stdatomic

来源： https://stackoverflow.com/questions/64113244/carries-dependency-what-it-means-and-how-to-implement

来源： https://stackoverflow.com/questions/64113244/carries-dependency-what-it-means-and-how-to-implement

来源： https://stackoverflow.com/questions/35534305/compare-exchange-strong-failing-despite-data-matching-expected-value

来源： https://stackoverflow.com/questions/35534305/compare-exchange-strong-failing-despite-data-matching-expected-value

问题 After seeing Herb Sutters excellent talk about "atomic weapons" I got a bit confused about the Relaxed Atomics examples. I took with me that an atomic in the C++ Memory Model (SC-DRF = Sequentially Consistent for Data Race Free) does an "acquire" on a load/read. I understand that for a load [and a store] the default is std::memory_order_seq_cst and therefore the two are the same: myatomic.load(); // (1) myatomic.load(std::memory_order_seq_cst); // (2) So far so good, no Relaxed Atomics

问题 If T is a C++ fundamental type, and if std::atomic<T>::is_lock_free() returns true , then is there anything in std::atomic<T> that is wait-free (not just lock-free)? Like, load , store , fetch_add , fetch_sub , compare_exchange_weak , and compare_exchange_strong . Can you also answer based on what is specified in the C++ Standard, and what is implemented in Clang and/or GCC (your version of choice). My favorite definitions for lock-free and wait-free are taken from C++ Concurrency in Action

问题 Say I have atomic<int> i; Thread A performs an atomic store/exchange with memory_order_release. Next, Thread B performs an atomic store with memory_order_release. Thread C performs an atomic fetch_add(0, memory_order_acquire); Does Thread C acquire dependencies from thread A and B or only thread B ? 回答1: Only B (I'm going to assume that by "next" you mean the modification order of the atomic is A -> B -> C so that by [atomics.order]p11 C 's RMW must read the value B wrote). See the note in

问题 Say I have atomic<int> i; Thread A performs an atomic store/exchange with memory_order_release. Next, Thread B performs an atomic store with memory_order_release. Thread C performs an atomic fetch_add(0, memory_order_acquire); Does Thread C acquire dependencies from thread A and B or only thread B ? 回答1: Only B (I'm going to assume that by "next" you mean the modification order of the atomic is A -> B -> C so that by [atomics.order]p11 C 's RMW must read the value B wrote). See the note in

问题 Whilst trying to work with std atomic pointer, I ran into the following. Say I do this: std::atomic<std::string*> myString; // <do fancy stuff with the string... also on other threads> //A can I do this? myString.load()->size() //B can I do this? char myFifthChar = *(myString.load()->c_str() + 5); //C can I do this? char myCharArray[255]; strcpy(myCharArray, myString.load()->c_str()); I'm pretty sure C is illegal because myString might be deleted in the meantime. However I'm unsure about A

问题 Whilst trying to work with std atomic pointer, I ran into the following. Say I do this: std::atomic<std::string*> myString; // <do fancy stuff with the string... also on other threads> //A can I do this? myString.load()->size() //B can I do this? char myFifthChar = *(myString.load()->c_str() + 5); //C can I do this? char myCharArray[255]; strcpy(myCharArray, myString.load()->c_str()); I'm pretty sure C is illegal because myString might be deleted in the meantime. However I'm unsure about A