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In many files, the following code is present (with slight variations,
but the important part is there) :
static PyObject *
objargs_mktuple(va_list va)
{
	int i, n = 0;
	va_list countva;
	PyObject *result, *tmp;
#ifdef VA_LIST_IS_ARRAY
	memcpy(countva, va, sizeof(va_list));
#else
#ifdef __va_copy
	__va_copy(countva, va);
#else
	countva = va;
#endif
#endif
...
memcpy() is accessing va_list before it is initialized.
Before the first access to a va_list type variable, and after the last
access to that variable, calls to va_start() and va_end() must be made
to initialize and free the variable.
Such behaviour should be corrected in the following files :
- Objects/abstract.c, line 1901
- Objects/stringobject.c, line 162
- getargs.c, line 66
- getargs.c, line 1188
- modsupport.c, line 479

Can you provide a patch for 2.6 against the latest svn checkout of the
trunk please?

This is not a bug. All the reported functions expect va_list argument 
to be initialized before being called. AFAICT, they are consistently 
used in this way. For example,
PyObject *
PyObject_CallFunctionObjArgs(PyObject *callable, ...)
{
 PyObject *args, *tmp;
 va_list vargs;
 if (callable == NULL)
 return null_error();
 /* count the args */
 va_start(vargs, callable);
 args = objargs_mktuple(vargs);
 va_end(vargs);
 if (args == NULL)
 return NULL;
 tmp = PyObject_Call(callable, args, NULL);
 Py_DECREF(args);
 return tmp;
}
This may need to be clarified in the docs. For example, PyString_FromFormatV does not mention that vargs needs to be 
initialized: <http://docs.python.org/dev/c-
api/string.html#PyString_FromFormatV>. On the other hand, this may be 
obvious to most C programmers.
I suggest to close this issue as invalid.

On the second thought the macro dance highlighted by OP belongs to 
pyport.h. Attached patch defines Py_VA_COPY macro and uses it to simplify 
va_list copying code.

Looks like a good idea to me

This is what I meant. The initialization should be done by calling 
va_start(count_va); as you described.
In the files and lines I reported though, this is not called.
I'll file a patch for it soon.
--Rolland Dudemaine
Alexander Belopolsky wrote:
> Alexander Belopolsky <belopolsky@users.sourceforge.net> added the comment:
>
> This is not a bug. All the reported functions expect va_list argument
> to be initialized before being called. AFAICT, they are consistently
> used in this way. For example,
>
> PyObject *
> PyObject_CallFunctionObjArgs(PyObject *callable, ...)
> {
> PyObject *args, *tmp;
> va_list vargs;
>
> if (callable == NULL)
> return null_error();
>
> /* count the args */
> va_start(vargs, callable);
> args = objargs_mktuple(vargs);
> va_end(vargs);
> if (args == NULL)
> return NULL;
> tmp = PyObject_Call(callable, args, NULL);
> Py_DECREF(args);
>
> return tmp;
> }
>
> This may need to be clarified in the docs. For example, PyString_FromFormatV does not mention that vargs needs to be
> initialized: <http://docs.python.org/dev/c-
> api/string.html#PyString_FromFormatV>. On the other hand, this may be
> obvious to most C programmers.
>
> I suggest to close this issue as invalid.
>
> ----------
> nosy: +belopolsky
>
> __________________________________
> Tracker <report@bugs.python.org>
> <http://bugs.python.org/issue2443>
> __________________________________
>

Actually, this thing is more complex to solve than I thought.
Specifically, as described in
http://www.opengroup.org/onlinepubs/007908775/xsh/stdarg.h.html stdarg
requires that variable argument functions have at least one fixed argument.
This is implied by the declaration of "void va_start(va_list ap, argN);".
As explained in the original ticket description, and also described
before in the above link, va_start() must be called before any call to
va_arg(), and this includes any access to the argument list using
__va_copy namely.
The problem is that at least objargs_mktuple(), line 2649 of
Objects/abstract.c does not have a first fixed argument.

What's the status of this?

Rolland,
The va_list is initialized by the function that calls objargs_mktuple. 
va_start() and va_end() need to be called in the function that takes
"..." as a parameter, and it is.
Not a bug, but +1 on Alexander's patch to consolidate all the #ifdef's
for cleanliness.

Reducing the priority and updating the target releases, since from the
discussion there doesn't appear to be a bug here.

Updated the patch. In a year one more copy of va_copy macro dance have cropped in. Any reason this is still languishing?

Rolland,
Please don't revert the title. The consensus is that there is no uninitialized access to va_list.

Looks like a good idea. Don't other compilers have __va_copy equivalents?
Apparently, C99 defines va_copy(), which we could use conditionally.

I updated the patch for 3.x. I agree that using va_copy where available makes sense, but let's leave this type of improvements for the future.

Committed in r83949.