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QUESTION
In C++20, we got the capability to sleep on atomic variables, waiting for their value to change.
We do so by using the std::atomic::wait method.
Unfortunately, while wait has been standardized, wait_for and wait_until are not. Meaning that we cannot sleep on an atomic variable with a timeout.
Sleeping on an atomic variable is anyway implemented behind the scenes with WaitOnAddress on Windows and the futex system call on Linux.
Working around the above problem (no way to sleep on an atomic variable with a timeout), I could pass the memory address of an std::atomic to WaitOnAddress on Windows and it will (kinda) work with no UB, as the function gets void* as a parameter, and it's valid to cast std::atomic to void*
On Linux, it is unclear whether it's ok to mix std::atomic with futex. futex gets either a uint32_t* or a int32_t* (depending which manual you read), and casting std::atomic to u/int* is UB. On the other hand, the manual says
The uaddr argument points to the futex word. On all platforms, futexes are four-byte integers that must be aligned on a four- byte boundary. The operation to perform on the futex is specified in the futex_op argument; val is a value whose meaning and purpose depends on futex_op.
Hinting that alignas(4) std::atomic should work, and it doesn't matter which integer type is it is as long as the type has the size of 4 bytes and the alignment of 4.
Also, I have seen many places where this trick of combining atomics and futexes is implemented, including boost and TBB.
So what is the best way to sleep on an atomic variable with a timeout in a non UB way? Do we have to implement our own atomic class with OS primitives to achieve it correctly?
(Solutions like mixing atomics and condition variables exist, but sub-optimal)
...ANSWER
Answered 2021-Jun-15 at 20:48You shouldn't necessarily have to implement a full custom atomic API, it should actually be safe to simply pull out a pointer to the underlying data from the atomic and pass it to the system.
Since std::atomic does not offer some equivalent of native_handle like other synchronization primitives offer, you're going to be stuck doing some implementation-specific hacks to try to get it to interface with the native API.
For the most part, it's reasonably safe to assume that first member of these types in implementations will be the same as the T type -- at least for integral values [1]. This is an assurance that will make it possible to extract out this value.
... and casting
std::atomictou/int*is UB
This isn't actually the case.
std::atomic is guaranteed by the standard to be Standard-Layout Type. One helpful but often esoteric properties of standard layout types is that it is safe to reinterpret_cast a T to a value or reference of the first sub-object (e.g. the first member of the std::atomic).
As long as we can guarantee that the std::atomic contains only the u/int as a member (or at least, as its first member), then it's completely safe to extract out the type in this manner:
QUESTION
This is the code I have written for the MPI's Group Communication Primitives-Brod cast example using c language try with Ubuntu system. I wrote a code for the string and variable concatenation here.
When I am compiling this code it shows error like that.(Please refer the image)
Can anyone help me to solve this?
...ANSWER
Answered 2021-Jun-15 at 12:43Change this line :
QUESTION
I'm trying to understand best practices for Golang concurrency. I read O'Reilly's book on Go's concurrency and then came back to the Golang Codewalks, specifically this example:
https://golang.org/doc/codewalk/sharemem/
This is the code I was hoping to review with you in order to learn a little bit more about Go. My first impression is that this code is breaking some best practices. This is of course my (very) unexperienced opinion and I wanted to discuss and gain some insight on the process. This isn't about who's right or wrong, please be nice, I just want to share my views and get some feedback on them. Maybe this discussion will help other people see why I'm wrong and teach them something.
I'm fully aware that the purpose of this code is to teach beginners, not to be perfect code.
Issue 1 - No Goroutine cleanup logic
...ANSWER
Answered 2021-Jun-15 at 02:48It is the
mainmethod, so there is no need to cleanup. Whenmainreturns, the program exits. If this wasn't themain, then you would be correct.There is no best practice that fits all use cases. The code you show here is a very common pattern. The function creates a goroutine, and returns a channel so that others can communicate with that goroutine. There is no rule that governs how channels must be created. There is no way to terminate that goroutine though. One use case this pattern fits well is reading a large resultset from a database. The channel allows streaming data as it is read from the database. In that case usually there are other means of terminating the goroutine though, like passing a context.
Again, there are no hard rules on how channels should be created/closed. A channel can be left open, and it will be garbage collected when it is no longer used. If the use case demands so, the channel can be left open indefinitely, and the scenario you worry about will never happen.
QUESTION
I know this question has been asked many times, but I still can't figure out what to do (more below).
I'm trying to spawn a new thread using std::thread::spawn and then run an async loop inside of it.
The async function I want to run:
...ANSWER
Answered 2021-Jun-14 at 17:28#[tokio::main] converts your function into the following:
QUESTION
I want to implement a datastructure in native memory using the Foreign Memory Access API of Project Panama.
In order to do that I need an underlying Object array (Object[]) for the entries.
In all the examples for the Foreign Memory Access API, MemorySegments are only used to store and retrieve primitives like so:
...ANSWER
Answered 2021-Jun-12 at 13:54Is there a way to store non primitives in a MemorySegment (e.g. Object)?
No, at least not directly. Objects are managed by the Java runtime, and they can not be safely stored in native memory (for instance because the garbage collector would not be able to trace object references inside objects in native memory).
However, as noted in the comments, for your purposes it might be enough to store the data inside an object in native memory. For instance, if an object contains only primitive fields (though, the same could be done recursively for object fields), it would be possible to write each such field separately to native memory. For example (with the JDK 16 API):
QUESTION
I am making a program which encrypts and decrypts texts. I am using Python 3.7 and cryptography.fernet library. I want to enter some information about my program's encryption standard to the GitHub page but I didn't understand which encryption does Fernet uses.
Here is my sample code which I am used in my project. I want to encrypt with 256-bit (AES-256) key but the key which this code generates is longer than 32 characters. It's 44 characters. But in official web site of cryptography library it says this code generates 128-bit key. What is the name of this 44 character (352-bit) key? Or is there any way for 256-bit symmetric encryption without PyCrypto?
...ANSWER
Answered 2021-May-03 at 13:26It is well written in the documentation;
Fernet is built on top of a number of standard cryptographic primitives. Specifically it uses:
- AES in CBC mode with a 128-bit key for encryption; using PKCS7 padding.
- HMAC using SHA256 for authentication.
- Initialization vectors are generated using os.urandom().
For complete details consult the specification.
Therefore you cannot use AES-256 with Fernet
- Cryptography.io library has other modes too, in the hazardous material layer including CBC, CTR, and GCM for AES-256, too.
PyCrypto can use a wide range of mode of operations for AES-256 including CBC, CTR, GCM, SIV, and OCB
Not clear how you get 44 bytes, here is the way to get the 32-bytes;
QUESTION
ANSWER
Answered 2021-May-28 at 09:46As @JérômeLaban answered in the comment:
you can find the solution here: github.com/unoplatform/uno/issues/3813
QUESTION
I trying to make a pure 3d cube, using my vectors. I thought it was perfect at once, but when rotate it, I can see some lines are not drawn correctly and it is not perfect.
I can't find why it is not perfect. Is some of my vectors wrong?
I'm using p5.js to draw it. I know they have methods of 3d rotation and some 3d primitives. But I don't want to use them. I want to draw my own 3d cube.
Here's the code I used as reference: https://github.com/OneLoneCoder/videos/blob/master/OneLoneCoder_olcEngine3D_Part1.cpp
...ANSWER
Answered 2021-Jun-09 at 15:54You need to close the outline around the triangles:
QUESTION
I have a relatively simple case where:
- My program will be receiving updates via Websockets, and will be using these updates to update it's local state. These updates will be very small (usually < 1-1000 bytes JSON so < 1ms to de-serialize) but will be very frequent (up to ~1000/s).
- At the same time, the program will be reading/evaluating from this local state and outputs its results.
- Both of these tasks should run in parallel and will run for the duration for the program, i.e. never stop.
- Local state size is relatively small, so memory usage isn't a big concern.
The tricky part is that updates need to happen "atomically", so that it does not read from a local state that has for example, written only half of an update. The state is not constrained to using primitives and could contain arbitrary classes AFAICT atm, so I cannot solve it by something simple like using Interlocked atomic operations. I plan on running each task on its own thread, so a total of two threads in this case.
To achieve this goal I thought to use a double buffer technique, where:
- It keeps two copies of the state so one can be read from while the other is being written to.
- The threads could communicate which copy they are using by using a lock. i.e. Writer thread locks copy when writing to it; reader thread requests access to lock after it's done with current copy; writer thread sees that reader thread is using it so it switches to other copy.
- Writing thread keeps track of state updates it's done on the current copy so when it switches to the other copy it can "catch up".
That's the general gist of the idea, but the actual implementation will be a bit different of course.
I've tried to lookup whether this is a common solution but couldn't really find much info, so it's got me wondering things like:
- Is it viable, or am I missing something?
- Is there a better approach?
- Is it a common solution? If so what's it commonly referred to as?
- (bonus) Is there a good resource I could read up on for topics related to this?
Pretty much I feel I've run into a dead-end where I cannot find (because I don't know what to search for) much more resources and info to see if this approach is "good". I plan on writing this in .NET C#, but I assume the techniques and solutions could translate to any language. All insights appreciated.
...ANSWER
Answered 2021-Jun-08 at 19:17If I understand correctly, the writes themselves are synchronous. If so, then maybe it's not necessary to keep two copies or even to use locks.
Maybe something like this could work?
QUESTION
I have a binary format that I'm writing encoders and decoders for. Almost all of the binary types directly map to primitives, except for two container types, a list and a map type that can contain any of the other types in the format including themselves.
These feel like they just want to be a typedef of std::variant
ANSWER
Answered 2021-Jun-07 at 19:18template
struct self_variant;
template
using self_variant_base =
std::variant<
std::vector...,
std::vector>
>;
template
struct self_variant:
self_variant_base
{
using self_variant_base::self_variant_base;
self_variant_base const& base() const { return *this; }
self_variant_base& base() { return *this; }
};
template
void print( T const& t ) {
std::cout << t << ",";
}
template
void print( std::vector const& v ) {
std::cout << "[";
for (auto const& e:v) {
print(e);
}
std::cout << "]\n";
}
template
void print( self_variant const& sv ) {
std::visit( [](auto& e){
print(e);
}, sv.base());
}
int main() {
self_variant bob = std::vector{1,2,3};
self_variant alice = std::vector>{ bob, bob, bob };
print(alice);
}
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