adds a process to a job


Terminates all processes in a job


Sets limits


Retrieves information about the job, such as CPU time, page fault count, number of processes, list of process IDs, quotas or limits, and security limits

The following are some of the CPU-related and memory-related limits you can specify for a job:

  • Maximum number of active processes Limits the number of concurrently existing processes in the job.

  • Jobwide user-mode CPU time limit Limits the maximum amount of user-mode CPU time that the processes in the job can consume (including processes that have run and exited). Once this limit is reached, by default all the processes in the job will be terminated with an error code and no new processes can be created in the job (unless the limit is reset). The job object is signaled, so any threads waiting for the job will be released. You can change this default behavior with a call to EndOfJobTimeAction.

  • Per-process user-mode CPU time limit Allows each process in the job to accumulate only a fixed maximum amount of user-mode CPU time. When the maximum is reached, the process terminates (with no chance to clean up).

  • Job scheduling class Sets the length of the time slice (or quantum) for threads in processes in the job. This setting applies only to systems running with long, fixed quantums (the default for Windows Server systems). The value of the job-scheduling class determines the quantum as shown here:

    Scheduling Class

    Quantum Units




















    Infinite if real-time; 60 otherwise

  • Job processor affinity Sets the processor affinity mask for each process in the job. (Individual threads can alter their affinity to any subset of the job affinity, but processes can’t alter their process affinity setting.)

  • Job process priority class Sets the priority class for each process in the job. Threads can’t increase their priority relative to the class (as they normally can). Attempts to increase thread priority are ignored. (No error is returned on calls to SetThreadPriority, but the increase doesn’t occur.)

  • Default working set minimum and maximum Defines the specified working set minimum and maximum for each process in the job. (This setting isn’t jobwide—each process has its own working set with the same minimum and maximum values.)

  • Process and job committed virtual memory limit Defines the maximum amount of virtual address space that can be committed by either a single process or the entire job.

Jobs can also be set to queue an entry to an I/O completion port object, which other threads might be waiting for, with the Windows GetQueuedCompletionStatus function.

You can also place security limits on processes in a job. You can set a job so that each process runs under the same jobwide access token. You can then create a job to restrict processes from impersonating or creating processes that have access tokens that contain the local administrator’s group. In addition, you can apply security filters so that when threads in processes contained in a job impersonate client threads, certain privileges and security IDs (SIDs) can be eliminated from the impersonation token.

Finally, you can also place user-interface limits on processes in a job. Such limits include being able to restrict processes from opening handles to windows owned by threads outside the job, reading and/or writing to the clipboard, and changing the many user-interface system parameters via the Windows SystemParametersInfo function.