_InterlockedCompareExchange intrinsic functions

  • Artikel

Microsoft Specific

Does an interlocked compare and exchange.

Syntax

long _InterlockedCompareExchange(
   long volatile * Destination,
   long Exchange,
   long Comparand
);
long _InterlockedCompareExchange_acq(
   long volatile * Destination,
   long Exchange,
   long Comparand
);
long _InterlockedCompareExchange_HLEAcquire(
   long volatile * Destination,
   long Exchange,
   long Comparand
);
long _InterlockedCompareExchange_HLERelease(
   long volatile * Destination,
   long Exchange,
   long Comparand
);
long _InterlockedCompareExchange_nf(
   long volatile * Destination,
   long Exchange,
   long Comparand
);
long _InterlockedCompareExchange_np(
   long volatile * Destination,
   long Exchange,
   long Comparand
);
long _InterlockedCompareExchange_rel(
   long volatile * Destination,
   long Exchange,
   long Comparand
);
char _InterlockedCompareExchange8(
   char volatile * Destination,
   char Exchange,
   char Comparand
);
char _InterlockedCompareExchange8_acq(
   char volatile * Destination,
   char Exchange,
   char Comparand
);
char _InterlockedCompareExchange8_nf(
   char volatile * Destination,
   char Exchange,
   char Comparand
);
char _InterlockedCompareExchange8_rel(
   char volatile * Destination,
   char Exchange,
   char Comparand
);
short _InterlockedCompareExchange16(
   short volatile * Destination,
   short Exchange,
   short Comparand
);
short _InterlockedCompareExchange16_acq(
   short volatile * Destination,
   short Exchange,
   short Comparand
);
short _InterlockedCompareExchange16_nf(
   short volatile * Destination,
   short Exchange,
   short Comparand
);
short _InterlockedCompareExchange16_np(
   short volatile * Destination,
   short Exchange,
   short Comparand
);
short _InterlockedCompareExchange16_rel(
   short volatile * Destination,
   short Exchange,
   short Comparand
);
__int64 _InterlockedCompareExchange64(
   __int64 volatile * Destination,
   __int64 Exchange,
   __int64 Comparand
);
__int64 _InterlockedCompareExchange64_acq(
   __int64 volatile * Destination,
   __int64 Exchange,
   __int64 Comparand
);
__int64 _InterlockedCompareExchange64_HLEAcquire (
   __int64 volatile * Destination,
   __int64 Exchange,
   __int64 Comparand
);
__int64 _InterlockedCompareExchange64_HLERelease(
   __int64 volatile * Destination,
   __int64 Exchange,
   __int64 Comparand
);
__int64 _InterlockedCompareExchange64_nf(
   __int64 volatile * Destination,
   __int64 Exchange,
   __int64 Comparand
);
__int64 _InterlockedCompareExchange64_np(
   __int64 volatile * Destination,
   __int64 Exchange,
   __int64 Comparand
);
__int64 _InterlockedCompareExchange64_rel(
   __int64 volatile * Destination,
   __int64 Exchange,
   __int64 Comparand
);

Parameters

Destination
[in, out] Pointer to the destination value. The sign is ignored.

Exchange
[in] Exchange value. The sign is ignored.

Comparand
[in] Value to compare to the value pointed at by Destination. The sign is ignored.

Return value

The return value is the initial value pointed at by the Destination pointer.

Requirements

Intrinsic Architecture Header
_InterlockedCompareExchange, _InterlockedCompareExchange8, _InterlockedCompareExchange16, _InterlockedCompareExchange64 x86, ARM, x64, ARM64 <intrin.h>
_InterlockedCompareExchange_acq, _InterlockedCompareExchange_nf, _InterlockedCompareExchange_rel, _InterlockedCompareExchange8_acq, _InterlockedCompareExchange8_nf, _InterlockedCompareExchange8_rel, _InterlockedCompareExchange16_acq, _InterlockedCompareExchange16_nf, _InterlockedCompareExchange16_rel, _InterlockedCompareExchange64_acq, _InterlockedCompareExchange64_nf, _InterlockedCompareExchange64_rel ARM, ARM64 <intrin.h>
_InterlockedCompareExchange_np, _InterlockedCompareExchange16_np, _InterlockedCompareExchange64_np x64 <intrin.h>
_InterlockedCompareExchange_HLEAcquire, _InterlockedCompareExchange_HLERelease, _InterlockedCompareExchange64_HLEAcquire, _InterlockedCompareExchange64_HLERelease x86, x64 <immintrin.h>

Remarks

_InterlockedCompareExchange does an atomic comparison of the value pointed at by Destination 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, does no operation.

_InterlockedCompareExchange provides compiler intrinsic support for the Win32 Windows SDK InterlockedCompareExchange function.

There are several variations on _InterlockedCompareExchange that vary based on the data types they involve and whether processor-specific acquire or release semantics are used.

While the _InterlockedCompareExchange function operates on 32-bit long integer values, _InterlockedCompareExchange8 operates on 8-bit integer values, _InterlockedCompareExchange16 operates on 16-bit short integer values, and _InterlockedCompareExchange64 operates on 64-bit integer values. For more information on similar intrinsics for 128-bit values, see _InterlockedCompareExchange128.

On all ARM platforms, use the intrinsics with _acq and _rel suffixes for acquire and release semantics, such as at the beginning and end of a critical section. The ARM intrinsics with an _nf ("no fence") suffix don't act as a memory barrier.

The intrinsics with an _np ("no prefetch") suffix prevent a possible prefetch operation from being inserted by the compiler.

On Intel platforms that support Hardware Lock Elision (HLE) instructions, the intrinsics with _HLEAcquire and _HLERelease suffixes include a hint to the processor that can accelerate performance by eliminating a lock write step in hardware. If these intrinsics are called on platforms that don't support HLE, the hint is ignored.

These routines are only available as intrinsics.

Example

In the following example, _InterlockedCompareExchange is used for simple low-level thread synchronization. The approach has its limitations as a basis for multithreaded programming; it's presented to illustrate the typical use of the interlocked intrinsics. For best results, use the Windows API. For more information about multithreaded programming, see Writing a Multithreaded Win32 Program.

// intrinExample.cpp
// compile with: /EHsc /O2
// Simple example of using _Interlocked* intrinsics to
// do manual synchronization
//
// Add [-DSKIP_LOCKING] to the command line to disable
// the locking. This will cause the threads to execute out
// of sequence.

#define _CRT_RAND_S

#include "windows.h"

#include <iostream>
#include <queue>
#include <intrin.h>

using namespace std;

// --------------------------------------------------------------------

// if defined, will not do any locking on shared data
//#define SKIP_LOCKING

// A common way of locking using _InterlockedCompareExchange.
// Refer to other sources for a discussion of the many issues
// involved. For example, this particular locking scheme performs well
// when lock contention is low, as the while loop overhead is small and
// locks are acquired very quickly, but degrades as many callers want
// the lock and most threads are doing a lot of interlocked spinning.
// There are also no guarantees that a caller will ever acquire the
// lock.
namespace MyInterlockedIntrinsicLock
{
    typedef unsigned LOCK, *PLOCK;

#pragma intrinsic(_InterlockedCompareExchange, _InterlockedExchange)

    enum {LOCK_IS_FREE = 0, LOCK_IS_TAKEN = 1};

    void Lock(PLOCK pl)
    {
#if !defined(SKIP_LOCKING)
        // If *pl == LOCK_IS_FREE, it is set to LOCK_IS_TAKEN
        // atomically, so only 1 caller gets the lock.
        // If *pl == LOCK_IS_TAKEN,
        // the result is LOCK_IS_TAKEN, and the while loop keeps spinning.
        while (_InterlockedCompareExchange((long *)pl,
                                           LOCK_IS_TAKEN, // exchange
                                           LOCK_IS_FREE)  // comparand
               == LOCK_IS_TAKEN)
        {
            // spin!
        }
        // This will also work.
        //while (_InterlockedExchange(pl, LOCK_IS_TAKEN) ==
        //                             LOCK_IS_TAKEN)
        //{
        //    // spin!
        //}

        // At this point, the lock is acquired.
#endif
    }

    void Unlock(PLOCK pl) {
#if !defined(SKIP_LOCKING)
        _InterlockedExchange((long *)pl, LOCK_IS_FREE);
#endif
    }
}

// ------------------------------------------------------------------
// Data shared by threads

queue<int> SharedQueue;
MyInterlockedIntrinsicLock::LOCK SharedLock;
int TicketNumber;

// ------------------------------------------------------------------

DWORD WINAPI
ProducerThread(
    LPVOID unused
    )
{
    unsigned int randValue;
    while (1) {
        // Acquire shared data. Enter critical section.
        MyInterlockedIntrinsicLock::Lock(&SharedLock);

        //cout << ">" << TicketNumber << endl;
        SharedQueue.push(TicketNumber++);

        // Release shared data. Leave critical section.
        MyInterlockedIntrinsicLock::Unlock(&SharedLock);

        rand_s(&randValue);
        Sleep(randValue % 20);
    }

    return 0;
}

DWORD WINAPI
ConsumerThread(
    LPVOID unused
    )
{
    while (1) {
        // Acquire shared data. Enter critical section
        MyInterlockedIntrinsicLock::Lock(&SharedLock);

        if (!SharedQueue.empty()) {
            int x = SharedQueue.front();
            cout << "<" << x << endl;
            SharedQueue.pop();
        }

        // Release shared data. Leave critical section
        MyInterlockedIntrinsicLock::Unlock(&SharedLock);

        unsigned int randValue;
        rand_s(&randValue);
        Sleep(randValue % 20);
    }
    return 0;
}

int main(
    void
    )
{
    const int timeoutTime = 500;
    int unused1, unused2;
    HANDLE threads[4];

    // The program creates 4 threads:
    // two producer threads adding to the queue
    // and two consumers taking data out and printing it.
    threads[0] = CreateThread(NULL,
                              0,
                              ProducerThread,
                              &unused1,
                              0,
                              (LPDWORD)&unused2);

    threads[1] = CreateThread(NULL,
                              0,
                              ConsumerThread,
                              &unused1,
                              0,
                              (LPDWORD)&unused2);

    threads[2] = CreateThread(NULL,
                              0,
                              ProducerThread,
                              &unused1,
                              0,
                              (LPDWORD)&unused2);

    threads[3] = CreateThread(NULL,
                              0,
                              ConsumerThread,
                              &unused1,
                              0,
                              (LPDWORD)&unused2);

    WaitForMultipleObjects(4, threads, TRUE, timeoutTime);

    return 0;
}

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END Microsoft Specific

See also

_InterlockedCompareExchange128
_InterlockedCompareExchangePointer intrinsic functions
Compiler intrinsics
Keywords
Conflicts with the x86 Compiler