Using Event Objects - Win32 Programmer's Reference

Custom Search

Using Event Objects

Win32-based applications use event objects in a number of situations to notify a waiting thread of the occurrence of an event. For example, overlapped I/O operations on files, named pipes, and communications devices use an event object to signal their completion. For more information about the use of event objects in overlapped I/O operations, see Synchronization and Overlapped Input and Output.

In the following example, an application uses event objects to prevent several threads from reading from a shared memory buffer while a master thread is writing to that buffer. First, the master thread uses the CreateEvent function to create a manual-reset event object. The master thread sets the event object to nonsignaled when it is writing to the buffer and then resets the object to signaled when it has finished writing. Then it creates several reader threads and an auto-reset event object for each thread. Each reader thread sets its event object to signaled when it is not reading from the buffer.

#define NUMTHREADS 4

HANDLE hGlobalWriteEvent;

oid CreateEventsAndThreads(void)

{

HANDLE hReadEvents[NUMTHREADS], hThread;

DWORD i, IDThread;

// Create a manual-reset event object. The master thread sets

// this to nonsignaled when it writes to the shared buffer.

hGlobalWriteEvent = CreateEvent(

NULL, // no security attributes

TRUE, // manual-reset event

TRUE, // initial state is signaled

"WriteEvent" // object name

);

if (hGlobalWriteEvent == NULL) {

// error exit

}

// Create multiple threads and an auto-reset event object

// for each thread. Each thread sets its event object to

// signaled when it is not reading from the shared buffer.

for(i = 1; i <= NUMTHREADS; i++)

{

// Create the auto-reset event.

hReadEvents[i] = CreateEvent(

NULL, // no security attributes

FALSE, // auto-reset event

TRUE, // initial state is signaled

NULL); // object not named

if (hReadEvents[i] == NULL)

{

// Error exit.

}

hThread = CreateThread(NULL, 0,

(LPTHREAD_START_ROUTINE) ThreadFunction,

&hReadEvents[i], // pass event handle

0, &IDThread);

if (hThread == NULL)

{

// Error exit.

}

Before the master thread writes to the shared buffer, it uses the ResetEvent function to set the state of hGlobalWriteEvent (an application-defined global variable) to nonsignaled. This blocks the reader threads from starting a read operation. The master then uses the WaitForMultipleObjects function to wait for all reader threads to finish any current read operations. When WaitForMultipleObjects returns, the master thread can safely write to the buffer. After it has finished, it sets hGlobalWriteEvent and all the reader-thread events to signaled, enabling the reader threads to resume their read operations.

VOID WriteToBuffer(VOID)

{

DWORD dwWaitResult, i;

// Reset hGlobalWriteEvent to nonsignaled, to block readers.

if (! ResetEvent(hGlobalWriteEvent) )

{

// Error exit.

}

// Wait for all reading threads to finish reading.

dwWaitResult = WaitForMultipleObjects(

NUMTHREADS, // number of handles in array

hReadEvents, // array of read-event handles

TRUE, // wait until all are signaled

INFINITE); // indefinite wait

switch (dwWaitResult)

{

// All read-event objects were signaled.

case WAIT_OBJECT_0:

// Write to the shared buffer.

break;

// An error occurred.

default:

printf("Wait error: %d\n", GetLastError());

ExitProcess(0);

}

// Set hGlobalWriteEvent to signaled.

if (! SetEvent(hGlobalWriteEvent) )

{

// Error exit.

}

// Set all read events to signaled.

for(i = 1; i <= NUMTHREADS; i++)

if (! SetEvent(hReadEvents[i]) ) {

// Error exit.

}

Before starting a read operation, each reader thread uses WaitForMultipleObjects to wait for the application-defined global variable hGlobalWriteEvent and its own read event to be signaled. When WaitForMultipleObjects returns, the reader thread's auto-reset event has been reset to nonsignaled. This blocks the master thread from writing to the buffer until the reader thread uses the SetEvent function to set the event's state back to signaled.

VOID ThreadFunction(LPVOID lpParam)

{

DWORD dwWaitResult, i;

HANDLE hEvents[2];

hEvents[0] = (HANDLE) *lpParam; // thread's read event

hEvents[1] = hGlobalWriteEvent;

dwWaitResult = WaitForMultipleObjects(

2, // number of handles in array

hEvents, // array of event handles

TRUE, // wait till all are signaled

INFINITE); // indefinite wait

switch (dwWaitResult)

{

// Both event objects were signaled.

case WAIT_OBJECT_0:

// Read from the shared buffer.

break;

// An error occurred.

default:

printf("Wait error: %d\n", GetLastError());

ExitThread(0);

}

// Set the read event to signaled.

if (! SetEvent(hEvents[0]) )

{

// Error exit.

}

Last news from Greatis Software

    Nostalgia .Net     .Net is powerful, but not all-powerful, so sometimes we need to use Win32 API for our .Net applications. It's simple enough with Platform Invoke if you have Win32 skill, but we do not always have time to dig the ancient documentation, declare the special types that are compatible with Win32, find the values of the Win32's constants and so on. Nostalgia .Net offers several simple-to-use classes, and components that will allow you to forget about the headache of Win32 and just use the power of Win32 in your application the same way as you use the native. Net classes. More »

Recommended software for developers

    Ultimate Pack     Component pack for Delphi and C++ Builder that contains runtime form designer, runtime object inspector, print suite and much more for the very special price. More »

    Form Designer .Net     Unique runtime form design solution that allows to edit any form in .Net WinForms application at runtime with full source codes for only 300 euro! More »

    Print Suite .Net     Print Suite .Net is a set of components for easy printing texts, images and grids from your WinForms applications. Full C# source codes are available More »

    Gradient Controls .Net     Gradient Controls .Net offers controls with gradient background feature. Labels, panels and so on... Full C# source codes are available More »

    Greatis iGrid     iGrid plots drawing grid right over your desktop, so you can use it everywhere, with any drawing application without any special plugins for different graphic editors. More »

All the contacts and projects

Dmitry Vasiliev (just.dmitry)

More Online Helps

Win32 Programmer's Reference
Win32 Multimedia Programmer's Reference
OLE Programmer's Reference
Microsoft Windows Pen API Programmer's Reference
Microsoft Windows Sockets 2 Reference
Microsoft Windows Telephony API (TAPI) Programmer's Reference
Unix Manual Pages

greatis just4fun network