Job Objects

A job object allows groups of processes to be managed as a unit. Job objects are namable, securable, sharable objects that control attributes of the processes associated with them. Operations performed on the job object affect all processes associated with the job object.

To create a job object, use the CreateJobObject function. When the job is created, there are no associated processes. To associate a process with a job, use the AssignProcessToJobObject function. After you associate a process with a job, the association cannot be broken. By default, processes created by a process associated with a job (child processes) are associated with the job. If the job has the extended limit JOB_OBJECT_LIMIT_BREAKAWAY_OK and the process was created with the CREATE_BREAKAWAY_FROM_JOB flag, its child processess are not associated with the job. If the job has the extended limit JOB_OBJECT_LIMIT_SILENT_BREAKAWAY_OK, no child processes are associated with the job.

A job can enforce limits on each associated process, such as the working set size, process priority, end-of-job time limit, and so on. To set limits for a job object, use the SetInformationJobObject function. If a process associated with a job attempts to increase its working set size or process priority, the function calls are silently ignored.

The job object also records basic accounting information for all its associated processes, including those that have terminated. To retrieve this accounting information, use the QueryInformationJobObject function.

To terminate all processes currently associated with a job object, use the TerminateJobObject function.

To close a job object handle, use the CloseHandle function. The job object is destroyed when its last handle has been closed. If there are running processes still associated with the job when it is destroyed, they will continue to run even after the job is destroyed.

If a tool is to manage a process tree that uses job objects, both the tool and the members of the process tree must cooperate. Use one of the following options:

1. The tool could use the JOB_OBJECT_LIMIT_SILENT_BREAKAWAY_OK limit. If the tool uses this limit, it cannot monitor an entire process tree. The tool can monitor only the processes it adds to the job. If these processes create child processes, they are not associated with the job. In this option, child processes can be associated with other job objects.
2. The tool could use the JOB_OBJECT_LIMIT_BREAKAWAY_OK limit. If the tool uses this limit, it can monitor the entire process tree, except for those processes that any member of the tree explicitly breaks away from the tree. A member of the tree can create a child process in a new job object by calling the CreateProcess function with the CREATE_BREAKAWAY_FROM_JOB flag, then calling the AssignProcessToJobObject function. Otherwise, the member must handle cases in which AssignProcessToJobObject fails.

   The CREATE_BREAKAWAY_FROM_JOB flag has no effect if the tree is not being monitored by the tool. Therefore, this is the preferred option, but it requires advance knowledge of the processes being monitored.

3. The tool could prevent breakaways of any kind. In this option, the tool can monitor the entire process tree. However, if a process associated with the job tries to call AssignProcessToJobObject, the call will fail. If the process was not designed to be associated with a job, this failure may be unexpected.