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I am testing a wrapper on Windows 64 and it seems to result in an null pointer access error ONLY when I insert a print statement in the C code. I have tested the wrapper with Python 2.6 and Python 2.7a4. In addition, I have compiled Python 2.6.5 source code and ONLY the release version results in an error. I think the issue is with memcpy(obj->b_ptr, *pArgs, dict->size) (callbacks.c). pArgs seem to be corrupted. However, I am only looking at the code for the first time and I might be off base.
The following is Python and C code to reproduce the bug. To resolve, please comment printf statement in jfunc (C function).
Python Code:
from ctypes import *
def fcn(m,n,x,f):
 print "IN Python function fcn ................"
 print f[0]
 
m=3
n=1
pydlltest=cdll.pydlltest
pydlltest.jfunc.restype = POINTER(c_double)
evalstring = 'pydlltest.jfunc('
TMP_FCN=CFUNCTYPE(None,c_int,c_int,POINTER(c_double),POINTER(c_double))
tmp_fcn=TMP_FCN(fcn)
state=[TMP_FCN,tmp_fcn]
evalstring += 'tmp_fcn'
evalstring +=','
evalstring +='c_int(m)'
evalstring +=','
evalstring +='c_int(n)'
evalstring += ')'
print "evalstring=",evalstring
result = eval(evalstring)
C code:
#include <stdio.h>
__declspec(dllexport) double *jfunc(void (*fcn) (int,int,double [],double[]),int m,int n);
double *jfunc(void (*fcn) (int,int,double [],double []),int m,int n)
{
	double *fvec=NULL;
	double *xguess = NULL;
	int i = 0;
	int j = 0;
	/* comment the line below to fix the resulting null pointer access error */
	printf("In j func .................\n");
	fvec = (double *) malloc (m * sizeof (double));
	xguess = (double *) malloc (n * sizeof (double));
	for (i = 0; i < n; i++){
	 xguess[i] = 0.123;
	}
	(*fcn) (m, n, xguess, fvec);
	return fvec;
}

I was able to fix the issue by modifying and re-compiling ctypes project in Visual Studio 2008. The following properties were modified:
Release:
configuration Properties->C/C++->Optimization->Optimization: Disabled(/Od)
configuration Properties->C/C++->Optimization->Enable Intrinsic Functions: No

Attaching a patch for the configuration changes mentioned in msg102544 

This patch fixes issue #9884 and possibly #9266.

Disabling optimizations doesn't really fix the issue, just hides it, for now.
The problem was an uninitialized variable. Attached is the patch that fixes it.

Would it be possible to add a test case to the patch?

Thank you. The new patch works and it also fixes a crash of the 	python-2.5.4.amd64 interpreter at startup when ctypes 1.0.2 and pyreadline 1.6.2 are installed.

attached patch with tests
I have no experience with python development so the tests will actually need a more detailed review. Also I only checked it on win64.

Patch verified to work under Linux and win32. It looks good, except for tab characters in the Modules/_ctypes/_ctypes_test.c (indentation of C files should use 4 spaces); that's a detail that we can fix ourselves.

ok, please also fix the first parameter of the tests. Because it has the same value in the first call and the callback it will always be right by accident, since the first call puts the value in both standard and floating point registers.

Ok, committed in r88284 (3.2), r88285 (3.1) and r88286 (2.7). Thank you!