[Python-Dev] Is it kosher to use a buffer after release? (original) (raw)

Larry Hastings larry at hastings.org
Sun May 10 20:28:19 CEST 2015

In Python's argument parsing code (convertsimple in Python/getargs.c), a couple of format units* accept "read-only bytes-like objects", aka read-only buffer objects. They call a helper function called convertbuffer() which uses the buffer protocol to extract a pointer to the memory.

Here's the relevant bit of code:

static Py_ssize_t
convertbuffer(PyObject *arg, void **p, char **errmsg)
{
Py_buffer view;
...

if (getbuffer(arg, &view, errmsg) < 0)
     return -1;
count = view.len;
*p = view.buf;
PyBuffer_Release(&view);
return count;
}

getbuffer() uses the buffer protocol to fill in the "view" buffer. If it's successful, "view" is a valid buffer. We store the pointer to the buffer's memory in output parameter p.

THEN WE RELEASE THE BUFFER.

THEN WE RETURN TO THE CALLER.

In case you missed the big helpful capital letters, we are returning a pointer given to us by PyObject_GetBuffer(), which we have already released by calling PyBuffer_Release(). The buffer protocol documentation for bf_releasebuffer makes it sound like this pointer could easily be invalid after the release call finishes.

Am I missing something, or is this code relying on an implementation detail it shouldn't--namely that you can continue using a pointer to some (most? all?) buffer memory even after releasing it?

//arry/

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