| FUNCTION |
Parameters:
- "varname" -
Specifies the name of the destination static variable. Static variable if exists must be scalar numeric type. If static variable is not initialized, the function assumes that it has value of zero.
- exchange - specifies the exchange value. Scalar numeric.
- comperand - specifies the value to compare to the destination static variable. Scalar numeric.
Return Values:
The return value is the initial value of the destination static variable. If variable did not exist, it returns zero.
The StaticVarCompareExchange function performs an atomic comparison of the "varname" static variable value with the Comperand value. If the static variable value is equal to the Comperand value, the Exchange value is stored in the static variable. Otherwise, no operation is performed.
The function StaticVarCompareExchange provides a simple mechanism for synchronizing access to static variables that are shared by multiple threads. The following examples show how to implement semaphore and critical section in AFL using StaticVarCompareExchange function. For more details see Tutorial: Efficient use of multithreading.
|
| EXAMPLE |
// EXAMPLE 1 : Simple semaphore
(no waiting)
if( StaticVarCompareExchange( "semaphore", 1, 0 )
== 0 ) //
obtain semaphore
{
// protected section here
// Here you have exclusive access (no other threads
that check for semaphore will enter simultaneously)
/////////////////////////
StaticVarSet("semaphore", 0 ); //
reset semaphore
}
else
{
_TRACE("Can
not obtain semaphore");
}
///////////////
// EXAMPLE 2 HOW TO IMPLEMENT CRITICAL SECTION IN
AFL
///////////////
function _TryEnterCS(
secname )
{
global _cursec;
_cursec= "";
// try obtaining semaphore for 1000 ms
for(
i = 0; i < 1000;
i++ )
if( StaticVarCompareExchange(
secname, 1, 0 )
== 0 )
{
_cursec = secname;
break;
}
else ThreadSleep( 1 ); //sleep
one millisecond
return _cursec
!= "";
}
// call it ONLY when _TryEnterCS returned TRUE !
function _LeaveCS()
{
global _cursec;
if( _cursec != "" )
{
StaticVarSet(
_cursec, 0 );
_cursec = "";
}
}
function TimeConsumingWork()
{
// WARNING: the Percentile is CPU hungry as it involves
lots of sorting, the loop below may take > 1 second to complete
for( i = 0;
i< 10; i++ ) Percentile( C, 100, 10 );
}
//_TRACE("Without CS Begin " + GetChartID() );
//TimeConsumingWork(); // some time consuming calculation
//_TRACE("Without CS End" + GetChartID() );
// Example usage (critical section)
if( _TryEnterCS( "mysemaphore" )
)
{
// you are inside critical section now
_TRACE("Begin
CS " + GetChartID()
);
TimeConsumingWork(); // some time consuming calculation
_TRACE("End
CS " + GetChartID()
);
_LeaveCS();
}
else
{
_TRACE("Unable
to enter CS");
} |