I have got a main function like this:
int main(){
....
Protocol SPI_conn;
SPI_conn.omap_SPI_init();
....
pthread_create(&rt_OneStep0_thread, NULL, rt_OneStep0, NULL);
....
}
where SPI_conn is an object of the class Protocol and omap_SPI_init() is a method of the same class. My thread-function looks like this:
extern "C" void * rt_OneStep0(void *)
{
while (1) {
sem_wait(&step0_semaphore);
SPI_do();
sem_wait(&step0_semaphore);
}
}
SPI_do() is also a function of the class Protocol. My question is, how can I use the object SPI_conn with the method SPI_do. Normally you can do it by reference, but here rt_OneStep0(void*) has to be defined like this, right?
I really appreciate your help!
Absolutely, your prototypes are correct.
It all lies in how you can use the last parameter of pthread_create. It's actually a pointer to anything you want, that will be passed as the parameter to your thread's entry point (so, here, rt_OneStep0).
Hence, if you create your thread like this :
pthread_create(&rt_OneStep0_thread, NULL, rt_OneStep0, &SPI_conn);
You will receive the address of your SPI_conn object as the void* argument of your rt_OneStep0 function. You'll just have to cast it back to the proper type and you can then use it normally.
extern "C" void * rt_OneStep0(void *arg)
{
Protocol *my_object = static_cast<Protocol*>(arg);
//...
}
However, since you're dealing with threads and you'll be sharing an object created on your main thread, be careful about concurrency and race conditions.
Related
My purpose is to call some C function from my C++ code and pass some C++ objects.
In fact I am using a integration routine from the GSL libray(written in C), see this link,
My code snippet:
// main.cpp
#include <stdio.h>
#include <gsl/gsl_integration.h>
#include <myclass.h>
/* my test function. */
double testfunction ( double x , void *param ) {
myclass *bar=static_cast<myclass*>(param);
/*** do something with x and bar***/
return val;
}
int main ( int argc , char *argv[] ) {
gsl_function F; // defined in GSL: double (* function) (double x, void * params)
/* initialize.*/
gsl_integration_cquad_workspace *ws =
gsl_integration_cquad_workspace_alloc( 200 ) ;
/* Prepare test function. */
myclass foo{}; // call myclass constructor
F.function = &testfunction;
F.params = &foo;
/* Call the routine. */
gsl_integration_cquad( &F, 0.0,1.0,1.0e-10,1.0e-10,ws, &res,&abserr,&neval);
/* Free the workspace. */
gsl_integration_cquad_workspace_free( ws );
return 0;
}
In my case, direct calling gsl_integration_cquad seems OK, provided the header includes sth like "ifdef __cplusplus", my concern is about the callback F,originally defined in C, am I allowed to pass the testfunction and also the C++ foo object in this way ? .
or is there any better way to do this kind of stuff, maybe overloading and use a functor?
P.S. Am I allowed to do exeption handling within the callback function? (use try catch inside "testfunction"). It works in my case but not sure if it's legal.
I'm not familiar with the library in question, but in general,
when passing a pointer to a callback and a void* to
a C routine, which will call the callback back with the void*,
there are two things you need to do to make it safe:
The function whose address you pass must be declared extern "C".
You'll get away with not doing this with a lot of compilers, but
it isn't legal, and a good compiler will complain.
The type you convert to the void* must be exactly the same
type as the type you cast it back to in the callback. The
classic error is to pass something like new Derived to the
C function, and cast it back to Base* in the callback. The
round trip Derived*→void*→Base* is undefined
behavior. It will work some of the time, but at other times, it
may crash, or cause any number of other problems.
And as cdhowie pointed out in a comment, you don't want to
allow exceptions to propagate accross the C code. Again, it
might work. But it might not.
For the exact example you posted, the only thing you need to do
is to declare testfunction as extern "C", and you're all
right. If you later start working with polymorphic objects,
however, beware of the second point.
You can use
myclass *bar=static_cast<myclass*>(param);
with void*.
If you meant something like transporting a c++ class pointer through a c callback's void* pointer, yes it's safe to do a static_cast<>.
There's no kind of losing c++ specific attributes of this class pointer, when passed through c code. Though passing a derived class pointer, and static casting back to the base class, won't work properly as #James Kanze pointed out.
The void* will likely just be passed trough by the C library without looking at the pointed-to data, so it's not a problem if this contains a C++ class. As log as you cast the void* to the correctly there shouldn't be any problems.
To make sure the callback function itself is compatible, you can declare it as extern "C". Additionally you should make sure that no exceptions are thrown from the callback function, since the C code calling the callback won't expect those.
All together I would split up the code into one function that does the real work and another function that is used as the callback and handles the interface with the C library, for example like this:
#include <math.h>
double testfunction ( double x ,myclass *param ) {
/*** do something with x and bar***/
return val;
}
extern "C" double testfunction_callback ( double x , void *param ) {
try {
myclass *bar=reinterpret_cast<myclass*>(param);
return testfunction(x, bar);
}
catch(...) {
std::cerr << "Noooo..." << std::endl;
return NAN;
}
}
I'm having some trouble making a callback wrapper class method that needs to be used by a third party library; the JackAudio library.
I have been able to make a wrapper for a JackAudio callback function that needs two arguments.
I'm just having trouble creating a callback function for a particular function that needs a const char * as an argument.
So far I have been able to make the JackAudio library jack_set_sample_rate_callback function use a custom class and can be executed like so:
SoundClass Sound;
SoundClass * SoundPointer = &Sound;
jack_set_sample_rate_callback(
client,
SoundClass::SampleRateCallbackWrapper,
SoundPointer
);
And the class looks something like this:
SoundClass
{
int SampleRateCallback( jack_nframes_t nframes )
{
//executes some code when called.
}
static int SampleRateCallbackWrapper( jack_nframes_t nframes, void * arg )
{
return static_cast < SoundClass* > ( arg )->SampleRateCallback( nframes );
}
};
All of the above works well, with no issues.
The problem I'm having now is with the JackAudio callback function jack_set_error_function
This is what I tried:
static void ErrorCallbackWrapper( const char * arg )
{
return static_cast < SoundClass*>( arg )->SomeErrorFunction();
}
But I get error: invalid static_cast from type ‘const char*’ to type ‘SoundClass*’
I get the gist why this is happening, I just have no idea what to do for a solution.
Thanks in advance for any help guys.
Assuming the Jack API is written for the C language, there is a formal problem already with the working callback that you have. Namely that it then needs to be extern "C", and that as a static member function it cannot be. So formally it needs to be a free-standing function.
The documentation that you link to for the jack_set_error_function gives this signature, presumably expressed in C:
void jack_set_error_function( void(*)(const char *) func);
For C++ the callback must be assumed to be extern "C", so,
extern "C" void MyErrorFunction( char const* errorMessage )
{
// Whatever, e.g. post a message to the GUI event queue, or terminate.
}
If you want this function to in turn call a method on an object, then unless the library provides some special mechanism to help you, you will just have to use one of the following techniques:
a namespace scope variable accessed by the callback, or
a dynamically generated callback.
C++ does not as of yet support the second approach, at all, so the first one is strongly indicated – if you want a callback on a method of an object.
EDIT: Sorry, I forgot to mention,
the function declarations in the API documentation are syntactically invalid.
E.g. the documentation’s signature
void jack_set_info_function( void(*)(const char *) func );
simply won’t compile with a standard-conforming compiler. Not as C, and not as C++. It’s syntactically invalid in both languages.
Instead it should be
void jack_set_info_function( void(*func)(const char *) );
Since the documentation apparently is generated by DOxygen, it stands to reason that it's been generated from source code that compiles. If so then this is a bug in DOxygen, and a problem with the quality assurance of the library provider. However it might be a problem that lies solely with the library provider, or, I might be mistaken in the assumption that this is a C library?
I'm a little confused about how to pass an object to the pthread_create function. I've found a lot of piecemeal information concerning casting to void*, passing arguments to pthread_create, etc., but nothing that ties it all together. I just want to make sure I've tied it all together and am not doing anything stupid. Let's say I have the following thread class:
Edit: fixed mis-matched static_cast.
class ProducerThread {
pthread_t thread;
pthread_attr_t thread_attr;
ProducerThread(const ProducerThread& x);
ProducerThread& operator= (const ProducerThread& x);
virtual void *thread_routine(void *arg) {
ProtectedBuffer<int> *buffer = static_cast<ProtectedBuffer<int> *> arg;
int randomdata;
while(1) {
randomdata = RandomDataGen();
buffer->push_back(randomdata);
}
pthread_exit();
}
public:
ProtectedBuffer<int> buffer;
ProducerThread() {
int err_chk;
pthread_attr_init(&thread_attr);
pthread_attr_setdetachstate(&thread_attr,PTHREAD_CREATE_DETACHED);
err_chk = pthread_create(&thread, &thread_attr, thread_routine, static_cast<void *> arg);
if (err_chk != 0) {
throw ThreadException(err_chk);
}
}
~ProducerThread() {
pthread_cancel(&thread);
pthread_attr_destroy(&thread_attr);
}
}
To clarify, the data in the ProtectedBuffer class can only be accessed with methods like ProtectedBuffer::push_back(int arg), which use mutexes to protect the actual data.
My main question is: am I using static_cast correctly? And my secondary question is do I need that first line in virtual void *thread_routine(void *arg) where I copy the passed void pointer to a pointer to ProtectedBuffer?
Also, if I've done anything else that might cause problems, I'd appreciate hearing it.
There are a number of problems with your code. For starters, I don't
see where the arg you are casting is declared, so I can't say whether
the case is appropriate.
Perhaps more importantly, thread_routine is a member function, so it
can't be converted to a pointer to a function. The function passed to
pthread_create must be extern "C", so it cannot be a member, period;
it must be a free function declare extern "C". If you want to call a
member function, pass a pointer to the object as the last argument, and
dereference it in the extern "C" function:
extern "C" void* startProducerThread( void* arg )
{
return static_cast<ProducerThread*>( arg )->thread_routine();
}
And to start the thread:
int status = pthread_create( &thread, &thread_attr, startProducerThread, this );
Just don't do this in a constructor. The other thread might start
running before the object is fully constructed, with disasterous
effects.
Also, be very sure that the cast in startProducerThread is to
exactly the same type as the pointer passed into pthread_create. If
you cast to a base class in startProducerThread, then be very, very
sure that it is a pointer to that base class that you pass to
pthread_create; use an explicit cast if necessary (to the type in
startProducerThread, not to void*).
Finally, while not relevant to your actual question: if
ProtectedBuffer has an interface like that of std::vector, and
returns references to internal data, there's no way you can make it
thread safe. The protection needs to be external to the class.
If you want to go this route, I believe you want something like this:
Edit: Based on James Kanze's answer, add a separate activate method to launch the thread after construction is finished.
class GenericThread {
protected:
GenericThread () {
//...
}
virtual ~GenericThread () {}
int activate () {
return pthread_create(..., GenericThreadEntry, this);
}
virtual void * thread_routine () = 0;
#if 0
// This code is wrong, because the C routine callback will do so using the
// C ABI, but there is no guarantee that the C++ ABI for static class methods
// is the same as the C ABI.
static void * thread_entry (void *arg) {
GenericThread *t = static_cast<GenericThread *>(arg);
return t->thread_routine();
}
#endif
};
extern "C" void * GenericThreadEntry (void *) {
GenericThread *t = static_cast<GenericThread *>(arg);
return t->thread_routine();
}
Then, ProducerThread would derive from GenericThread.
Edit: Searching for extern "C" in the C++ Standard. revealed no requirement that a function pointer must point to a function with C linkage to be callable by a C library routine. Since pointers are being passed, linkage requirements do not apply, as linkage is used to resolve names. A pointer to a static method is a function pointer, according to C++ 2011 draft (n3242), Sec. 3.9.2p3:
Except for pointers to static members, text referring to pointers does not apply to pointers to members.
Edit: Mea culpa. The C library will invoke the callback function assuming the C application binary interface. A function with C++ linkage may use a different ABI than the C ABI. This is why it is required to use a function with extern "C" linkage when passing to a callback function to a C library. My sincere apologies to James Kanze for doubting him, and my sincere thanks to Loki Astari for setting me straignt.
I'm trying to pass 2 pointers as an arguement for another function typecasted into (void*)
How do I seperate those two in the final function?
Example:
class Backgrounder{
public:
MESSAGE_QUEUE* m_pMsgQueue;
LockSection* m_pLc;
static void __cdecl Run( void* args){
MESSAGE_QUEUE* s_pMsgQueue = (MESSAGE_QUEUE*)args[0]; // doesn't work
LockSection* s_pLc = (LockSection*)args[1]; // doesn't work
}
Backgrounder(MESSAGE_QUEUE* pMsgQueue,LockSection* pLc) {
m_pMsgQueue = pMsgQueue;
m_pLc = pLc;
_beginthread(Run,0,(void*)(m_pMsgQueue,m_pLc));
}
~Backgrounder(){ }
};
You should create a struct with these two pointer types as members, and pass a pointer to that around.
The expression (m_pMsgQueue,m_pLc) doesn't do what you think it does; it invokes the comma operator, which simply evaluates to the second argument.
Bundle the arguments into a struct and pass that.
You could wrap them together in a struct and pass a pointer to that struct. Be careful though, because that struct should not be declared locally to the Backgrounder constructor - that would cause undefined behaviour because the thread may still be running after the function that started it has terminated. It should either be dynamically allocated, a static class member, or a global variable.
Actually, I would pass the this pointer since you essentially want to be able to access the fields of the object within the Run function:
class Backgrounder{
public:
MESSAGE_QUEUE* m_pMsgQueue;
LockSection* m_pLc;
static void __cdecl Run (void *pThis) {
MESSAGE_QUEUE* s_pMsgQueue = ((Backgrounder *) pThis)->m_pMsgQueue;
LockSection* s_pLc = ((Backgrounder *) pThis)->m_pLc;
}
Backgrounder(MESSAGE_QUEUE* pMsgQueue,LockSection* pLc) {
m_pMsgQueue = pMsgQueue;
m_pLc = pLc;
_beginthread(Run, 0, (void *) this);
}
~Backgrounder(){ }
};
Of course, you'll need to make sure that the newly created Backgrounder object is not prematurely destroyed, that is, the thread should be finished before the destruction.
Also, if these fields are later modified from the parent thread, you'll need to employ the appropriate synchronisation mechanisms.
Using C++.
pthread_t threads[STORAGE]; // 0-99
...
void run()
Error>>> int status = pthread_create(&threads[0], NULL, updateMessages, (void *) NULL);
if (status != 0)
{
printf("pthread_create returned error code %d\n", status);
exit(-1);
}
...
void ClientHandler::updateMessages(void *)
{
string reqUpdate = "91"; // Request for update
string recvMSG;
while (true)
{
sleep(5);
sending(sock,reqUpdate); // send
recvMSG = receiving(sock); // receive
QString output(recvMSG);
emit signal_chat(output, 0); // Print message to text box
}
}
...
Compile Error:
TCPClient.cpp:109: error: argument of type ‘void (ClientHandler::)(void*)’ does not match ‘void* (*)(void*)’
I can't figure out whats wrong.
Thanks in advance.
A pointer to a member function is different from a global function with the same signature since the member function needs an additional object on which it operates. Therefore pointers to these two types of functions are not compatible.
In this case this means that you cannot pass a member function pointer to pthread_create but only a pointer to a non-member (or static) function. A work around for this problem is to use the forth parameter of pthread_create to pass a pointer to a object to a global function which then calls the method of the passed object:
class ClientHandler {
public:
void updateMessages();
void run();
};
// Global function that will be the threads main function.
// It expects a pointer to a ClientHandler object.
extern "C"
void *CH_updateMessages(void *ch) {
// Call "real" main function
reinterpret_cast<ClientHandler*>(ch)->updateMessages();
return 0;
}
void ClientHandler::run() {
// Start thread and pass pointer to the current object
int status = pthread_create(&threads[0], NULL, CH_updateMessages, (void*)this);
...
}
It's nothing to do with threads, it's a normal C++ error, you're just passing an incompatible type of function pointer.
A function pointer is not the same as a member instance function pointer, even if their signature is the same; this is because there is an implicit reference to *this passed. You can't avoid this.
As pthread_create takes a free function, create a static function(is a free function) inside ClientHandler
static void Callback(void * this_pointer,int other_arg) {
ClientHandler* self = static_cast< ClientHandler*>(this_pointer);
self-> updateMessages(other_arg);
}
and call pthread_create as follows
pthread_create(&threads[0], NULL, &ClientHandler::Callback, (void *) pointer_to_ClientHandler,int other_arg);
That works because Callback is free function
YoLinux has a nice pthread tutorial that my help you in learning about threads.
As others have already said, the problem is that the signatures between the functions are different. Class member functions always have a "secret" extra parameter, the this pointer. So you can never pass a member function where a global function is expected. You can hack around this either with libraries such as Boost.Bind, or by making the function a static member of the class.
But the simplest, and most elegant solution is to use a different threading API.
Boost.Thread is a very nice threading library for C++ (pthreads is designed for C, and that's why it doesnt play well with C++ features such as class methods).
I'd recommend using that.
Your code could be rewritten as something like this:
class ClientHandler {
public:
ClientHandler(/* All the parameters you want to pass to the thread. Unlike pthreads you have complete type safety and can pass as many parameters to this constructor as you like */){...}
void operator()() // boost.thread calls operator() to run the thread, with no parameters. (Since all parameters were passed in the constructor and saved as member variables
{
string reqUpdate = "91"; // Request for update
string recvMSG;
while (true)
{
sleep(5);
sending(sock,reqUpdate); // send
recvMSG = receiving(sock); // receive
QString output(recvMSG);
emit signal_chat(output, 0); // Print message to text box
}
}
// whatever arguments you want to pass to the thread can be stored here as member variables
};
boost::threead_group gr; // can store all your threads here, rather than being limited to your fixed-size array
gr.create_thread(ClientHandler(/* construct a ClientHandler object with the parameters you like*/));
You're passing a member function instead of a global, normal, one.
Just define:
void updateMessages(void *) {
static ClientHandler c;
// use c..
}