Threads and Processes

Threads and processes are related concepts in computer science. Both represent sequences of instructions that must execute in a specific order. Instructions in separate threads or processes, however, can execute in parallel.

Processes exist in the operating system and correspond to what users see as programs or applications. A thread, on the other hand, exists within a process. For this reason, threads are sometimes referred to as light-weight processes. Each process consists of one or more threads.

The existence of multiple processes enables a computer to perform more than one task at a time. The existence of multiple threads enables a process to separate work to be performed in parallel. On a computer with multiprocessors, processes or threads can run on different processors. This enables true parallel processing.

Perfect parallel processing is not always possible. Threads sometimes must be synchronized. One thread may have to wait for a result from another thread, or one thread may need exclusive access to a resource that another thread is using. Synchronization problems are a common cause of bugs in multithreaded applications. Sometimes threads may end up waiting for a resource that never becomes available. This results in a condition called deadlock.

The Visual Studio debugger provides powerful but easy-to-use tools for debugging threads and processes.

Tools for Debugging Threads and Processes in Visual Studio

The primary tools for working with processes in Visual Studio are the Attach to Process dialog box, the Processes window, and the Debug Location toolbar. The primary tools for debugging threads are the Threads window, thread markers in source windows, and the Debug Location toolbar.

The primary tools for debugging multithreaded applications are the Parallel Stacks and Parallel Tasks windows.

The following table shows the information available and the actions you can perform in each of these places:

User Interface	Information Available	Actions You Can Perform
Attach to Process dialog box	Available Processes you can attach to: Process name (.exe) Process ID number Menubar Title Type (Managed v4.0; Managed v2.0, v1.1, v1.0; x86; x64; IA64) User Name (account name) Session number	Select a process to attach to Select a remote computer Change transport type for connecting to remote computers
Processes window	Attached Processes: Process Name Process ID number Path to process .exe Menubar Title State (Break. Running) Debugging (Native, Managed, and so on.) Transport type (default, native with no authentication) Transport Qualifier (remote computer)	Tools: Attach Detach Terminate Shortcut menu: Attach Detach Detach when debugging stopped Terminate
Threads window	Threads in current process: Thread ID Managed ID Category (main thread, interface thread, remote procedure call handler, or worker thread) Thread Name Location where thread is created Priority Affinity Mask Suspended Count Process Name Flag Indicator Suspended indicator	Tools: Search Search Call Stack Flag Just My Code Flag Custom Module Selection Group by Columns Expand/Collapse callstacks Expand/Collapse groups Freeze/Thaw Threads Shortcut menu: Show threads in source Switch to a thread Freeze a running thread Thaw a frozen thread Flag a thread for additional study Unflag a thread Rename a thread Show and hide threads Other actions: View the call stack for a thread in a DataTip
Source window	Thread indicators in left gutter indicate single or multiple threads (off by default, turned on by using shortcut menu in Threads window)	Shortcut menu: Switch to a thread Flag a thread for additional study Unflag a thread
Debug Location toolbar	Current process Current thread Current stack frame	Switch to another process Switch to another thread in current process Switch to another stack frame in current thread Flag or unflag current threads Show only flagged threads
Parallel Stacks window	Call stacks for multiple threads in one window. Active stack frame for each thread. Callers and callees for any method.	Filter out specified threads Switch to Parallel Tasks view Flag or unflag a thread Zoom
Parallel Tasks window	View information about Task objects including task ID, task status (scheduled, running, waiting, deadlocked), and which thread is assigned to the task. Current location in call stack. Delegate passed to the task at creation time	Switch to current task Flag or unflag a task Freeze or thaw a task

See Also

Tasks

How to: Use the Processes Window

Other Resources

Attaching to Running Processes

Debugging Multithreaded Applications