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PyCapsule example for a C function that consumes a previously created C ptr to a struct object created by a void function

void function declaration from .pxd

void initMsgHeader(msgHeaderType *msgHeader, int msgVersion) char *readMsgHeader(char *buf, msgHeaderType *msgHeader)

PyCapsule passing msgHeader struct pointer

void function initializes the struct in C space

initMsgHeader(&c_msgHeader, _msgVersion)

  • void function initializes the struct for c_msgHeader

Passing the pointer to the initialized C struct to Python space

ptr_msgdata = pycapsule.PyCapsule_New(readMsgHeader(c_buf, &c_msgHeader), NULL, NULL)

  • readMsgHeader function is called to generate the C pointer to the struct
  • PyCapsule passes the c_msgHeader struct to Python to allow access to the struct's data content

Accessing the C struct members from Python space

originator = c_msgHeader.originator sequenceNum = c_msgHeader.sequencenumber

''' The Cython code creates a Python capsule (similar to a smart pointer in C++) to store a pointer to some data. The data being pointed to is obtained by calling the C function readMsgHeader and passing it a buffer (c_buf) and a pointer to a c_msgHeader structure.

The capsule is created using the PyCapsule_New function from the pycapsule module. The first argument is a pointer to the data, the second argument is a destructor function that should be called when the capsule is destroyed (NULL indicates that no such function is needed), and the third argument is a NULL-terminated string identifying the capsule type (not used by the Python interpreter, but may be used by the calling code).

Once the capsule is created, it can be passed around between Cython code and Python code as a pointer, and the data it contains can be accessed using the PyCapsule_GetPointer function in Cython or the PyCapsule_GetPointer function in Python. '''