What is the differance between boost::details::pool::pthread_mutex and boost::details::pool::null_mutex.
I see that in latest boost version - 1.42, the class boost::details::pool::pthread_mutex was deleted. What should I use instead?
boost::details::pool::null_mutex is a mutex that does nothing (a lock always succeeds immediately). It's appropriate when you're not using threads. The Boost pool library selects what kind of mutex it will use to synchronize access to critical sections with a typedef for the mutex type based on the following snippet from boost\pool\detail\mutex.hpp:
#if !defined(BOOST_HAS_THREADS) || defined(BOOST_NO_MT) || defined(BOOST_POOL_NO_MT)
typedef null_mutex default_mutex;
#else
typedef boost::mutex default_mutex;
#endif
In other words, if the configuration says that no threading is involved (either for Boost as a whole, or for the pool library in particular), then the null_mutex will be used (which is basically a nop).
If threading is to be supported, then the boost::mutex type will be used, which comes from the Boost thread library (and will be a pthread-based mutex if your system uses pthreads).
Related
C++/CLI is known to block the mutex header when a project is compiled using the -clr:pure or clr flag.
The error is reported here
https://social.msdn.microsoft.com/Forums/vstudio/en-US/d4d082ff-ce43-478d-8386-0effed04b108/ccli-and-stdmutex?forum=vclanguage
The recommended solution seems to be to use the pimpI pattern. See here
Turn off clr option for header file with std::mutex
The problem I see is when using other std functions.
For example consider the std::condition_variable
mutexPimpI _mut;
std::unique_lock<mutexPimpI> lk(_mut); //Fine std::unique_lock is templated.
std::condition_variable _gate1;
_gate1.wait(lk); //Error wait expects std::unique_lock<std::mutex> as argument
Is there any easy way to resolve / work around this problem?
you can try using recursive__mutex class since objects wont be locked.
https://msdn.microsoft.com/en-us/library/hh921466.aspx refer this as well.
I solved it by forward declare std::condition_variable.
The problem with Visual Studio's compiler for mutex includes is only for headers.
Including in the source files still worked.
I'm very new to boost, and just trying to understand a small part of it - the interprocess mutex.
AFAIU, there's a robust mutex emulation in the cross platform implementation using file lock: http://www.boost.org/doc/libs/1_62_0/boost/interprocess/detail/robust_emulation.hpp
This is for emulating the robust mutex that's available in posix standard, but the generic approach will only be used when platform specific versions are not available.
#if !defined(BOOST_INTERPROCESS_FORCE_GENERIC_EMULATION) && defined (BOOST_INTERPROCESS_POSIX_PROCESS_SHARED)
#include <boost/interprocess/sync/posix/mutex.hpp>
#define BOOST_INTERPROCESS_USE_POSIX
//Experimental...
#elif !defined(BOOST_INTERPROCESS_FORCE_GENERIC_EMULATION) && defined (BOOST_INTERPROCESS_WINDOWS)
#include <boost/interprocess/sync/windows/mutex.hpp>
#define BOOST_INTERPROCESS_USE_WINDOWS
#elif !defined(BOOST_INTERPROCESS_DOXYGEN_INVOKED)
#include <boost/interprocess/sync/spin/mutex.hpp>
#define BOOST_INTERPROCESS_USE_GENERIC_EMULATION
But on posix system that actually supports robust mutexes (I guess it depends on the kernel version), robust mutex is actually not enabled. http://www.boost.org/doc/libs/1_62_0/boost/interprocess/sync/posix/pthread_helpers.hpp
So I don't really understand the logic here, is there any serious performance penalty with posix robust mutex implementation so that we don't want to use it? Even so, there should be at least an option to optionally enable this feature.
And to solve this problem, as I'm noob to boost, is there any way to make the generic implementation available over the posix implementation so that I can make use of the robustness?
I just looked into boost implementation for wrapper class for intercrosses mutex in file boost/interprocess/sync/posix/pthread_helpers.hpp.
I saw PTHREAD_MUTEX_ROBUST is missing.
Please add ROBUST attributes as below and try.
mutexattr_wrapper(bool recursive = false)
{
if(pthread_mutexattr_init(&m_attr)!=0 ||
pthread_mutexattr_setpshared(&m_attr, PTHREAD_PROCESS_SHARED)!= 0 ||
(recursive &&
pthread_mutexattr_settype(&m_attr, PTHREAD_MUTEX_RECURSIVE)!= 0 ) ||
pthread_mutexattr_setrobust(&m_attr, PTHREAD_MUTEX_ROBUST)!= 0)
throw interprocess_exception("pthread_mutexattr_xxxx failed");
}
I have a property which is similar to the following:
private:
Foo* myFoo_m;
public:
Foo getMyFoo() const
{
if (myFoo_m == NULL)
{
myFoo_m = new Foo();
// perform initialization
This works well in a single-threaded environment, but how do I handle this in a multi-threaded environment? Most of the info I've found deals with static singletons, but in this case, myFoo is a public instance property.
I am porting this over from C# (where I can use Lazy) and Java (where I can use double check locking), but it doesn't seem that there is a straightforward way to do this in C++. I cannot rely on any external libraries (no BOOST), and this needs to work on windows and linux. I also cannot use C++11.
Any insight would be good. I am new to C++.
If you have access to c++11 you can use std::mutex to lock prevent multiple threads from initializing the lazy section. (Note: std::mutex only became available on Windows with VS2012)
You can even perform a scoped aquisition of the mutex with std::lock_guard:
private:
std::mutex m_init_mutex;
public:
Foo getMyFoo() const
{
{
std::lock_guard<std::mutex> lock(m_init_mutex);
if (myFoo_m == NULL)
{
myFoo_m = new Foo();
// perform initialization
}
}
EDIT: The OPs now stated that C++11 isn't an option, but perhaps this answer will be useful in the future
By saying "no C++11", "no Boost or other third-party code", "must work on Windows and Linux", you have restricted yourself to using implementation-specific locking mechanisms.
I think your best option is to define a simple lock class for yourself, and implement it to use pthread_mutex on Linux and a CriticalSection on Windows. Possibly you already have some platform-specific code, to start the threads in the first place.
You could try something like Windows Services for UNIX to avoid writing platform-specific code, but it's probably not worth it for one lock. And although it's supplied by Microsoft, you'd probably consider it an external library anyway.
Warning: I didn't see the "no C++11" requirement, so please disregard the answer.
Since C++11 mandates that static variable initialization be thread-safe, here's a simple way that you might consider "cheating":
Foo init_foo()
{
// initialize and return a Foo
}
Foo & get_instance_lazily()
{
static Foo impl = init_foo();
return impl;
}
The instance will be initialized the first time that you call get_instance_lazily(), and thread-safely so.
I was wondering how to execute two processes in a dual-core processor in c++.
I know threads (or multi-threading) is not a built-in feature of c++.
There is threading support in Qt, but I did not understand anything from their reference. :(
So, does anyone know a simple way for a beginner to do it. Cross-platform support (like Qt) would be very helpful since I am on Linux.
Try the Multithreading in C++0x part 1: Starting Threads as a 101. If you compiler does not have C++0x support, then stay with Boost.Thread
Take a look at Boost.Thread. This is cross-platform and a very good library to use in your C++ applications.
What specifically would you like to know?
The POSIX thread (pthreads) library is probably your best bet if you just need a simple threading library, it has implementations both on Windows and Linux.
A guide can be found e.g. here. A Win32 implementation of pthreads can be downloaded here.
Edit: Didn't see you were on Linux. In that case I'm not 100% sure but I think the libraries are probably already bundled in with your GCC installation.
I'd recommend using the Boost libraries Boost.Thread instead. This will wrap platform specifics of Win32 and Posix, and give you a solid set of threading and synchronization objects. It's also in very heavy use, so finding help on any issues you encounter on SO and other sites is easy.
You can search for a free PDF book "C++-GUI-Programming-with-Qt-4-1st-ed.zip" and read Chapter 18 about Multi-threading in Qt.
Concurrent programming features supported by Qt includes (not limited to) the following:
Mutex
Read Write Lock
Semaphore
Wait Condition
Thread Specific Storage
However, be aware of the following trade-offs with Qt:
Performance penalties vs native threading libraries. POSIX thread (pthreads) has been native to Linux since kernel 2.4 and may not substitute for < process.h > in W32API in all situations.
Inter-thread communication in Qt is implemented with SIGNAL and SLOT constructs. These are NOT part of the C++ language and are implemented as macros which requires proprietary code generators provided by Qt to be fully compiled.
If you can live with the above limitations, just follow these recipes for using QThread:
#include < QtCore >
Derive your own class from QThread. You must implement a public function run() that returns void to contain instructions to be executed.
Instantiate your own class and call start() to kick off a new thread.
Sameple Code:
#include <QtCore>
class MyThread : public QThread {
public:
void run() {
// do something
}
};
int main(int argc, char** argv) {
MyThread t1, t2;
t1.start(); // default implementation from QThread::start() is fine
t2.start(); // another thread
t1.wait(); // wait for thread to finish
t2.wait();
return 0;
}
As an important note in c++14, the use of concurrent threading is available:
#include<thread>
class Example
{
auto DoStuff() -> std::string
{
return "Doing Stuff";
}
auto DoStuff2() -> std::string
{
return "Doing Stuff 2";
}
};
int main()
{
Example EO;
std::string(Example::*func_pointer)();
func_pointer = &Example::DoStuff;
std::future<string> thread_one = std::async(std::launch::async, func_pointer, &EO); //Launching upon declaring
std::string(Example::*func_pointer_2)();
func_pointer_2 = &Example::DoStuff2;
std::future<string> thread_two = std::async(std::launch::deferred, func_pointer_2, &EO);
thread_two.get(); //Launching upon calling
}
Both std::async (std::launch::async, std::launch::deferred) and std::thread are fully compatible with Qt, and in some cases may be better at working in different OS environments.
For parallel processing, see this.
I'm looking for a good reader/writer lock in C++. We have a use case of a single infrequent writer and many frequent readers and would like to optimize for this. Preferable I would like a cross-platform solution, however a Windows only one would be acceptable.
Since C++ 17 (VS2015) you can use the standard:
#include <shared_mutex>
typedef std::shared_mutex Lock;
typedef std::unique_lock< Lock > WriteLock;
typedef std::shared_lock< Lock > ReadLock;
Lock myLock;
void ReadFunction()
{
ReadLock r_lock(myLock);
//Do reader stuff
}
void WriteFunction()
{
WriteLock w_lock(myLock);
//Do writer stuff
}
For older compiler versions and standards you can use boost to create a read-write lock:
#include <boost/thread/locks.hpp>
#include <boost/thread/shared_mutex.hpp>
typedef boost::shared_mutex Lock;
typedef boost::unique_lock< Lock > WriteLock;
typedef boost::shared_lock< Lock > ReadLock;
Newer versions of boost::thread have read/write locks (1.35.0 and later, apparently the previous versions did not work correctly).
They have the names shared_lock, unique_lock, and upgrade_lock and operate on a shared_mutex.
Using standard pre-tested, pre-built stuff is always good (for example, Boost as another answer suggested), but this is something that's not too hard to build yourself. Here's a dumb little implementation pulled out from a project of mine:
#include <pthread.h>
struct rwlock {
pthread_mutex_t lock;
pthread_cond_t read, write;
unsigned readers, writers, read_waiters, write_waiters;
};
void reader_lock(struct rwlock *self) {
pthread_mutex_lock(&self->lock);
if (self->writers || self->write_waiters) {
self->read_waiters++;
do pthread_cond_wait(&self->read, &self->lock);
while (self->writers || self->write_waiters);
self->read_waiters--;
}
self->readers++;
pthread_mutex_unlock(&self->lock);
}
void reader_unlock(struct rwlock *self) {
pthread_mutex_lock(&self->lock);
self->readers--;
if (self->write_waiters)
pthread_cond_signal(&self->write);
pthread_mutex_unlock(&self->lock);
}
void writer_lock(struct rwlock *self) {
pthread_mutex_lock(&self->lock);
if (self->readers || self->writers) {
self->write_waiters++;
do pthread_cond_wait(&self->write, &self->lock);
while (self->readers || self->writers);
self->write_waiters--;
}
self->writers = 1;
pthread_mutex_unlock(&self->lock);
}
void writer_unlock(struct rwlock *self) {
pthread_mutex_lock(&self->lock);
self->writers = 0;
if (self->write_waiters)
pthread_cond_signal(&self->write);
else if (self->read_waiters)
pthread_cond_broadcast(&self->read);
pthread_mutex_unlock(&self->lock);
}
void rwlock_init(struct rwlock *self) {
self->readers = self->writers = self->read_waiters = self->write_waiters = 0;
pthread_mutex_init(&self->lock, NULL);
pthread_cond_init(&self->read, NULL);
pthread_cond_init(&self->write, NULL);
}
pthreads not really being Windows-native, but the general idea is here. This implementation is slightly biased towards writers (a horde of writers can starve readers indefinitely); just modify writer_unlock if you'd rather the balance be the other way around.
Yes, this is C and not C++. Translation is an exercise left to the reader.
Edit
Greg Rogers pointed out that the POSIX standard does specify pthread_rwlock_*. This doesn't help if you don't have pthreads, but it stirred my mind into remembering: Pthreads-w32 should work! Instead of porting this code to non-pthreads for your own use, just use Pthreads-w32 on Windows, and native pthreads everywhere else.
Whatever you decide to use, benchmark your work load against simple locks, as read/write locks tend to be 3-40x slower than simple mutex, when there is no contention.
Here is some reference
C++17 supports std::shared_mutex . It is supported in MSVC++ 2015 and 2017.
Edit: The MSDN Magazine link isn't available anymore. The CodeProject article is now available on https://www.codeproject.com/Articles/32685/Testing-reader-writer-locks and sums it up pretty nicely. Also I found a new MSDN link about Compound Synchronisation Objects.
There is an article about reader-writer locks on MSDN that presents some implementations of them. It also introduces the Slim reader/writer lock, a kernel synchronisation primitive introduced with Vista. There's also a CodeProject article about comparing different implementations (including the MSDN article's ones).
Intel Thread Building Blocks also provide a couple of rw_lock variants:
http://www.threadingbuildingblocks.org/
They have a spin_rw_mutex for very short periods of contention and a queueing_rw_mutex for longer periods of contention. The former can be used in particularly performance sensitive code. The latter is more comparable in performance to that provided by Boost.Thread or directly using pthreads. But profile to make sure which one is a win for your access patterns.
Boost.Thread has since release 1.35.0 already supports reader-writer locks. The good thing about this is that the implementation is greatly cross-platform, peer-reviewed, and is actually a reference implementation for the upcoming C++0x standard.
I can recommend the ACE library, which provides a multitude of locking mechanisms and is ported to various platforms.
Depending on the boundary conditions of your problem, you may find the following classes useful:
ACE_RW_Process_Mutex
ACE_Write_Guard and ACE_Read_Guard
ACE_Condition
http://www.codeproject.com/KB/threads/ReaderWriterLock.aspx
Here is a good and lightweight implementation suitable for most tasks.
Multiple-Reader, Single-Writer Synchronization Lock Class for Win32 by Glenn Slayde
http://www.glennslayden.com/code/win32/reader-writer-lock
#include <shared_mutex>
class Foo {
public:
void Write() {
std::unique_lock lock{mutex_};
// ...
}
void Read() {
std::shared_lock lock{mutex_};
// ...
}
private:
std::shared_mutex mutex_;
};
You could copy Sun's excellent ReentrantReadWriteLock. It includes features such as optional fairness, lock downgrading, and of course reentrancy.
Yes it's in Java, but you can easily read and transpose it to C++, even if you don't know any Java. The documentation I linked to contains all the behavioral properties of this implementation so you can make sure it does what you want.
If nothing else, it's a guide.