check a receiver socket is connected: tcp^udp

Q: given a TCP receiver socket, how do you tell if it’s connected to a session or disconnected?

Shanyou said when you recv() on the socket but got 0 it means disconnected.

http://man7.org/linux/man-pages/man2/recv.2.html#RETURN_VALUE shows recv() return value of 0 indicates dead connection i.e. disconnected.

https://stackoverflow.com/questions/4142012/how-to-find-the-socket-connection-state-in-c uses getsockopt()

Q: given a UDP multicast receiver socket, how do you tell if it’s still has a live subscription to the multicast group?

%%A: I guess you can use getsockopt() to check socket /aliveness/. If alive but no data, then the group is quiet

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non-blocking socket readiness: alternatives to periodic polling

Some interviewer once asked me —

Q: After your non-blocking send() fails due to a full buffer, what can you do to get your data sent ASAP?

Simple solution is retrying after 0 or more millisecond. Zero would be CPU spinning. Non-zero means unwanted latency.

A 1st alternative is poll()/select() with a timeout, and immediately retry the same. There’s basically no latency. No spinning either. The linux proprietary epoll() is more efficient than poll()/select() and a popular solution for asynchronous IO

2nd alternative is SIGIO. http://compgeom.com/~piyush/teach/4531_06/project/hell.html says it doesn’t waste CPU. P52 [[tcp/ip sockets in C]] also picked this solution to go with non-blocking sockets.

 

## Y avoid blocking design

There are many contexts. I only know a few.

1st, let’s look at an socket context. Suppose there are many (like 500 or 50) sockets to process. We don’t want 50 threads. We prefer fewer, perhaps 1 thread to check each “ready” socket, transfer whatever data can be transferred then go back to waiting. In this context, we need either

  • /readiness notification/, or
  • polling
  • … Both are compared on P51 [[TCP/IP sockets in C]]

2nd scenario — GUI. Blocking a UI-related thread (like the EDT) would freeze the screen.

3rd, let’s look at some DB request client. The request thread sends a request and it would take a long time to get a response. Blocking the request thread would waste some memory resource but not really CPU resource. It’s often better to deploy this thread to other tasks, if any.

Q: So what other tasks?
A: ANY task, in the thread pool design. The requester thread completes the sending task, and returns to the thread pool. It can pick up unrelated tasks. When the DB server responds, any thread in the pool can pick it up.

This can be seen as a “server bound” system, rather than IO bound or CPU bound. Both the CPU task queue and the IO task queue gets drained quickly.

 

Wells IV #socket/c++/threading

This is one of the longest tech interviews and one of the most enriching 🙂 even though I don’t “like” all their questions — very academic/theoretical, text-book driven, too low-level to be practical, not testing practical zbs.

Q: Any consistently reliable technique to detect stale order in your book?

Q: what’s your frame size if your tcp/ucp buffers sizes are so large?

Q: experience storing ticks?

—— C++ questions:
Q1: how can you avoid the cost of virtual function?
%%A: enum/switch or CRTP

Q1b: what’s CRTP

Q: what if your copy ctor signature is missing the “const”?
%%A: you can’t pass in temporaries or literal values. Such arguments must pass into “const &” parameter or “const” parameter. Correct.

Q: double delete?

Q: policy class? traits class?

Q: STL binders.. use case?

Q: how many types of smart pointers do you use?

Q: difference between java generics vs c++ templates?
%%A: type erasure. Java Compiler enforces many rules, but bytecode saves no info about the type argument, so we can get strange runtime errors.
%%A: template specialization. Template meta-programming
%%A (accepted): code bloat since each template instantiation is a separate chunk of code in the object file and in memory.
A: I have a dedicated blog post on this.

Q: what’s returned by a std::queue’s dequeue operation when it’s empty?
AA: undefined behavior, so we must check empty() before attempting dequeue. I believe ditto for a std::stack

Q: why template specialization?
%%A: customize behavior for this particular vector since the standard implementation is not suitable.

Q: how do you implement a thread-safe c++singleton
A: not obvious. See concurrent lazy singleton using static-local var

Q12: in a simple function I have
vector v1 = {“a”, “b”}; vector v2 = v1; cout<<….
What happens to the ctor, dtor etc?
A: 2 std::strings constructed on heap, vector constructed on stack; 2nd vector copy-constructed on stack; 2 new strings constructed on heap; vector destructors deletes all four strings
A: the actual char array is allocated only once for each actual value, due to reference counting in the current std::string implementation.

Q12b: you mean it’s interned?

Coding question: implement void remove_spaces(char * s) //modify the s char array in place. See %%efficient removeSpaces(char*) #Wells

—— threading, mostly in java
Q: What are the problems of CAS solutions?
A: too many retries. CAS is optimistic, but if there are too many concurrent writes, then the assumption is invalid.
%%A: missed update? Not a common issue so far.

%%Q: Synchronized keyword usage inside a static method?
AA: you need be explicit about the target object, like synchronized(MyClass.class)

Q21: Name 3 effects of java volatile keyword — Advanced. See 3effects@volatile ] java5.

Q21b: analyze the double-checking singleton implementation.
staticInst = new Student(); // inside synchronized block, this can assign an incomplete object’s address to staticInst variable, which will be visible to an unsynchronized reader. Solution – declare staticInst as volatile static field

—— System programming (ANSI-C) questions
Q: have you used kernel bypass?

Q5: how do you construct a Student object in a memory-mapped-file?
%%A: placement new

Q5b: what if we close the file and map it again in the same process?
%%A: the ptr I got from malloc is now a dangling ptr. Content will be intact, as in Quest

Q5c: what if Student has virtual functions and has a vptr.
%%A: I see no problem.

Q: you mentioned non-blocking TCP send(), so when your send fails, what can you do?
%%A: retry after completing some other tasks.

Q4: why is UDP faster than TCP
%%A: no buffering; smaller envelopes; no initial handshake; no ack;
%%A: perhaps most important for market data — multiple recipient with TCP is very heavy on sender and on network

Q4b: why buffering in tcp?
%%A: resend

Q: can tcp client use bind()?
%%A: bind to a specific client port, rather than a random port

Q6: is there socket buffer overflow in TCP?
A: probably not

Q6b: what if I try to send a big file by TCP, and the receiver’s buffer is full? Will sender care about it? Is that reliable transmission?
A: Aquis sends 100MB file. See no overflow]TCP slow receiver #non-blocking sender

Q: in your ticking risk system, how does the system get notified with new market data?
%%A: we use polling instead of notification. The use case doesn’t require notification.

Q: any network engineering techniques?

Q: what kernel parameters did you tune?

 

—— Stupid best-practice questions:
Q: what’s the benefit of IOC?

Q: Fitnesse, mock objects

Q6: motivation of factory pattern?
Q6b: why prevent others calling your new()?
%%A: there are too many input parameters to my ctor and I want to provide users a simplified façade
%%A: some objects are expensive to construct (DbConnection) and I need tight control.
%%A: similarly, after construction, I have some initialization in my factory, but I may not be allowed to modify the ctor, or our design doesn’t favor doing such things in the ctor. I make my factory users’ life easier if they don’t call new() directly.
%%A: I want to centralize the logic of how to decide what to return. Code reuse rather than duplication
%%A: I can have some control over the construction. I could implement some loose singleton

select^poll # phrasebook

Based on https://www.ulduzsoft.com/2014/01/select-poll-epoll-practical-difference-for-system-architects/, which I respect.

* descriptor count — up to 200 is fine with select(); 1000 is fine with poll(); Above 1000 consider epoll

* single-threaded app — poll is just as fast as epoll. epoll() excels in MT.

* time-out precision — poll/epoll has millisec precision. select() has nanosec, but only embedded devices need such precision.

* linux-only — epoll

sharing port or socket #index page

Opening example – we all know that once a local endpoint is occupied by a tcp server process, another process can’t bind to it.

However, various scenarios exist to allow some form of sharing.

https://bintanvictor.wordpress.com/2017/04/29/socket-shared-between-2-processes/

https://bintanvictor.wordpress.com/2017/04/29/so_reuseport-socket-option/

https://bintanvictor.wordpress.com/2017/04/29/2-tcp-clients-connected-to-a-single-server-socket/

https://bintanvictor.wordpress.com/2017/03/29/multiple-sockets-can-attach-to-the-same-addressport/

## IV favorites ] sockets

There are dozens of sub-topics but in my small sample of interviews, the following sub-topics have received disproportionate attention:

  1. blocking vs non-blocking
  2. select (or epoll) on multiple sockets
  3. multicast
  4. add basic reliability over UDP (many blog posts); how is TCP transmission control implemented
  5. throughput tuning
  6. accept() + threading

Background — The QQ/ZZ framework was first introduced in this post on c++ learning topics

Only c/c++ positions need socket knowledge. However, my perl/py/java experience with socket API is still relevant.

Socket is a low-level subject. Socket tough topics feel not as complex as concurrency, algorithms, probabilities, OO design, MOM … Interview is mostly knowledge test; but to do well in real projects, you probably need experience.

Coding practice? no need. Just read and blog.

Socket knowledge is seldom the #1 selection criteria for a given position, but could be #3. (In contrast, concurrency or algorithm skill could be #1.)

  • [ZZ] tweaking
  • [ZZ] exception handling in practice
  • —-Above topics are still worth studying to some extent—–
  • [QQ] tuning
  • [QQ] buffer management

 

socket accept() key points often missed

I have studied accept() many times but still unfamiliar.

Useful as zbs, and perhaps QQ, rarely for GTD…

Based on P95-97 [[tcp/ip socket in C]]

  • (probably) used in tcp only
  • (probably) used on server side only
  • usually called inside an endless loop
  • blocks most of the time, when there’s no incoming new connections. The existing clients don’t bother us as they communicate with the “child” sockets independently. The accept() “show” starts only upon a new incoming connection
    • thread remains blocked, starting from receiving the incoming until a newborn socket is fully Established.
    • at that juncture the new remote client is probably connected to the newborn socket, so the “parent thread[2]” have the opportunity/license to let-go and return from accept()
    • now, parent thread has the newborn socket, it needs to pass it to a child thread/process
    • after that, parent thread can go back into another blocking accept()
  • new born or other child sockets all share the same local port, not some random high port! Until now I still find this unbelievable. https://stackoverflow.com/questions/489036/how-does-the-socket-api-accept-function-work confirms it.
  • On a host with a single IP, 2 sister sockets would share the same local ip too, but luckily each socket structure has at least 4 [1] identifier keys — local ip:port / remote ip:port. So our 2 sister sockets are never identical twins.
  • [1] I omitted a 5th key — protocol as it’s a distraction from the key point.
  • [2] 2 variations — parent Thread or parent Process.

in-depth article: epoll illustrated #SELECT

(source code is available for download in the article)

Compared to select(), the newer linux system call epoll() is designed to be more performant.

Ticker Plant uses epoll. No select() at all.

https://banu.com/blog/2/how-to-use-epoll-a-complete-example-in-c/ is a nice article with sample code of a TCP server.

  • bind(), listen(), accept()
  • main() function with an event loop. In the loop
  • epoll_wait() to detect
    • new client
    • new data on existing clients
    • (Using the timeout parameter, it could also react to a timer events.)

I think this toy program is more readable than a real-world epoll server with thousands of lines.

##which common UDP/TCP functions are blocking

A non-blocking send fails when it can’t send a single byte, usually because destination send buffer (for TCP) is full. For UDP, see [4]

Note read() and write() has similar behavior. See send()recv() ^ write()read() @sockets and http://www.mathcs.emory.edu/~cheung/Courses/455/Syllabus/7-transport/flow-control.html

[1] meaning of non-blocking connect() on TCP is special. See P51[[tcp/ip sokets in C]] and https://www.scottklement.com/rpg/socktut/nonblocking.html
[2] non-blocking accept() is obscure knowledge — See https://www.scottklement.com/rpg/socktut/nonblocking.html
[3] select() on a non-blocking socket is obscure knowledge —  See https://www.scottklement.com/rpg/socktut/nonblocking.html
[4] UDP has no send buffer but some factors can still cause blocking

default flags arg to func fcntl on entire socket touching TCP buffers?
select blocking not supported? still blocking! [3]  no
epoll blocking not supported?  no
recv blocking can change to NB can change to NB  yes
send/write TCP blocking frequently can change to NB can change to NB  yes
recvfrom blocking can change to NB can change to NB  yes
sendto UDP blocking sometimes [4] can change to NB can change to NB  yes
accept blocking not supported? can change to NB [2]  yes
connect()TCP blocking not supported? can change to NB [1]  no
connect()UDP NB. Saves the destination
for future transfers
Not Applicable