APC (Asynchronous Procedure Call)

When the system creates a new thread, and this will also create a queue associated with the thread that an asynchronous procedure call (APC) queue.

Some asynchronous operation can be achieved by adding APC, for example, I now learn the IO request / completion.

BOOL ReadFileEx(
  HANDLE                          hFile,
  LPVOID                          lpBuffer,
  DWORD                           nNumberOfBytesToRead,
  LPOVERLAPPED                    lpOverlapped,
  LPOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
);

When the IO completion, the APC system queue is added to a thread, and comprising lpCompleteionRoutine lpOverlapped. When a thread is in a state of non-execution state is alert and the system will remove the entry in the APC, and wherein a thread of execution so that the callback function. This action is repeated until the queue is empty, I guess the thread may also be interrupted normal wake.

Non-execution states are thread calls wait, sleep function, like

DWORD SleepEx(DWORD dwMilliseconds, bool bAlertable );
DWORD WaitForSigleObjectEx(HANDLE hObject,DWORD dwMilliseconds,bool bAlertable);

bAlertable = true; alert status is!

APC call another piece of code, it is an example of a waitableTimer.

#include <iostream>
#include<process.h>
#include<Windows.h>
#include<tchar.h>
#include<string.h>
void APIENTRY TimerAPCRoutine(PVOID pvArgToCompleteRoutine, DWORD dwTimerLowValue, DWORD dwTimerHighValue);
void SomeFunc()
{
    HANDLE hTimer = CreateWaitableTimer(NULL, TRUE, NULL);
    LARGE_INTEGER li = { 0 };
    SetWaitableTimer(hTimer, &li, 5000, TimerAPCRoutine, NULL, false);
    SleepEx(INFINITE, true);
    CloseHandle(hTimer);
}
void APIENTRY TimerAPCRoutine(PVOID pvArgToCompleteRoutine, DWORD dwTimerLowValue, DWORD dwTimerHighValue)
{
    FILETIME ftUTC, ftLocal;
    SYSTEMTIME st;
    TCHAR szBuf[256];
    ftUTC.dwHighDateTime = dwTimerHighValue;
    ftUTC.dwLowDateTime = dwTimerLowValue;
    FileTimeToLocalFileTime(&ftUTC, &ftLocal);
    FileTimeToSystemTime(&ftLocal, &st);
    GetDateFormat(LOCALE_USER_DEFAULT, DATE_LONGDATE, &st, NULL, szBuf, _countof(szBuf));
    _tcscat_s(szBuf, _countof(szBuf), " ");
    GetTimeFormat(LOCALE_USER_DEFAULT, 0, &st, NULL, _tcschr(szBuf, TEXT('\0')), (int)(_countof(szBuf) - _tcslen(szBuf)));
    MessageBox(NULL, szBuf, TEXT("Timer went off at ..."), MB_OK);
}

int wmain(int argc, wchar_t* argv[])
{
    SomeFunc();
    char c;
    std::cin >> c;
    return 0;
}

 

 

When the thread ran APC callback function,

 

 

to sum up:

APC is a queue management system associated with the threads, it can be used to perform asynchronous operations.

APC callback function is invoked in the original thread in the original thread to sleep.