A Windows process is essentially container that hosts the execution of an executable image file. It is represented with a kernel process object and Windows uses the process object and its associated data structures to store and track information about the image’s execution. For example, a process has a virtual address space that holds the process’s private and shared data and into which the executable image and its associated DLLs are mapped. Windows records the process’s use of resources for accounting and query by diagnostic tools and it registers the process’s references to operating system objects in the process’s handle table. Processes operate with a security context, called a token, that identifies the user account, account groups, and privileges assigned to the process. Finally, a process includes one or more threads that actually execute the code in the process (technically, processes don’t run, threads do) and that are represented with kernel thread objects. There are several reasons applications create threads in addition to their default initial thread: processes with a user interface typically create threads to execute work so that the main thread remains responsive to user input and windowing commands; applications that want to take advantage of multiple processors for scalability or that want to continue executing while threads are tied up waiting for synchronous I/O operations to complete also benefit from multiple threads. Thread Limits Besides basic information about a thread, including its CPU register state, scheduling priority, and resource usage accounting, every thread has a portion of the process address space assigned to it, called a stack, which the thread can use as scratch storage as it executes program code to pass function parameters, maintain local variables, and save function return addresses. So that the system’s virtual memory isn’t unnecessarily wasted, only part of the stack is initially allocated, or committed and the rest is simply reserved. Because stacks grow downward in memory, the system places guard pages beyond the committed part of the stack that trigger an automatic commitment of additional memory (called a stack expansion) when accessed. Process Information The system maintains a list of running processes. You can retrieve the identifiers for these processes by calling the EnumProcesses function. This function fills an array of DWORD values with the identifiers of all processes in the system. Many functions in PSAPI require a process handle. To obtain a process handle for a running process, pass its process identifier (obtained from EnumProcesses) to the OpenProcess function. Remember to call the CloseHandle function when you are finished with the process handle.
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