I have the following encapsulation for my pthread_t threads:
#include <pthread.h>
class Thread
{
public:
void run(const int);
static void *run_helper(void *);
bool startThread(int);
void joinThread();
pthread_t th;
};
Where run is my thread routine, and run_helper is the following:
void *Thread::run_helper(int num)
{
return (Thread *)this->run(num);
}
I start my threads like such:
bool Thread::startThread(intptr_t arg)
{
return (pthread_create(&this->th, NULL, &run_helper(arg), (void *)(intptr_t)arg));
}
But when I compile, I get the following errors:
error: lvalue required as unary ‘&’ operand
return (pthread_create(&this->th, NULL, &run_helper(arg), (void *)(intptr_t)arg));
error: ‘this’ is unavailable for static member functions
return (Thread *)this->run(num);
And despite trying, I can't seem to make this encapsulation work.
I think your issue might specifically be &this->th. & has higher precedence than ->. Perhaps try &(this->th).
For the first error, you need to cast the third argument into (void*(*)(void*)) and remove & (maybe the casting isn't neccessary).
pthread_create(&this->th, NULL, (void*(*)(void*))run_helper(arg), (void *)(intptr_t)arg);
The second error, you are trying to use pointer to this in a static function, but the function isn't called on any object, therefore you can't use this in a function like that. The solution is to cast arg into Thread* and then call the non-static run function.
Related
Okay so i'm looking at the documentation for pthread_create and I just don't understand at all how to do what I want to do.
I want to call pthread_create which will obv pass in a struct of pthread_t. But the function I pass to it takes in a pointer to a of MyNode*. How would I pass the function as a parameter and pass it "this" as a parameter to that function.
//MyNode field and method in class file
pthread_t myTrd;
static void compute(MyNode* node);
////////////////////////////////////////////////////////////
//Actual code in header file below
static void MyNode::compute(*MyNode node){ //L61
//code
}
void MyNode::run(){ //run function in header file
pthread_create(&(this->thread),NULL,MyNode::compute, this);
}
outcome:
myNode.cpp:61: error: 'static' may not be used when defining (as opposed to declaring) a static data member
myNode.cpp:61: error: 'int MyProjectGraph::MyNode::compute' is not a static member of 'class MyProjectGraph::MyNode'
myNode.cpp:61: error: expected primary-expression before 'node'
myNode.cpp:61: error: expected ',' or ';' before '{' token
myNode.cpp:134: error: expected `}' at end of input
The function passed to pthread_create() should match the prototype:
void *function(void *arg);
If your function does not match that, you have to use brute force and ignorance (and a cast) to make the function pointer acceptable — and then hope that the alternative interface doesn't break anything.
It is far better to make your function match the specification:
void *function(void *arg)
{
MyNode *mnp = (MyNode *)arg;
…
return 0;
}
The return can return some more meaningful value if you have one available, but returning a null (you could probably write nullptr given that you're mainly using C++).
Note that pthread_create() is usually a C function itself and expects C function semantics in the function pointer it is passed.
Having a thread per object isn't really a good way to go. I presume that
since you are calling your object a node, you have a bunch of them
and want to do something to them on a thread.
I typically do the following, which is a classic idiom:
class Worker
{
struct ThreadStr
{
Worker * worker;
// put parameters here
MyNode * node;
};
public:
static void *StaticHandler(void *pvoid)
{
ThreadStr * data = (ThreadStr*)pvoid;
data->worker->Compute(data->node);
delete data;
return NULL;
}
void Compute(MyNode *node)
{
// whatever you want to compute on a node.
}
// start a thread to execute Worker::Compute(MyNode*)
void Run(MyNode *node)
{
ThreadStr * data = new ThreadStr();
data->worker = this;
data->node = node;
pthread_t threadId;
pthread_create(&threadId, NULL, Worker::StaticHandler, data);
}
};
I am a little confused on how pthread works - specifically, I am pretty sure that pthread takes in a pointer to a function that takes a void pointer as an argument (correct me if I am wrong), and I have declared my function in that way, but I am still getting an error. Here is the code I am struggling with:
void eva::OSDAccessibility::_resumeWrapper(void* x)
{
logdbg("Starting Connection.");
_listener->resume();
logdbg("Connected.");
pthread_exit(NULL);
}
void eva::OSDAccessibility::resumeConnection()
{
long t;
_listener->setDelegate(_TD);
pthread_t threads[1];
pthread_create(&threads[0], NULL, &eva::OSDAccessibility::_resumeWrapper, (void *)t);
}
The error I'm getting is:
No matching function for call to pthread_create.
You don't necessarily have to tell me how to fix the code (although that would be appreciated of course), I'm more interested in why this error is coming up and if my understanding of pthread is correct. Thanks! :)
Your function signature must be void * function (void*)
If called from c++ code, the method must be static:
class myClass
{
public:
static void * function(void *);
}
A solution to use methods that are not static is the following:
class myClass
{
// the interesting function that is not an acceptable parameter of pthread_create
void * function();
public:
// the thread entry point
static void * functionEntryPoint(void *p)
{
((myClass*)p)->function();
}
}
And to launch the thread:
myClass *p = ...;
pthread_create(&tid, NULL, myClass::functionEntryPoint, p);
This question already has answers here:
pthread function from a class
(9 answers)
Closed 9 years ago.
I have a class named qwerty and a function called compute_ans inside it which takes a void pointer and returns a void pointer. Now when I try to compile, the following statement throws an error
pthread_create (thread_a, NULL, compute_ans, (void*) struct_left);
The definition of the function is void* compute_ans (void* struct_input)
The error is
cannot convert ‘qwerty::compute_ans’ from type ‘void* (qwerty::)(void*)’ to type ‘void* ()(void)’*
You cannot convert a pointer to a non-static member function to a pointer to function, C++ does not allow it. The reason is that member functions take an implicit this pointer as a parameter. Essentially this changes the signature of your function to be something like void* compute_ans(qwerty*, void*). In order to pass the function to pthread_create you need to make the member function static.
class qwerty
{
public:
// ... other member functions and variables ...
// thread start function
static void* compute_ans(void*);
};
If you cannot make this a static member function you will need to pass a pointer to a qwerty object to the thread start function. Looking at the code in your question you also need to pass additional data to the thread function. To do this you can use an additional data structure that contains all the necessary data and pass a pointer to that instead.
class qwerty; // forward declaration
// Structure passed to pthread_create and our helper function
struct thread_data
{
qwerty* qptr; // pointer to qwerty object
void* data; // pointer to other data. change void to your data type.
};
class qwerty
{
public:
// thread start function
static void* start_compute_ans(void* param)
{
// get a pointer to the thread data
thread_data* tdata = static_cast<thread_data*>(param);
// Call the real compute_ans
tdata->qptr->compute_ans(tdata->data);
// Delete the data (use an appropriate smart pointer if possible)
delete tdata;
return NULL;
}
// the real
void compute_ans(void*)
{
// do stuff here
}
};
// Create our thread startup data
thread_data* tdata = new thread_data();
tdata->qptr = qwerty_pointer;
tdata->data = struct_left;
// start the thread data
pthread_create (thread_a, NULL, &qwerty::start_compute_ans, tdata);
You can find the answer here.
You should use static functions to pass to pthread.
class qwerty
{
public:
void compute_ans(void)
{
std::cout << "Compute result!" << std::endl;
return
}
static void hello_helper(void *context)
{
return ((qwerty *)context)->compute_answer();
}
};
...
qwerty c;
pthread_t t;
pthread_create(&t, NULL, &qwerty::hello_helper, &c);
I am trying to use pthread_create() but it always gives me this error invalid conversion from void* to void* ( * )(void*)
This error is in the 3rd argument. Could someone help me with this error ?
void Print_data(void *ptr) {
cout<<"Time of Week = " <<std::dec<<iTOW<<" seconds"<<endl;
cout<<"Longitude = "<<lon<<" degree"<<endl;
cout<<"Latitude = "<<lat<<" degree"<<endl;
cout<<"Height Above Sea = "<<alt_MSL<<" meters"<<endl;
}
int call_thread()
{
pthread_create(&thread, NULL, (void *) &Print_data, NULL);
return 0;
}
The error is that you're converting the function pointer (void* (*)(void*)) to an object pointer (void*), when pthread_create expects a function pointer for that argument. There's no implicit conversion to undo the dodgy conversion you've done, hence the error.
The answer is to not do that:
pthread_create(&thread, NULL, &Print_data, NULL);
and you'll also need to modify Print_data to return void* to match the Posix threads interface:
void *Print_data(void *ptr) {
// print stuff
return NULL; // or some other return value if appropriate
}
As noted in the comments, there are various other issues with using this C library directly from C++; in particular, for portability, the thread entry function should be extern "C". Personally, I'd recommend using the standard C++ thread library (or Boost's implementation, if you're stuck with a pre-2011 version of the language).
You're trying to convert a function pointer into a void* here: (void *) &Print_data
According to pthread_create you need to pass in a function that takes a void* and returns a void*
So your function signature should be
void* Print_data(void *ptr)
And your call should be
pthread_create(&thread, NULL, &Print_data, NULL);
pthread_create takes the third argument as
int pthread_create(pthread_t *thread,
const pthread_attr_t *attr,
void *(*start_routine)(void*),
void *arg);
This, void *(*start_routine)(void*) is a pointer to a function that takes a void* pointer and returns a void* pointer.
When you do &Print_data and convert the pointer to void * , it means you are passing a pointer of type void* and not a pointer of type void *(*start_routine)(void*) [function pointer].
To be correct, you need to make your return type as void* and make the call as pthread_create(&thread, NULL, &Print_data, NULL);
You must return void*
void* Print_data(void *ptr) {
to satisfy the needs.
The signature of the function to be passed is
void* function(void*);
then call pthread_create using
pthread_create(&thread, NULL, &Print_data, NULL);
add header file #include
and compile g++ -lpthread
I'm doing to implement thread class for my own using pthread. So, I create Thread class as below :
class Thread
{
public:
Thread()
{
}
virtual void* run(void *params) = 0;
void start(void *params)
{
pthread_create (&threadId, 0, run, params);
pthread_join (threadId, 0);
}
private:
pthread_t threadId;
};
After implementing this class and override virtual run function, I do compile this project. But error: argument of type ‘void* (Thread::)(void*)’ does not match ‘void* (*)(void*)’ occurs. What is wrong in my code?
Thanks in advance :)
Exactly what the compiler is telling you.
pthread_create is expecting a function with the signature :
void* (*)(void*)
Which is a function pointer.
However, you are providing something with this signature:
void* (Thread::)(void*)
Which is not a function pointer, but a pointer to member function. There is a difference : a pointer to member function needs an instance of an object in order to work properly (here, it would need an instance of Thread).
A usual solution would be to make your function run static : it would not be a member function anymore - it doesn't NEED an instance of Thread in order to work properly anymore, and you could pass your current instance as the last parameter of pthread_create in order to act on it once the thread is launched.
You would just need to save the parameters in the class itself.
public:
void start(void *params)
{
this->my_thread_params = params;
pthread_create (&threadId, 0, run, static_cast<void*>(this));
}
private:
static void *run(void *my_object)
{
// here, my_object already contains the params you passed to the function start
static_cast<Thread*>(my_object)->my_member_function();
}
pthread_create is a C function, and knows nothing of C++ member functions. You'll need to give it a static or non-member function, and pass a pointer to your Thread object via the final argument of pthread_create; something like:
class Thread
{
virtual void* run(void *params) = 0;
void start(void * params)
{
this->params = params;
pthread_create(&threadId, 0, &Thread::static_run, this);
}
static void * static_run(void * void_this)
{
Thread * thread_this = static_cast<Thread*>(void_this);
return thread_this->run(thread_this->params);
}
private:
pthread_t threadId;
void *params;
};
Of course in modern C++, this is rather more straightforward:
std::thread thread;
void start(void * params)
{
thread = std::thread([this]{run(params);});
}
(Although of course you shouldn't be using void* to pass your parameters, and there's probably no good reason to wrap the thread up in a class in the first place.)
The error message is telling you that a pointer to a member function in Thread that takes and returns a void* (void* (Thread::*)(void*)) is not convertible to a pointer to function taking and returning the same void*.
While the declaration of the member function may look similar to the type that you need, there is an implicit this pointer of type Thread that needs to be injected on any call to Thread::run