pthread_join() retriev`return value: !! best practice

Most pthreads programs don’t retrieve the return value via pthread_join(). has a comment by the author of boost::thread (reference implementation for c++11 thread library). He said

(3) I never use the return value of a thread in raw POSIX threads. However, I tend to use higher level facilities such as the Boost thread library, and more recently the C++0x thread library, which provide alternative means for transferring values between threads such as futures, which avoid the problems associated with memory management that you allude to.

Therefore, even though you felt it was unnatural to store the per-thread computed results in a global array, in practice it’s not bad. It is inherently thread-safe because there’s no data sharing, in a truly parallel mode.

That was my preferred solution, but to experiment, I also used new to return a value to pthread_join(). Personally, I am always wary of using new() in one function and the corresponding delete() in another function … unreliable. As much as possible, I use smart pointers to manage new/delete. shows both solutions


CVA c++ IV 2 #oq

void func(ResourceMgr & rm){ 
  int connId = rm.getConn();
  double d=externalFunc(connId); 
  rm.reclaim(connId); //release the resource to the mgr
  • Q5: In the code above, external func can throw, so write a RAII wrapper to prevent resource leak.
  • Q5b: what if you aren’t allowed to use RAII? Ok you said catch-all. Is an empty catch block enough?
    AA: need to re-throw the original exception, to mimic the RAII behavior.
  • Q(paper coding): Iterate over two vectors in lock steps. Which is faster — iterator vs vector index?
  • Q (bond math): Is there any uncertainty in the present value calc on an pre-existing vanilla IRS?
  • q: what design patterns do you use?
  • q: write a singleton skeleton
  • Q: how do we make a class “final” in pre-c++11
    %%A: either make dtor private or make all constructors private
  • Q: is shared_ptr thread safe?
  • Q: any difference — const shared_ptr<T> vs shared_ptr<const T>?
  • %%Q: does it make sense to pass a shared_ptr by const reference? I feel it’s cleaner to pass by raw ptr
    AA!: pass by const-reference is faster and recommended


  • Q: why use weak_ptr instead of raw ptr. Valid question.
    A: See [[std c++lib]] P84.
    A: See last paragraph in
  • Q: you said atomic<int> operator++ probably wraps a CAS in a while loop internally, so is it spinlock?
    %%A: I think so. is a detailed explanation
  • Q34: various synchronization techniques?
  • Q34b: what’s the c++ function to force a memory barrier?
    A: std::atomic_thread_fence(),
    but i feel if an application (not a library) uses this function then something is seriously wrong.
  • Q: can we implement a mutex using CAS?
    AA: blockingMutex implementation ] kernel
    AA: spinlock, not blockingMutex, can be implemented as in
  • ? Q (paper coding): write a piecewise linear interpolation Curve class with a ctor taking a vector<pair<double, double>>.  See
  • Q: What are r-values and r-value references
  • Q: Explain your use of SFINAE. See
  • Q: what’s the c++ library function for cmpxch?
    A: atomic::compare_exchange_weak()
  • Q: is your barcap vol surface used for EOD risk only?
    %%A: no. A trader might suspect a lesser known product is currently mis-priced. She could confirm the live bid/ask are obvious outliers on the fitted surface.
    %%A: a dealer desk price new deals based on the fitted surface, whether or not the strike/expiry requires interpolation.

–Simon Ma:

  • Q (paper coding): implement Fib() recursively and iteratively
  • Q: what’s inline?
  • Q: semaphore vs mutex
  • Q: how did you compute greeks like gamma?
  • Q (bond math): given 6M spot IR = 5% and 12M = 10%, what’s the 6M rate 6M forward?
  • Q: Assign first half of a vector to another vector
    %%A: vector::assign() probably takes two iterators so I can pass v.begin(), v.begin()+5
  • Q (obscure): What data structure to represent a directed graph of N nodes? See NxN matrix for graph of N nodes
    %%A: create a Node class with a single ptr field…
  • Q: parallel accumulating on a vector<int>
    %%A: only the last stage global aggregation has a shared mutable that needs protection. The sub-aggregation can be single-threaded.
  • Q (probability problem): Both Alan and Bob agree to show up at Cafe9 sometime between 12 and 2pm. Whoever arrives first would wait for 15 minutes only. What’s the probability of them actually meeting
  • Q: what’s thread_local?
    %%A (correct): a new storage class that’s similar to static
  • Q (paper coding): A natural number is Good if it is a product of only 3, 5 and 7. The ascending sequence of Good numbers looks like 1,3,5,7,9,15,21,25,27,35… Write a print(int N) to print the Nth Good number. Hint: write a isGood(int k). See
  • Q (unclear): implement subtract(int a, int b) using only operator+ and comparison operators. I feel this question is unclear. Can we use bitwise? Can we multiply?
    %%A: just try all the integers (i) one by one a == b+i
  • Q (obscure): what operators can’t be overloaded?
    %%A: q(?:) correct
    AA: address-of CAN be overloaded!

–Mikhail the mgr

  • Q: inserting 1000,000 items into a list vs a vector without reserve()
    A: vector wins
  • Q: do you define your exception classes?
  • Q: in what context is a deque completely unusable whereas vector is perfectly fine?
    A: a C function taking an array (i.e. a ptr + a count). Vector::data() is designed for this. In Pre-c++11, we can pass &v[0] and v.size()
  • Q89 (coding question): given a vector of T [passed in by const ref], write a template function to return [by const reference] the minimum element.
  • Q89b: how do you handle an empty vector?
  • ? Q89c (obscure): given q(vector const & vec), what can you say about q{auto it = vec.begin()} versus q{vector::const_iterator it=vec.begin()}

atomic{int} offers operator+=()

Background — on a machine with lock-free CPU…

My friend Shanyou asked me — with c++11 atomic data types, can we simply say

myAtomicInt ++; //without any mutex

A: Yes according to [[c++StdLib]]

Q: Is there some hidden CAS while-loop therein?
A: Yes I am 99% sure because other threads could be updating the same shared mutable object concurrently on other CPU cores.

Q: is there a CAS while-loop in a basic store/load operation?
A: I don’t think so

concurrent lazy singleton using static-local var has 12 upvotes and looks substantiated. It also considers double-checking, std::call_once, atomics, CAS…

GCC uses platform-specific tricks to ensure a static local variable is initialized only once, on first use. 

If it works, this is the easiest solution.

As explained in another blog post, static local is a shared mutable.