Fetch-and-addfetch-and-add是CPU指令（FAA），对内存位置执行增加一个数量的原子操作。具体内容为：
1991年，Maurice Herlihy证明fetch-and-add具有一个有限的consensus数，能解决不超过两个并发进程的无等待consensus问题。[1] 用途下述伪代码用ticket lock算法实现了互斥锁： record locktype {
int ticketnumber
int turn
}
procedure LockInit( locktype* lock ) {
lock.ticketnumber := 0
lock.turn := 0
}
procedure Lock( locktype* lock ) {
int myturn := FetchAndIncrement( &lock.ticketnumber ) //must be atomic, since many threads might ask for a lock at the same time
while lock.turn ≠ myturn
skip // spin until lock is acquired
}
procedure UnLock( locktype* lock ) {
FetchAndIncrement( &lock.turn ) //this need not be atomic, since only the possessor of the lock will execute this
}
硬件软件支持C++11标准定义了原子的fetch_add函数。[2] GCC把它作为对C语言的扩展。[3] x86实现从8086起，以内存为目的操作数的ADD指令就是fetch-and-add。如果使用LOCK前缀，那么它对多处理器是原子操作。但不能返回原值，直至486引入XADD指令。 void __fastcall atomic_inc (volatile int* pNum)
{
__asm
{
lock inc dword ptr [ECX]
ret
}
}
下述GCC编译的C语言函数，在x86的32位与64位平台上，使用扩展asm语法： static inline int fetch_and_add(int* variable, int value)
{
__asm__ volatile("lock; xaddl %0, %1"
: "+r" (value), "+m" (*variable) // input+output
: // No input-only
: "memory"
);
return value;
}
参见参考文献
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