std::atomic...hange_strong(3)

🌸 NAME 🌸

okay so bestie, this page is std::atomic...hange_strong(3) from the C++ Standard Libary (yes, that’s the manual name, we’re not fixing the spelling, periodt 💅). It’s all about std::atomic::compare_exchange_weak,std::atomic::compare_exchange_strong — the atomic compare-and-swap girlies. 💖

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✨ Synopsis ✨

Here’s the whole overload lineup, no cap. All of these are (since C++11) and all are noexcept:

(1) (since C++11)
bool compare_exchange_weak( T& expected, T desired,
                            std::memory_order success,
                            std::memory_order failure ) noexcept;

(2) (since C++11)
bool compare_exchange_weak( T& expected, T desired,
                            std::memory_order success,
                            std::memory_order failure ) volatile noexcept;

(3) (since C++11)
bool compare_exchange_weak( T& expected, T desired,
                            std::memory_order order =
                                std::memory_order_seq_cst ) noexcept;

(4) (since C++11)
bool compare_exchange_weak( T& expected, T desired,
                            std::memory_order order =
                                std::memory_order_seq_cst ) volatile noexcept;

(5) (since C++11)
bool compare_exchange_strong( T& expected, T desired,
                              std::memory_order success,
                              std::memory_order failure ) noexcept;

(6) (since C++11)
bool compare_exchange_strong( T& expected, T desired,
                              std::memory_order success,
                              std::memory_order failure ) volatile noexcept;

(7) (since C++11)
bool compare_exchange_strong( T& expected, T desired,
                              std::memory_order order =
                                  std::memory_order_seq_cst ) noexcept;

(8) (since C++11)
bool compare_exchange_strong( T& expected, T desired,
                              std::memory_order order =
                                  std::memory_order_seq_cst ) volatile noexcept;

So what’s the vibe? It atomically compares the object representation (until C++20) / value representation (since C++20) of *this with that of expected. If those are bitwise-equal, it replaces the former with desired — that’s the read-modify-write operation 💅. Otherwise, it loads the actual value stored in *this into expected — that’s the load operation. Slay.

Here’s the memory-order tea ☕:

• Overloads (1,2,5,6): the read-modify-write operation uses success; the load operation uses failure.
• Overloads (3,4,7,8): the read-modify-write operation uses order; the load operation uses:
  • std::memory_order_acquire if order is std::memory_order_acq_rel
  • std::memory_order_relaxed if order is std::memory_order_release
  • otherwise order

⚠️ If failure is stronger than success, or (until C++17) is one of std::memory_order_release and std::memory_order_acq_rel, the behavior is undefined. Like, bestie, do not do that. 😭

🫠 It is deprecated if std::atomic<T>::is_always_lock_free is false and (since C++20) any volatile overload participates in overload resolution.

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🎀 Parameters 🎀

• expected — reference to the value expected to be found in the atomic object
• desired — the value to store in the atomic object if it is as expected
• success — the memory synchronization ordering for the read-modify-write operation if the comparison succeeds
• failure — the memory synchronization ordering for the load operation if the comparison fails
• order — the memory synchronization ordering for both operations

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💖 Return value 💖

true if the underlying atomic value was successfully changed, false otherwise. Simple as that, fr fr.

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🧚 Notes 🧚

• The comparison and copying are bitwise (similar to std::memcmp and std::memcpy); no constructor, assignment operator, or comparison operator are used.
• compare_exchange_weak is allowed to fail spuriously, that is, acts as if *this != expected even if they are equal. When a compare-and-exchange is in a loop, compare_exchange_weak will yield better performance on some platforms.
• When compare_exchange_weak would require a loop and compare_exchange_strong would not, compare_exchange_strong is preferable unless the object representation of T may include padding bits, (until C++20) trap bits, or offers multiple object representations for the same value (e.g. floating-point NaN). In those cases, compare_exchange_weak typically works because it quickly converges on some stable object representation.
• For a union with bits that participate in the value representations of some members but not the others, compare-and-exchange might always fail because such padding bits have indeterminate values when they do not participate in the value representation of the active member.
• Padding bits that never participate in an object’s value (since C++20) representation are ignored.

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🦋 Example 🦋

Compare-and-exchange operations are often used as basic building blocks of lock-free data structures. Iconic behavior, honestly. 👑

// Run this code

#include <atomic>

template<typename T>
struct node
{
    T data;
    node* next;
    node(const T& data) : data(data), next(nullptr) {}
};

template<typename T>
class stack
{
    std::atomic<node<T>*> head;
public:
    void push(const T& data)
    {
        node<T>* new_node = new node<T>(data);

        // put the current value of head into new_node->next
        new_node->next = head.load(std::memory_order_relaxed);

        // now make new_node the new head, but if the head
        // is no longer what's stored in new_node->next
        // (some other thread must have inserted a node just now)
        // then put that new head into new_node->next and try again
        while (!head.compare_exchange_weak(new_node->next, new_node,
                                           std::memory_order_release,
                                           std::memory_order_relaxed))
            ; // the body of the loop is empty

// Note: the above use is not thread-safe in at least
// GCC prior to 4.8.3 (bug 60272), clang prior to 2014-05-05 (bug 18899)
// MSVC prior to 2014-03-17 (bug 819819). The following is a workaround:
//      node<T>* old_head = head.load(std::memory_order_relaxed);
//      do
//      {
//          new_node->next = old_head;
//      }
//      while (!head.compare_exchange_weak(old_head, new_node,
//                                         std::memory_order_release,
//                                         std::memory_order_relaxed));
    }
};

int main()
{
    stack<int> s;
    s.push(1);
    s.push(2);
    s.push(3);
}

Demonstrates how std::compare_exchange_strong either changes the value of the atomic variable or the variable used for comparison.

Incomplete section alert: This section is incomplete Reason: more practical use of the strong CAS would be nice, such as where Concurrency in Action uses it

// Run this code

#include <atomic>
#include <iostream>

std::atomic<int> ai;

int tst_val = 4;
int new_val = 5;
bool exchanged = false;

void valsout()
{
    std::cout << "ai = " << ai
              << "  tst_val = " << tst_val
              << "  new_val = " << new_val
              << "  exchanged = " << std::boolalpha << exchanged
              << '\n';
}

int main()
{
    ai = 3;
    valsout();

    // tst_val != ai   ==>  tst_val is modified
    exchanged = ai.compare_exchange_strong(tst_val, new_val);
    valsout();

    // tst_val == ai   ==>  ai is modified
    exchanged = ai.compare_exchange_strong(tst_val, new_val);
    valsout();
}

Output:

ai = 3  tst_val = 4  new_val = 5  exchanged = false
ai = 3  tst_val = 3  new_val = 5  exchanged = false
ai = 5  tst_val = 3  new_val = 5  exchanged = true

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🔥 See also 🔥

• atomic_compare_exchange_weak (C++11) — function template
• atomic_compare_exchange_weak_explicit (C++11) — function template
• atomic_compare_exchange_strong (C++11) — function template
• atomic_compare_exchange_strong_explicit (C++11) — function template

All of these 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). Obsessed. ✨

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🏷️ Category 🏷️

• * Todo with reason

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xoxo, your fave manpage bestie 💖 — source: http://cppreference.com, version/date: 2024.06.10, page: std::atomic...hange_strong(3).