Difference between revisions of "cpp/atomic/atomic exchange"
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===Exceptions=== | ===Exceptions=== | ||
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===Example=== | ===Example=== | ||
Revision as of 11:56, 31 March 2017
| Defined in header <atomic>
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| (1) | (since C++11) | |
| template< class T > T atomic_exchange( std::atomic<T>* obj, T desr ); |
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| template< class T > T atomic_exchange( volatile std::atomic<T>* obj, T desr ); |
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| (2) | (since C++11) | |
| template< class T > T atomic_exchange_explicit( std::atomic<T>* obj, T desr, |
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| template< class T > T atomic_exchange_explicit( volatile std::atomic<T>* obj, T desr, |
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1) Atomically replaces the value pointed to by
obj with the value of desr and returns the value obj held previously, as if by obj->exchange(desr)2) Atomically replaces the value pointed to by
obj with the value of desr and returns the value obj held previously, as if by obj->exchange(desr, order)Contents |
Parameters
| obj | - | pointer to the atomic object to modify |
| desr | - | the value to store in the atomic object |
| order | - | the memory synchronization ordering for this operation: all values are permitted. |
Return value
The value held previously by the atomic object pointed to by obj
Exceptions
noexcept specification:
noexcept
Example
A spinlock mutex can be implemented in userspace using an atomic exchange operation, similar to std::atomic_flag_test_and_set:
Run this code
#include <thread> #include <vector> #include <iostream> #include <atomic> std::atomic<bool> lock(false); // holds true when locked // holds false when unlocked void f(int n) { for (int cnt = 0; cnt < 100; ++cnt) { while(std::atomic_exchange_explicit(&lock, true, std::memory_order_acquire)) ; // spin until acquired std::cout << "Output from thread " << n << '\n'; std::atomic_store_explicit(&lock, false, std::memory_order_release); } } int main() { std::vector<std::thread> v; for (int n = 0; n < 10; ++n) { v.emplace_back(f, n); } for (auto& t : v) { t.join(); } }
Output:
Output from thread 2 Output from thread 6 Output from thread 7 ...<exactly 1000 lines>...
See also
| atomically replaces the value of the atomic object and obtains the value held previously (public member function of std::atomic<T>)
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| atomically compares the value of the atomic object with non-atomic argument and performs atomic exchange if equal or atomic load if not (function template) | |
| specializes atomic operations for std::shared_ptr (function template) | |
| C documentation for atomic_exchange, atomic_exchange_explicit
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