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SetThreadPriority function

Sets the priority value for the specified thread. This value, together with the priority class of the thread's process, determines the thread's base priority level.

Syntax


BOOL WINAPI SetThreadPriority(
  _In_  HANDLE hThread,
  _In_  int nPriority
);

Parameters

hThread [in]

A handle to the thread whose priority value is to be set.

The handle must have the THREAD_SET_INFORMATION or THREAD_SET_LIMITED_INFORMATION access right. For more information, see Thread Security and Access Rights.

Windows Server 2003 and Windows XP:  The handle must have the THREAD_SET_INFORMATION access right.

nPriority [in]

The priority value for the thread. This parameter can be one of the following values.

PriorityMeaning
THREAD_MODE_BACKGROUND_BEGIN
0x00010000

Begin background processing mode. The system lowers the resource scheduling priorities of the thread so that it can perform background work without significantly affecting activity in the foreground.

This value can be specified only if hThread is a handle to the current thread. The function fails if the thread is already in background processing mode.

Windows Server 2003 and Windows XP:  This value is not supported.

THREAD_MODE_BACKGROUND_END
0x00020000

End background processing mode. The system restores the resource scheduling priorities of the thread as they were before the thread entered background processing mode.

This value can be specified only if hThread is a handle to the current thread. The function fails if the thread is not in background processing mode.

Windows Server 2003 and Windows XP:  This value is not supported.

THREAD_PRIORITY_ABOVE_NORMAL
1

Priority 1 point above the priority class.

THREAD_PRIORITY_BELOW_NORMAL
-1

Priority 1 point below the priority class.

THREAD_PRIORITY_HIGHEST
2

Priority 2 points above the priority class.

THREAD_PRIORITY_IDLE
-15

Base priority of 1 for IDLE_PRIORITY_CLASS, BELOW_NORMAL_PRIORITY_CLASS, NORMAL_PRIORITY_CLASS, ABOVE_NORMAL_PRIORITY_CLASS, or HIGH_PRIORITY_CLASS processes, and a base priority of 16 for REALTIME_PRIORITY_CLASS processes.

THREAD_PRIORITY_LOWEST
-2

Priority 2 points below the priority class.

THREAD_PRIORITY_NORMAL
0

Normal priority for the priority class.

THREAD_PRIORITY_TIME_CRITICAL
15

Base priority of 15 for IDLE_PRIORITY_CLASS, BELOW_NORMAL_PRIORITY_CLASS, NORMAL_PRIORITY_CLASS, ABOVE_NORMAL_PRIORITY_CLASS, or HIGH_PRIORITY_CLASS processes, and a base priority of 31 for REALTIME_PRIORITY_CLASS processes.

 

If the thread has the REALTIME_PRIORITY_CLASS base class, this parameter can also be -7, -6, -5, -4, -3, 3, 4, 5, or 6. For more information, see Scheduling Priorities.

Return value

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

Windows Phone 8.1:  Windows Phone Store apps may call this function but it has no effect. The function will return a nonzero value indicating success.

Remarks

Every thread has a base priority level determined by the thread's priority value and the priority class of its process. The system uses the base priority level of all executable threads to determine which thread gets the next slice of CPU time. Threads are scheduled in a round-robin fashion at each priority level, and only when there are no executable threads at a higher level does scheduling of threads at a lower level take place.

The SetThreadPriority function enables setting the base priority level of a thread relative to the priority class of its process. For example, specifying THREAD_PRIORITY_HIGHEST in a call to SetThreadPriority for a thread of an IDLE_PRIORITY_CLASS process sets the thread's base priority level to 6. For a table that shows the base priority levels for each combination of priority class and thread priority value, see Scheduling Priorities.

For IDLE_PRIORITY_CLASS, BELOW_NORMAL_PRIORITY_CLASS, NORMAL_PRIORITY_CLASS, ABOVE_NORMAL_PRIORITY_CLASS, and HIGH_PRIORITY_CLASS processes, the system dynamically boosts a thread's base priority level when events occur that are important to the thread. REALTIME_PRIORITY_CLASS processes do not receive dynamic boosts.

All threads initially start at THREAD_PRIORITY_NORMAL. Use the GetPriorityClass and SetPriorityClass functions to get and set the priority class of a process. Use the GetThreadPriority function to get the priority value of a thread.

Use the priority class of a process to differentiate between applications that are time critical and those that have normal or below normal scheduling requirements. Use thread priority values to differentiate the relative priorities of the tasks of a process. For example, a thread that handles input for a window could have a higher priority level than a thread that performs intensive calculations for the CPU.

When manipulating priorities, be very careful to ensure that a high-priority thread does not consume all of the available CPU time. A thread with a base priority level above 11 interferes with the normal operation of the operating system. Using REALTIME_PRIORITY_CLASS may cause disk caches to not flush, cause the mouse to stop responding, and so on.

The THREAD_PRIORITY_* values affect the CPU scheduling priority of the thread. For threads that perform background work such as file I/O, network I/O, or data processing, it is not sufficient to adjust the CPU scheduling priority; even an idle CPU priority thread can easily interfere with system responsiveness when it uses the disk and memory. Threads that perform background work should use the THREAD_MODE_BACKGROUND_BEGIN and THREAD_MODE_BACKGROUND_END values to adjust their resource scheduling priorities; threads that interact with the user should not use THREAD_MODE_BACKGROUND_BEGIN.

When a thread is in background processing mode, it should minimize sharing resources such as critical sections, heaps, and handles with other threads in the process, otherwise priority inversions can occur. If there are threads executing at high priority, a thread in background processing mode may not be scheduled promptly, but it will never be starved.

Windows Server 2008 and Windows Vista:  While the system is starting, the SetThreadPriority function returns a success return value but does not change thread priority for applications that are started from the system Startup folder or listed in the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run registry key. These applications run at reduced priority for a short time (approximately 60 seconds) to make the system more responsive to user actions during startup.

Windows 8.1 and Windows Server 2012 R2: This function is supported for Windows Store apps.

Windows Phone 8.1:Windows Phone Store apps may call this function but it has no effect.

Examples

The following example demonstrates the use of thread background mode.


#include <windows.h>
#include <tchar.h>

int main( void )
{
   DWORD dwError, dwThreadPri;

   if(!SetThreadPriority(GetCurrentThread(), THREAD_MODE_BACKGROUND_BEGIN))
   {
      dwError = GetLastError();
      if( ERROR_THREAD_MODE_ALREADY_BACKGROUND == dwError)
         _tprintf(TEXT("Already in background mode\n"));
      else _tprintf(TEXT("Failed to enter background mode (%d)\n"), dwError);
      goto Cleanup;
   } 

   // Display thread priority

   dwThreadPri = GetThreadPriority(GetCurrentThread());

   _tprintf(TEXT("Current thread priority is 0x%x\n"), dwThreadPri);

   //
   // Perform background work
   //
   ;

   if(!SetThreadPriority(GetCurrentThread(), THREAD_MODE_BACKGROUND_END))
   {
      _tprintf(TEXT("Failed to end background mode (%d)\n"), GetLastError());
   }

Cleanup:
   // Clean up
   ;
return 0;
}


Requirements

Minimum supported client

Windows XP [desktop apps only]

Minimum supported server

Windows Server 2003 [desktop apps only]

Minimum supported phone

Windows Phone 8.1

Header

WinBase.h on Windows XP, Windows Server 2003, Windows Vista, Windows 7, Windows Server 2008, and Windows Server 2008 R2 (include Windows.h);
Processthreadsapi.h on Windows 8, Windows Server 2012, and Windows Phone 8.1

Library

Kernel32.lib;
WindowsPhoneCore.lib on Windows Phone 8.1

DLL

Kernel32.dll;
KernelBase.dll on Windows Phone 8.1

See also

GetPriorityClass
GetThreadPriority
Process and Thread Functions
Scheduling Priorities
SetPriorityClass
Threads

 

 

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