Question

I need to write a python call that has a callback that will be invoked by C function. The C header file has the following

...
typedef struct _myResponse {
   char * data, 
   void (*setResponseFunc)(const char * const  req, char ** dataptr);
} MyResponse_t

The C library calls the callback like this

void process(const char * const req , MyResponse_t *response) {
   response->SetResponseFunc("some request", &response->data);
}

Sample callback implementation of the callback in C:

void SetResponse(const char *respData, char **dataptr) {
      char *ptr = malloc(strlen(respData));
      strcpy(ptr, respData);
      *dataptr= ptr;
}

I then use ctypesgen to generate the python code. The generated file contains the following:

agent.py:

def String:
 ...

class struct__myResponse(Structure):
    pass

struct__myResponse.__slots__ = [
    'data',
    'SetResponseFunc',
]
struct__myResponse._fields_ = [
    ('data', String),
    ('SetResponseFunc', CFUNCTYPE(UNCHECKED(None), String, POINTER(String))),
]

this is how I tried to use it

CALLBACK_FUNC = CFUNCTYPE(c_agent.UNCHECKED(None), agent.String, POINTER(agent.String))

def PyCopyResponse(a,b):
    print "XXXXXX" // here, I haven't tried to implement the proper code, just want to see if the callback is called from the C library

copy_response = CALLBACK_FUNC(PyCopyResponse)

Calling it

    agentResp = agent.MyResponse_t(None,copy_response)

    agent.process("some value",agentResp)

I dont’ see my python callback implementation to be called at all. I have verified that the C library is calling the callback appropriately. Can anyone please help?

Answer 1

Ok, found the issue. The process takes a pointer to a structure, I was passing the structure directly.

data = agent.String() 
agentResp = agent.MyResponse_t(data,copy_response)
agentResp_p = pointer(agentResp)

agent.process("some value",agentResp_p)