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• gcc and reentrant code

  - by S.M

  Does GCC generate reentrant code for all scenarios ?

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• What exactly is a reentrant function?

  - by eSKay

  Most of the times, the definition of reentrance is quoted from Wikipedia: A computer program or routine is described as reentrant if it can be safely called again before its previous invocation has been completed (i.e it can be safely executed concurrently). To be reentrant, a computer program or routine: Must hold no static (or global) non-constant data. Must not return the address to static (or global) non-constant data. Must work only on the data provided to it by the caller. Must not rely on locks to singleton resources. Must not modify its own code (unless executing in its own unique thread storage) Must not call non-reentrant computer programs or routines. How is safely defined? If a program can be safely executed concurrently, does it always mean that it is reentrant? What exactly is the common thread between the six points mentioned that I should keep in mind while checking my code for reentrant capabilities? Also, Are all recursive functions reentrant? Are all thread-safe functions reentrant? Are all recursive and thread-safe functions reentrant? While writing this question, one thing comes to mind: Are the terms like reentrance and thread safety absolute at all i.e. do they have fixed concrete definations? For, if they are not, this question is not very meaningful. Thanks!

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• Non-reentrant C# timer

  - by Oak

  I'm trying to invoke a method f() every t time, but if the previous invocation of f() has not finished yet, wait until it's finished. I've read a bit about the available timers (this is a useful link) but couldn't find any good way of doing what I want, save for manually writing it all. Any help about how to achieve this will be appreciated, though I fear I might not be able to find a simple solution using timers. To clarify, if x is one second, and f() runs the arbitrary durations I've written below, then: Step Operation Time taken 1 wait 1s 2 f() 0.6s 3 wait 0.4s (because f already took 0.6 seconds) 4 f() 10s 5 wait 0s (we're late) 6 f() 0.3s 7 wait 0.7s (we can disregard the debt from step 4) Notice that the nature of this timer is that f() will not need to be safe regarding re-entrance, and a thread pool of size 1 is enough here.

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• Mixing synchronized() with ReentrantLock.lock()

  - by yarvin

  In Java, do ReentrantLock.lock() and ReetrantLock.unlock() use the same locking mechanism as synchronized()? My guess is "No," but I'm hoping to be wrong. Example: Imagine that Thread 1 and Thread 2 both have access to: ReentrantLock lock = new ReentrantLock(); Thread 1 runs: synchronized (lock) { // blah } Thread 2 runs: lock.lock(); try { // blah } finally { lock.unlock(); } Assume Thread 1 reaches its part first, then Thread 2 before Thread 1 is finished: will Thread 2 wait for Thread 1 to leave the synchronized() block, or will it go ahead and run?

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• Writing re-entrant lexer with Flex

  - by Viet

  I'm newbie to flex. I'm trying to write a simple re-entrant lexer/scanner with flex. The lexer definition goes below. I get stuck with compilation errors as shown below (yyg issue): reentrant.l: /* Definitions */ digit [0-9] letter [a-zA-Z] alphanum [a-zA-Z0-9] identifier [a-zA-Z_][a-zA-Z0-9_]+ integer [0-9]+ natural [0-9]*[1-9][0-9]* decimal ([0-9]+\.|\.[0-9]+|[0-9]+\.[0-9]+) %{ #include <stdio.h> #define ECHO fwrite(yytext, yyleng, 1, yyout) int totalNums = 0; %} %option reentrant %option prefix="simpleit_" %% ^(.*)\r?\n printf("%d\t%s", yylineno++, yytext); %% /* Routines */ int yywrap(yyscan_t yyscanner) { return 1; } int main(int argc, char* argv[]) { yyscan_t yyscanner; if(argc < 2) { printf("Usage: %s fileName\n", argv[0]); return -1; } yyin = fopen(argv[1], "rb"); yylex(yyscanner); return 0; } Compilation errors: vietlq@mylappie:~/Desktop/parsers/reentrant$ gcc lex.simpleit_.c reentrant.l: In function ‘main’: reentrant.l:44: error: ‘yyg’ undeclared (first use in this function) reentrant.l:44: error: (Each undeclared identifier is reported only once reentrant.l:44: error: for each function it appears in.)

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• Java ReentrantReadWriteLocks - how to safely acquire write lock?

  - by Andrzej Doyle

  I am using in my code at the moment a ReentrantReadWriteLock to synchronize access over a tree-like structure. This structure is large, and read by many threads at once with occasional modifications to small parts of it - so it seems to fit the read-write idiom well. I understand that with this particular class, one cannot elevate a read lock to a write lock, so as per the Javadocs one must release the read lock before obtaining the write lock. I've used this pattern successfully in non-reentrant contexts before. What I'm finding however is that I cannot reliably acquire the write lock without blocking forever. Since the read lock is reentrant and I am actually using it as such, the simple code lock.getReadLock().unlock(); lock.getWriteLock().lock() can block if I have acquired the readlock reentrantly. Each call to unlock just reduces the hold count, and the lock is only actually released when the hold count hits zero. EDIT to clarify this, as I don't think I explained it too well initially - I am aware that there is no built-in lock escalation in this class, and that I have to simply release the read lock and obtain the write lock. My problem is/was that regardless of what other threads are doing, calling getReadLock().unlock() may not actually release this thread's hold on the lock if it acquired it reentrantly, in which case the call to getWriteLock().lock() will block forever as this thread still has a hold on the read lock and thus blocks itself. For example, this code snippet will never reach the println statement, even when run singlethreaded with no other threads accessing the lock: final ReadWriteLock lock = new ReentrantReadWriteLock(); lock.getReadLock().lock(); // In real code we would go call other methods that end up calling back and // thus locking again lock.getReadLock().lock(); // Now we do some stuff and realise we need to write so try to escalate the // lock as per the Javadocs and the above description lock.getReadLock().unlock(); // Does not actually release the lock lock.getWriteLock().lock(); // Blocks as some thread (this one!) holds read lock System.out.println("Will never get here"); So I ask, is there a nice idiom to handle this situation? Specifically, when a thread that holds a read lock (possibly reentrantly) discovers that it needs to do some writing, and thus wants to "suspend" its own read lock in order to pick up the write lock (blocking as required on other threads to release their holds on the read lock), and then "pick up" its hold on the read lock in the same state afterwards? Since this ReadWriteLock implementation was specifically designed to be reentrant, surely there is some sensible way to elevate a read lock to a write lock when the locks may be acquired reentrantly? This is the critical part that means the naive approach does not work.

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• Reentrancy and recursion

  - by Tony

  Would it be a true statement to say that every recursive function needs to be reentrant?

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• JUnit Testing in Multithread Application

  - by e2bady

  This is a problem me and my team faces in almost all of the projects. Testing certain parts of the application with JUnit is not easy and you need to start early and to stick to it, but that's not the question I'm asking. The actual problem is that with n-Threads, locking, possible exceptions within the threads and shared objects the task of testing is not as simple as testing the class, but testing them under endless possible situations within threading. To be more precise, let me tell you about the design of one of our applications: When a user makes a request several threads are started that each analyse a part of the data to complete the analysis, these threads run a certain time depending on the size of the chunk of data (which are endless and of uncertain quality) to analyse, or they may fail if the data was insufficient/lacking quality. After each completed its analysis they call upon a handler which decides after each thread terminates if the collected analysis-data is sufficient to deliver an answer to the request. All of these analysers share certain parts of the applications (some parts because the instances are very big and only a certain number can be loaded into memory and those instances are reusable, some parts because they have a standing connection, where connecting takes time, ex.gr. sql connections) so locking is very common (done with reentrant-locks). While the applications runs very efficient and fast, it's not very easy to test it under real-world conditions. What we do right now is test each class and it's predefined conditions, but there are no automated tests for interlocking and synchronization, which in my opionion is not very good for quality insurances. Given this example how would you handle testing the threading, interlocking and synchronization?

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• Learning OO for a C Programmer

  - by Holysmoke

  I've been programming professionally in C, and only C, for around 10 years in a variety of roles. As would be normal to expect, I understand the idioms of the language fairly well and beyond that also some of the design nuances - which APIs to make public, who calls what, who does what, what is supposed to reentrant and so on. I grew up reading 'Writing Solid Code', it's early C edition, not the one based on C++. However, I've never ever programmed in an OO language. Now, I want to migrate to writing applications for iPhone (maybe android), so want to learn to use Objective-C and use it with a degree of competence fitting a professional programmer. How do I wrap my head around the OO stuff? What would be your smallest reading list suggestion to me. Is there a book that carries some sort of relatively real world example OO design Objective-C? Besides, the reading what source code would you recommend me to go through. How to learn OO paradigm using Objective-C?

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• Async friendly DispatcherTimer wrapper/subclass

  - by Simon_Weaver

  I have a DispatcherTimer running in my code that fire every 30 seconds to update system status from the server. The timer fires in the client even if I'm debugging my server code so if I've been debugging for 5 minutes I may end up with a dozen timeouts in the client. Finally decided I needed to fix this so looking to make a more async / await friendly DispatcherTimer. Code running in DispatcherTimer must be configurable whether it is reentrant or not (i.e. if the task is already running it should not try to run it again) Should be task based (whether or not this requires I actually expose Task at the root is a gray area) Should be able to run async code and await on tasks to complete Whether it wraps or extends DispatcherTimer probably doesn't really matter but wrapping it may be slightly less ambiguous if you don't know how to use it Possibly expose bindable properties for IsRunning for UI

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• WCF Blocking problem with mutiple clients!!

  - by Marcel

  Hi I seem to have a blocking issue with WCF. Say I have two users and each have created their own instance of a class exposed on a WCF host using net.tcp with endpoint something like this "net.tcp://localhost:32000/SymHost/". The class is PerSession context and concurrency is reentrant. The class exposes two methods Alive() which return a bool of true straight away and an AliveWait which I inserted which does a Thread.Sleep for 4 seconds before returning true (testing purposes). Now client 1 calls AliveWait() during which time he is blocked which is fair enough but then if client 2 makes a call to Alive() on its own instance he has to wait until client 1's call is returned - this behaviour is not what I would have expected? I would have expected client 2 to carry on as if nothing has happened or is this to do with the fact that they both share the same endpoint? Can anyone explain what is going on and how I can make sure that client 2 can call its own instance uninterrupted? Any help much appreciated!

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• How can I change what happens when "enter" key is pressed on a DataGridView?

  - by SO give me back my rep

  when I am editing a cell and press enter the next row is automatically selected, I want to stay with the current row... I want to happen nothing except the EndEdit. I have this: private void dtgProductos_CellEndEdit(object sender, DataGridViewCellEventArgs e) { dtgProductos[e.ColumnIndex, e.RowIndex].Selected = true; //this line is not working var index = dtgProductos.SelectedRows[0].Cells.IndexOf(dtgProductos.SelectedRows[0].Cells[e.ColumnIndex]); switch (index) { case 2: { dtgProductos.SelectedRows[0].Cells[4].Selected = true; dtgProductos.BeginEdit(true); } break; case 4: { dtgProductos.SelectedRows[0].Cells[5].Selected = true; dtgProductos.BeginEdit(true); } break; case 5: { btnAddProduct.Focus(); } break; default: break; } } so when I edit a row that is not the last one I get this error: Operation is not valid because it results in a reentrant call to the SetCurrentCellAddressCore function.

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• WCF: How to find out when a session is ending?

  - by TomTom

  I have a WCF application that is using sessions. Is there any central event to get thrown when a session ends? How can I find out when a session is ending WITHOUT (!) calling a method (network disconnect, client crashing - so no "logout" method call)? The server is hosted as: [ServiceBehavior( InstanceContextMode = InstanceContextMode.PerSession, ConcurrencyMode = ConcurrencyMode.Reentrant, UseSynchronizationContext = false, IncludeExceptionDetailInFaults = true )] Basically because it is using a callback interface. Now, I basically need to decoubple the instance created from the backend store when the session terminates ;) Any ideas?

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• Remote Socket Read In Multi-Threaded Application Returns Zero Bytes or EINTR (104)

  - by user39891

  Hi. Am a c-coder for a while now - neither a newbie nor an expert. Now, I have a certain daemoned application in C on a PPC Linux. I use PHP's socket_connect as a client to connect to this service locally. The server uses epoll for multiplexing connections via a Unix socket. A user submitted string is parsed for certain characters/words using strstr() and if found, spawns 4 joinable threads to different websites simultaneously. I use socket, connect, write and read, to interact with the said webservers via TCP on their port 80 in each thread. All connections and writes seems successful. Reads to the webserver sockets fail however, with either (A) all 3 threads seem to hang, and only one thread returns -1 and errno is set to 104. The responding thread takes like 10 minutes - an eternity long:-(. *I read somewhere that the 104 (is EINTR?), which in the network context suggests that ...'the connection was reset by peer'; or (B) 0 bytes from 3 threads, and only 1 of the 4 threads actually returns some data. Isn't the socket read/write thread-safe? I use thread-safe (and reentrant) libc functions such as strtok_r, gethostbyname_r, etc. *I doubt that the said webhosts are actually resetting the connection, because when I run a single-threaded standalone (everything else equal) all things works perfectly right, but of course in series not parallel. There's a second problem too (oops), I can't write back to the client who connect to my epoll-ed Unix socket. My daemon application will hang and hog CPU 100% for ever. Yet nothing is written to the clients end. Am sure the client (a very typical PHP socket application) hasn't closed the connection whenever this is happening - no error(s) detected either. Any ideas? I cannot figure-out whatever is wrong even with Valgrind, GDB or much logging. Kindly help where you can.

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• WCF deadlock when using callback channel

  - by mafutrct

  This is probably a simple mistake, but I could not figure out what was wrong. I basically got a method like this: [ServiceBehavior ( ConcurrencyMode = ConcurrencyMode.Reentrant, InstanceContextMode = InstanceContextMode.PerSession, IncludeExceptionDetailInFaults = true) ] public class Impl : SomeContract { public string Foo() { _CallbackChannel.Blah(); return ""; } } Its interface is decorated: [ServiceContract ( Namespace = "http://MyServiceInterface", SessionMode = SessionMode.Required, CallbackContract = typeof (WcfCallbackContract)) ] public interface SomeContract { [OperationContract] string Foo (); } The service is hosted like this: ServiceHost host = new ServiceHost (typeof (Impl)); var binding = new NetTcpBinding (); var address = new Uri ("net.tcp://localhost:8000/"); host.AddServiceEndpoint ( typeof (SomeContract), binding, address); host.Open (); The client implements the callback interface and calls Foo. Foo runs, calls the callback method and returns. However, the client is still struck in the call to Foo and never returns. The client callback method is never run. I guess I made a design mistake somewhere. If needed, I can post more code. Any help is appreciated.

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• gcc: Do I need -D_REENTRANT with pthreads?

  - by stefanB

  On Linux (kernel 2.6.5) our build system calls gcc with -D_REENTRANT. Is this still required when using pthreads? How is it related to gcc -pthread option? I understand that I should use -pthread with pthreads, do I still need -D_REENTRANT? On a side note, is there any difference that you know off between the usage of REENTRANT between gcc 3.3.3 and gcc 4.x.x ? When I use -pthread gcc option I can see that _REENTRANT gets defined. Will omitting -D_REENTRANT from command line make any difference, for example could some objects be compiled without multithreaded support and then linked into binary that uses pthreads and will cause problems? I assume it should be ok just to use: g++ -pthread > echo | g++ -E -dM -c - > singlethreaded > echo | g++ -pthread -E -dM -c - > multithreaded > diff singlethreaded multithreaded 39a40 > #define _REENTRANT 1 We're compiling multiple static libraries and applications that link with the static libraries, both libraries and application use pthreads. I believe it was required at some stage in the past but want to know if it is still required. Googling hasn't returned any recent information mentioning -D_REENTRANT with pthreads. Could you point me to links or references discussing the use in recent version of kernel/gcc/pthread? Clarification: At the moment we're using -D_REENTRANT and -lpthread, I assume I can replace them with just g++ -pthread, looking at man gcc it sets the flags for both preprocessor and linker. Any thoughts?

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• Socket Read In Multi-Threaded Application Returns Zero Bytes or EINTR (104)

  - by user309670

  Hi. Am a c-coder for a while now - neither a newbie nor an expert. Now, I have a certain daemoned application in C on a PPC Linux. I use PHP's socket_connect as a client to connect to this service locally. The server uses epoll for multiplexing connections via a Unix socket. A user submitted string is parsed for certain characters/words using strstr() and if found, spawns 4 joinable threads to different websites simultaneously. I use socket, connect, write and read, to interact with the said webservers via TCP on their port 80 in each thread. All connections and writes seems successful. Reads to the webserver sockets fail however, with either (A) all 3 threads seem to hang, and only one thread returns -1 and errno is set to 104. The responding thread takes like 10 minutes - an eternity long:-(. *I read somewhere that the 104 (is EINTR?), which in the network context suggests that ...'the connection was reset by peer'; or (B) 0 bytes from 3 threads, and only 1 of the 4 threads actually returns some data. Isn't the socket read/write thread-safe? I use thread-safe (and reentrant) libc functions such as strtok_r, gethostbyname_r, etc. *I doubt that the said webhosts are actually resetting the connection, because when I run a single-threaded standalone (everything else equal) all things works perfectly right, but of course in series not parallel. There's a second problem too (oops), I can't write back to the client who connect to my epoll-ed Unix socket. My daemon application will hang and hog CPU 100% for ever. Yet nothing is written to the clients end. Am sure the client (a very typical PHP socket application) hasn't closed the connection whenever this is happening - no error(s) detected either. Any ideas? I cannot figure-out whatever is wrong even with Valgrind, GDB or much logging. Kindly help where you can.

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• Is it safe to reuse javax.xml.ws.Service objects

  - by Noel Ang

  I have JAX-WS style web service client that was auto-generated with the NetBeans IDE. The generated proxy factory (extends javax.xml.ws.Service) delegates proxy creation to the various Service.getPort methods. The application that I am maintaining instantiates the factory and obtains a proxy each time it calls the targetted service. Creating the new proxy factory instances repeatedly has been shown to be expensive, given that the WSDL documentation supplied to the factory constructor, an HTTP URI, is re-retrieved for each instantiation. We had success in improving the performance by caching the WSDL. But this has ugly maintenance and packaging implications for us. I would like to explore the suitability of caching the proxy factory itself. Is it safe, e.g., can two different client classes, executing on the same JVM and targetting the same web service, safely use the same factory to obtain distinct proxy objects (or a shared, reentrant one)? I've been unable to find guidance from either the JAX-WS specification nor the javax.xml.ws API documentation. The factory-proxy multiplicity is unclear to me. Having Service.getPort rather than Service.createPort does not inspire confidence.

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• How to restart a wcf server from within a client?

  - by djerry

  Hey guys, I'm using Wcf for server - client communication. The server will need to run as a service, so there's no GUI. The admin can change settings using the client program and for those changes to be made on server, it needs to restart. This is my server setup NetTcpBinding binding = new NetTcpBinding(SecurityMode.Message); Uri address = new Uri("net.tcp://localhost:8000"); //_svc = new ServiceHost(typeof(MonitoringSystemService), address); _monSysService = new MonitoringSystemService(); _svc = new ServiceHost(_monSysService, address); publishMetaData(_svc, "http://localhost:8001"); _svc.AddServiceEndpoint(typeof(IMonitoringSystemService), binding, "Monitoring Server"); _svc.Open(); MonitoringSystemService is a class i'm using to handle client - server comm. It looks like this: [CallbackBehavior(ConcurrencyMode = ConcurrencyMode.Reentrant)] [ServiceBehavior(InstanceContextMode = InstanceContextMode.Single, MaxItemsInObjectGraph = 2147483647)] public class MonitoringSystemService : IMonitoringSystemService {} So i need to call a restart method on the client to the server, but i don't know how to restart (even stop - start) the server. I hope i'm not missing any vital information. Thanks in advance.

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• Socket Read In Multi-Threaded Application Returns Zero Bytes or EINTR (-1)

  - by user309670

  Hi. Am a c-coder for a while now - neither a newbie nor an expert. Now, I have a certain daemoned application in C on a PPC Linux. I use PHP's socket_connect as a client to connect to this service locally. The server uses epoll for concurrent connections via a Unix socket. A user submitted string is parsed for certain characters/words using strstr() and if found, spawns 4 joinable threads to different websites simultaneously. I use socket, connect, write and read, to interact with the said webservers via TCP on port 80 in each thread. All connections and writes seems successful. Reads to the webserver sockets fail however, with either (A) all 3 threads seem to hang, and only one thread returns -1 and errno is set to 104. The responding thread takes like 10 minutes - an eternity long:-(. *I read somewhere that the 104 (is EINTR) suggests that ...'the connection was reset by peer', or (B) 0 bytes from 3 threads, and only 1 of the 4 threads actually returns some data. Isn't the socket read/write thread-safe? Otherwise, use thread-safe (and reentrant) libc functions such as strtok_r, gethostbyname_r, etc. *I doubt that the said webhosts are actually resetting the connection, because when I run a single-threaded standalone (everything else equal) all things works perfectly right. There's a second problem too (oops), I can't write back to the client who connect to my epoll-ed Unix socket. My daemon application will hang and hog CPU 100% for ever. Yet nothing is written to the clients end. Am sure the client (a very typical PHP socket application) hasn't closed the connection whenever this is happening - no error(s) detected either. I cannot figure-out whatever is wrong even with Valgrind or GDB

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• Does oneway declaration in Android .aidl guarantee that method will be called in a separate thread?

  - by Dan Menes

  I am designing a framework for a client/server application for Android phones. I am fairly new to both Java and Android (but not new to programming in general, or threaded programming in particular). Sometimes my server and client will be in the same process, and sometimes they will be in different processes, depending on the exact use case. The client and server interfaces look something like the following: IServer.aidl: package com.my.application; interface IServer { /** * Register client callback object */ void registerCallback( in IClient callbackObject ); /** * Do something and report back */ void doSomething( in String what ); . . . } IClient.aidl: package com.my.application; oneway interface IClient { /** * Receive an answer */ void reportBack( in String answer ); . . . } Now here is where it gets interesting. I can foresee use cases where the client calls IServer.doSomething(), which in turn calls IClient.reportBack(), and on the basis of what is reported back, IClient.reportBack() needs to issue another call to IClient.doSomething(). The issue here is that IServer.doSomething() will not, in general, be reentrant. That's OK, as long as IClient.reportBack() is always invoked in a new thread. In that case, I can make sure that the implementation of IServer.doSomething() is always synchronized appropriately so that the call from the new thread blocks until the first call returns. If everything works the way I think it does, then by declaring the IClient interface as oneway, I guarantee this to be the case. At least, I can't think of any way that the call from IServer.doSomething() to IClient.reportBack() can return immediately (what oneway is supposed to ensure), yet IClient.reportBack still be able to reinvoke IServer.doSomething recursively in the same thread. Either a new thread in IServer must be started, or else the old IServer thread can be re-used for the inner call to IServer.doSomething(), but only after the outer call to IServer.doSomething() has returned. So my question is, does everything work the way I think it does? The Android documentation hardly mentions oneway interfaces.

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