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Thu, 19 Jun 2008 09:10:27 . . . . afh [new page]

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= createCriticalSection =

== Syntax ==

:int createCriticalSection((void)?)

== Arguments ==


== Return ==

:Returns a handle to a newly created Critical Section, or 0 if the Critical Section could not be created.

== Description ==

:This function creates a new Critical Section, which may be used to enforce mutually exclusive access to a resource or set of resources. At most one thread can "enter" a Critical Section at any given time. Any other threads wishing to enter the same Critical Section must wait until the first thread "leaves" the Critical Section. If you have more that one set of resources that need protection, you can create a separate Critical Section to guard each resource, thus allowing a higher degree of parallel processing, since a thread using one such resource does not lock out other threads from using other guarded resources at the same time.



:For more information about Critical Sections, see the MSDN documentation about [Critical Section Objects].

== Example[[Anchor example]] ==




// Sample filter creates 5 worker threads. Each thread is assigned

// the "next available" line "y" to process, until all lines

// have been processed. Since multiple threads are attempting

// to read and increment the "nextY" variable at the same time,

// we use a Critical Section to serialize access to "nextY".


// N.B. This example assumes global variables are implemented in

// such a manner that they are shared among all threads (which is

// not yet true as of FM 1.0 Beta 9e).

int nextY; //Global int to hold next line to be processed

int csNEXTY; //Critical Section handle for guarding access to "nextY"


if (e==FME_CUSTOMEVENT && n==666) {

// Perform function #666

while(1) {

//Need to serialize access to "nextY"; else

//two threads may try to increment nextY at

//the same time, with indeterminate results.


int y = nextY++; //"nextY" is a shared global var


if (y >= y_end) return true; //exit thread if no more lines to process

//process this line y...

for (int x = x_start; x < x_end; x++) {

for (int z = 0; z < Z; z++) {

//invert this pixel...

pset(x,y,z, 255-src(x,y,z));

//do other useful work here...

}//for z

}//for x



return false; //other events not yet processed



// Create a Critical Section to guard access to nextY

csNEXTY = createCriticalSection();

if (!csNEXTY) {

ErrorOk("Failed to create Critical Section csNEXTY");

return false;


// Set initial value of "nextY" to y_start

nextY = y_start;

// Start 5 worker threads...

for (int i=0; i < 5; i++)


// Wait for all threads to finish

waitForThread(0, INFINITE, 0);

// Delete the Critical Section


return true; //finished processing this tile



== Also see ==

:System Functions, Critical Section Functions, enterCriticalSection, tryEnterCriticalSection, leaveCriticalSection, deleteCriticalSection

== Comments ==

:Everyone can add her comments here about her experiences with this function. Tips for using it are welcome, too.