Applies to: desktop apps | Metro style apps
Performs an atomic compare-and-exchange operation on the specified values. The function compares two specified 32-bit values and exchanges with another 32-bit value based on the outcome of the comparison.
If you are exchanging pointer values, this function has been superseded by the InterlockedCompareExchangePointer function.
To operate on 64-bit values, use the InterlockedCompareExchange64 function.
Syntax
LONG __cdecl InterlockedCompareExchange( __inout LONG volatile *Destination, __in LONG Exchange, __in LONG Comparand );
Parameters
- Destination [in, out]
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A pointer to the destination value.
- Exchange [in]
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The exchange value.
- Comparand [in]
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The value to compare to Destination.
Return value
The function returns the initial value of the Destination parameter.
Remarks
The function compares the Destination value with the Comparand value. If the Destination value is equal to the Comparand value, the Exchange value is stored in the address specified by Destination. Otherwise, no operation is performed.
The parameters for this function must be aligned on a 32-bit boundary; otherwise, the function will behave unpredictably on multiprocessor x86 systems and any non-x86 systems. See _aligned_malloc.
The interlocked functions provide a simple mechanism for synchronizing access to a variable that is shared by multiple threads. This function is atomic with respect to calls to other interlocked functions.
This function is implemented using a compiler intrinsic where possible. For more information, see the WinBase.h header file and _InterlockedCompareExchange.
This function generates a full memory barrier (or fence) to ensure that memory operations are completed in order.
Itanium-based systems: For performance-critical applications, use InterlockedCompareExchangeAcquire or InterlockedCompareExchangeRelease instead.
Requirements
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Minimum supported client | Windows XP |
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Minimum supported server | Windows Server 2003 |
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Header |
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Library |
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DLL |
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See also
- Interlocked Variable Access
- InterlockedCompare64Exchange128
- InterlockedCompareExchange64
- InterlockedCompareExchangeAcquire
- InterlockedCompareExchangePointer
- InterlockedCompareExchangeRelease
- Synchronization Functions
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Build date: 3/7/2012
InterlockedCompareExchange can be used to implement any interlocked operation that operates on a single word of data. The pattern is as follows:
- Atomically read variable into old_value
- Operate on old_value to produce new_value
- Set variable to new_value if the current value of variable is still old_value
- otherwise, go back to 1
For example, here's how a Lehmer PRNG can be implemented in a lock-free way:
LONG Random(LONG seed)
{
return (LONG)(DWORD)(0x41C64E6D * (DWORD64)(DWORD)seed + 0x3039) % ((DWORD64)MAXDWORD + 1);
}
LONG InterlockedRandom(volatile LONG * seed)
{
LONG old_value = *seed;
LONG cur_value = old_value;
LONG new_value;
for(;;)
{
// calculate the function
new_value = Random(old_value);
// set the new value if the current value is still the expected one
cur_value = InterlockedCompareExchange(seed, new_value, old_value);
// we found the expected value: the exchange happened
if(cur_value == old_value)
break;
// recalculate the function on the unexpected value
old_value = cur_value;
}
// success
return new_value;
}
InterlockedCompareExchange can be understood as _atomic_ implementation of the following snippet:
LONG CmpXchg(LONG * Destination, LONG Exchange, Long Comparand) {
LONG retval = *Destination
if (*Destination == Comparand)
*Destination = Exchange;
return retval;
}