Py_INCREF() incomprehension

[Stefan Behnel]

Ervin Hegedüs, 26.04.2011 11:48:
> Hello Python users,
>
> I'm working on a Python module in C - that's a cryptographic module,
> which uses a 3rd-party lib from a provider (a bank).
> This module will encrypt and decrypt the messages for the provider web service.
>
> Here is a part of source:
>
> static PyObject*
> mycrypt_encrypt(PyObject *self, PyObject *args)
> {
>      int cRes = 0;
>      int OutLen = 0;
>
>      char *  url;
>      char *  path;
>
>      if (!PyArg_ParseTuple(args, "ss",&url,&path)) {

Use the "s#" format instead to get the length as well.


>          return NULL;
>      }
>
>      OutLen = strlen(url)*4;
>      outdata=calloc(OutLen, sizeof(char));
>
>      if (!outdata) {
>          handle_err(UER_NOMEM);

I assume this raises MemoryError?


>          return NULL;
>      }
>      cRes = ekiEncodeUrl (url, strlen(url)+1, outdata,&OutLen, 1, path);
>
>      if (cRes == 0) {
>          return Py_BuildValue("s", outdata);

You are leaking the memory allocated for outdata here.

And, again, use the "s#" format.


>      } else {
>          handle_err(cRes);
>          return NULL;
>      }

I assume this raises an appropriate exception?


>      return Py_None;

This is unreachable code.


> where ekiEncodeUrl is in a 3rd-party library.
> I should call this function from Python like this:
>
> import mycrypt
>
> message = "PID=IEB0001&MSGT=10&TRID=000000012345678"
> crypted = mycrypt(mymessage, "/path/to/key");
>
> Everything works fine, but sorry for the recurrent question: where
> should I use the Py_INCREF()/Py_DECREF() in code above?

These two functions are not your problem here.

In any case, I recommend using Cython instead of plain C. It keeps you from 
getting distracted too much by reference counting and other CPython C-API 
details, so that you can think more about the functionality you want to 
implement.

Stefan