多线程编程之原子操作

　　在多线程环境中，对共享的变量的访问，可以使用基于Compare And Swap这种lock free的技术进行实现，这种实现的好处是效率高。

一、原子操作摘录

1.1 Android

　　源码：system/core/libcutils /atomic.c（针对X86）：

#elif defined(__i386__) || defined(__x86_64__)

void android_atomic_write(int32_t value, volatile int32_t* addr) {
    int32_t oldValue;
    do {
        oldValue = *addr;
    } while (android_atomic_cmpxchg(oldValue, value, addr));
}

int32_t android_atomic_inc(volatile int32_t* addr) {
    int32_t oldValue;
    do {
        oldValue = *addr;
    } while (android_atomic_cmpxchg(oldValue, oldValue+1, addr));
    return oldValue;
}

int32_t android_atomic_dec(volatile int32_t* addr) {
    int32_t oldValue;
    do {
        oldValue = *addr;
    } while (android_atomic_cmpxchg(oldValue, oldValue-1, addr));
    return oldValue;
}

int32_t android_atomic_add(int32_t value, volatile int32_t* addr){
    int32_t oldValue;
    do {
        oldValue = *addr;
    } while (android_atomic_cmpxchg(oldValue, oldValue+value, addr));
    return oldValue;
}

int android_atomic_cmpxchg(int32_t oldvalue, int32_t newvalue, volatile int32_t* addr) {
    int xchg;
    asm volatile
    (
        " lock; cmpxchg %%ecx, (%%edx);"
        " setne %%al;"
        " andl $1, %%eax"
        : "=a" (xchg)
        : "a" (oldvalue), "c" (newvalue), "d" (addr)
    );
    return xchg;
}

　　android_atomic_cmpxchg是使用GNU C嵌入汇编实现，使用X86提供的对CAS的原子支持的指令。oldvalue放在eax寄存器中，newvalue放在ecx中，addr（pointer）放在edx中。cmpxchg指令首先比较addr指向的内存与oldvalue(eax)，如果二者相等，将newvalue(ecx)放到addr所指向的内存中，同时设置Z标志1。setne与andl 指令的操作的结果很简单：如果Z标志被设置，则eax为0，否则为1。程序执行最终eax放到xchg变量里。

　　可以看出自增、自减操作都是基于android_atomic_cmpxchg实现的，这个操作成功返回1，失败返回0，例如：

int32_t a = 5;
int b = android_atomic_cmpxchg( a, a+1, &a );
printf("%d, %d
", a, b);        
// 成功时输出：6,1
// 失败时输出：5,0

 1.2 iOS

　　头文件：#include <libkern/OSAtomic.h>

Operation	Function name	Description
Add	OSAtomicAdd32 OSAtomicAdd32Barrier OSAtomicAdd64 OSAtomicAdd64Barrier	Adds two integer values together and stores the result in one of the specified variables.
Increment	OSAtomicIncrement32 OSAtomicIncrement32Barrier OSAtomicIncrement64 OSAtomicIncrement64Barrier	Increments the specified integer value by 1.
Decrement	OSAtomicDecrement32 OSAtomicDecrement32Barrier OSAtomicDecrement64 OSAtomicDecrement64Barrier	Decrements the specified integer value by 1.
Logical OR	OSAtomicOr32 OSAtomicOr32Barrier	Performs a logical OR between the specified 32-bit value and a 32-bit mask.
Logical AND	OSAtomicAnd32 OSAtomicAnd32Barrier	Performs a logical AND between the specified 32-bit value and a 32-bit mask.
Logical XOR	OSAtomicXor32 OSAtomicXor32Barrier	Performs a logical XOR between the specified 32-bit value and a 32-bit mask.
Compare and swap	OSAtomicCompareAndSwap32 OSAtomicCompareAndSwap32Barrier OSAtomicCompareAndSwap64 OSAtomicCompareAndSwap64Barrier OSAtomicCompareAndSwapPtr OSAtomicCompareAndSwapPtrBarrier OSAtomicCompareAndSwapInt OSAtomicCompareAndSwapIntBarrier OSAtomicCompareAndSwapLong OSAtomicCompareAndSwapLongBarrier	Compares a variable against the specified old value. If the two values are equal, this function assigns the specified new value to the variable; otherwise, it does nothing. The comparison and assignment are done as one atomic operation and the function returns a Boolean value indicating whether the swap actually occurred.
Test and set	OSAtomicTestAndSet OSAtomicTestAndSetBarrier	Tests a bit in the specified variable, sets that bit to 1, and returns the value of the old bit as a Boolean value. Bits are tested according to the formula (0×80 >> (n & 7)) of byte((char*)address + (n >> 3)) where n is the bit number and address is a pointer to the variable. This formula effectively breaks up the variable into 8-bit sized chunks and orders the bits in each chunk in reverse. For example, to test the lowest-order bit (bit 0) of a 32-bit integer, you would actually specify 7 for the bit number; similarly, to test the highest order bit (bit 32), you would specify 24 for the bit number.
Test and clear	OSAtomicTestAndClear OSAtomicTestAndClearBarrier	Tests a bit in the specified variable, sets that bit to 0, and returns the value of the old bit as a Boolean value. Bits are tested according to the formula (0×80 >> (n & 7)) of byte((char*)address + (n >> 3)) where n is the bit number and address is a pointer to the variable. This formula effectively breaks up the variable into 8-bit sized chunks and orders the bits in each chunk in reverse. For example, to test the lowest-order bit (bit 0) of a 32-bit integer, you would actually specify 7 for the bit number; similarly, to test the highest order bit (bit 32), you would specify 24 for the bit number.

　　这些操作中，可能关心的地方就是函数的返回值，这个返回值在编程的时看头文件说明就好了，比较方便。官网文档说明是这样的：The arithmetic operations return the new value, after the operation has been performed.  The boolean operations come in two styles, one of which returns the new value, and one of which (the "Orig" versions) returns the old.  The compare-and-swap operations return true if the comparison was equal, ie if the swap occured.  The bit test and set/clear operations return the original value of the bit.  The dequeue operation returns the most recently enqueued element, or NULL if the list in empty.

1.3 Windows

　　Windows平台上的操作其实也是大体类似的，查看InterLockedIncrement、InterLockedDecrement系列的函数就知道了，需要注意的是这两个函数返回操作后的新值，而不是操作前的原始值。

Header	windows.h
Library	coredll.lib
Windows Embedded CE	Windows CE .NET 4.0 and later
Windows Mobile	Windows Mobile Version 5.0 and later