从C调用蟒蛇法++（或C）回调 [英] Calling python method from C++ (or C) callback

问题描述

我试图调用从C ++ Python类的方法。 C ++的方法，从这个被称为是一个C ++回调。

在这个方法中，当我试图调用Python方法，它是给分段故障。

我在一个全局变量保存Python函数的一个实例像

  //（pFunc的类型的PyObject的全局变量*）
pFunc = PyDict_GetItemString（pDictPlxMsgWrapper）;
 

其中， PlxMsgWrapper 是一个Python方法，它会在回调中使用。

在回调，参数创建为

 的PyObject * ARGS = PyTuple_Pack（2 PyString_FromString（header.c_str（）），
                                 PyString_FromString（payload.c_str（）））;
 

创建当

 的PyObject * pInstance = PyObject_CallObject（pFunc，参数）;
 

在该行给予其分段故障。在此之后，实际的Python方法称为

 的PyObject * recv_msg_func = PyObject_GetAttrString（模块（字符*）recvCallback）;
ARGS = PyTuple_Pack（1，pInstance）;
PyObject_CallObject（recv_msg_func，参数）;
 

解决方案

有你需要做的，如果你是从C / C ++回调调用Python函数的几件事情。首先，当你保存了你的Python函数对象，你需要用递增引用计数：

  Py_INCREF（pFunc）
 

否则Python有不知道你持有到一个对象的引用，它可以收集垃圾的，造成了分段错误，当您尝试从回调中使用它。

这时旁边，你需要关心的事情是被调用的C / C ++的回调时，线程运行的产品。如果您收到来自其他非Python中叫回创建的线程（即C / C ++线程套接字接收数据），那么您必须获得Python的全球国米preTER锁（GIL）前调用任何的Python API函数。否则，你的程序的行为是不确定的。为了获取GIL你做的：

 的回调（）{
    PyGILState_STATE gstate;
    gstate = PyGILState_Ensure（）;    //获取ARGS等。    //调用你的Python函数对象
    *的PyObject = pInstance PyObject_CallObject（pFunc，参数）;    //是否有任何其他的Python所需的API操作    //释放线程。没有的Python API允许超出了这一点。
    PyGILState_Release（gstate）;
}
 

另外，在扩展模块的初始化函数，你应该做到以下几点，以确保线程正确初始化：

  //确保GIL已经创建，因为我们需要获得它在我们的
//回调，安全地调用到Python应用程序。
如果（！PyEval_ThreadsInitialized（））{
    PyEval_InitThreads（）;
}
 

另外，当您试图从非Python的线程获得GIL崩溃和奇怪的行为可能会随之而来。

请参阅Non-Python创建的线程的详细的这一点。

I am trying to call methods in a python class from C++. The C++ method from which this is called is a C++ callback.

Within this method when I am trying to call python method, it was giving segmentation fault.

I have saved an instance of python function in a global variable like

// (pFunc is global variable of type PyObject*)
pFunc = PyDict_GetItemString(pDict, "PlxMsgWrapper");

where PlxMsgWrapper is a python method, which will be used in the callback.

In the callback, the arguments are created as

PyObject* args = PyTuple_Pack(2, PyString_FromString(header.c_str()),
                                 PyString_FromString(payload.c_str()));

When creating the

PyObject * pInstance = PyObject_CallObject(pFunc, args);

In this line its giving segmentation fault. After this the actual python method is called as

PyObject* recv_msg_func = PyObject_GetAttrString(module, (char *)"recvCallback");
args = PyTuple_Pack(1, pInstance);
PyObject_CallObject(recv_msg_func, args);

解决方案

There are a few things you need to do if you are invoking a Python function from a C/C++ callback. First when you save off your python function object, you need to increment the reference count with:

Py_INCREF(pFunc)

Otherwise Python has no idea you are holding onto an object reference, and it may garbage collect it, resulting in a segmentation fault when you try to use it from your callback.

Then next thing you need to be concerned about is what thread is running when your C/C++ callback is invoked. If you are getting called back from another non-Python created thread (i.e. a C/C++ thread receiving data on a socket), then you MUST acquire Python's Global Interpreter Lock (GIL) before calling any Python API functions. Otherwise your program's behavior is undefined. To acquire the GIL you do:

void callback() {
    PyGILState_STATE gstate;
    gstate = PyGILState_Ensure();

    // Get args, etc.

    // Call your Python function object
    PyObject * pInstance = PyObject_CallObject(pFunc, args);

    // Do any other needed Python API operations

    // Release the thread. No Python API allowed beyond this point.
    PyGILState_Release(gstate);
}

Also, in your extension module's init function, you should do the following to ensure that threading is properly initialized:

// Make sure the GIL has been created since we need to acquire it in our
// callback to safely call into the python application.
if (! PyEval_ThreadsInitialized()) {
    PyEval_InitThreads();
}

Otherwise, crashes and strange behavior may ensue when you attempt to acquire the GIL from a non-Python thread.

See Non-Python Created Threads for more detail on this.

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