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How set a C char** pointer in Python

I want to set a C char** pointer, called results, in Python. The variable is in a dll I have loaded. I want to set results so that it points to a string in Python. I want to get the string I created in Python (or at least a copy of it since ctypes does a lot of copying) to be pointed to by the C variable results. So I have in Python product_class = (ctypes.c_char_p)(b"321"). I want to set results to the value "321".
Here is the code I have written. It does not work. It does not even change the C-variable results.
# py_parse_pdl_func function is a callback which is called from a c dll which has been loaded into the python prorgram.
# Here is the declaration of the callback in c
# typedef int (*tsl_pdl_cb_t)(void *pz_prv, const char **results, const char* query);
# so am trying to set results to point to a string "321"
def py_parse_pdl_func(pz_prv, py_results, query):
global product_class_void
product_class = (ctypes.c_char_p)(b"321")
product_class_void = ctypes.cast(product_class, ctypes.c_void_p)
py_results.contents = ctypes.c_long(product_class_void.value)
return 1

Here's a reproducible example. You may need to keep a reference to the string returned since Python could deallocate it at any time.
test.c
#include <stdio.h>
typedef int (*tsl_pdl_cb_t)(void *pz_prv, const char **results, const char* query);
__declspec(dllexport)
int function_using_callback(tsl_pdl_cb_t callback) {
char* results = NULL;
int retval = 0;
if(callback)
retval = callback(NULL, &results, "the query");
printf("results = '%s'\n", results);
return retval;
}
test.py
import ctypes as ct
CALLBACK = ct.CFUNCTYPE(ct.c_int, ct.c_void_p, ct.POINTER(ct.c_char_p), ct.c_char_p)
dll = ct.CDLL('./test')
dll.function_using_callback.argtypes = CALLBACK,
dll.function_using_callback.restype = ct.c_int
#CALLBACK
def py_parse_pdl_func(pz_prv, py_results, query):
py_results[0] = b'321'
return 1
retval = dll.function_using_callback(py_parse_pdl_func)
print('retval =', retval)
Output:
results = '321'
retval = 1

Python ctypes throws TypeError: one character bytes, bytearray or integer expected for functions with pointer references

I am new to both Python and ctypes the module. Trying to call C++ a function by loading a shared library. Here is the prototype of the function I want to call.
void foo_func(const char *binary, size_t binsz, size_t memsz, void *params, size_t paramssz, void *settings);
And here is the code I have written to call this function.
import ctypes
import pathlib
class virt_buff(ctypes.Structure):
_fields_ = [("x", ctypes.c_int), ("y", ctypes.c_int)]
if __name__ == "__main__":
libname = pathlib.Path().absolute() / "build/libfoo.so"
c_lib = ctypes.CDLL(libname)
func_param = virt_buff(7, 0)
with open("build/fib.bin", mode='rb') as file: # b is important -> binary
binary = file.read()
c_lib.foo_func(ctypes.c_char(binary), file.tell(), 0x9000 + (file.tell() & ~0xFFF), func_param, 4, NULL)
But when I run this code, it gives me the following output.
Traceback (most recent call last):
File "ct_test.py", line 32, in <module>
c_lib.foo_func(ctypes.c_char(binary), file.tell(), 0x9000 + (file.tell() & ~0xFFF), virt_param, 4, NULL)
TypeError: one character bytes, bytearray or integer expected
I tried a lot of things and nothing seems to work. Can anyone help me find out what the actual problem is?

The first error message is due to trying to pass a single character (c_char) and initializing it with a multiple character data blob. You want c_char_p for C char* instead. But there are other issues after that. Declaring .argtypes and .restype for your function correctly will help ctypes catch errors.
Make the following additional commented fixes:
import ctypes as ct
import pathlib
class virt_buff(ct.Structure):
_fields_ = [("x", ct.c_int), ("y", ct.c_int)]
if __name__ == "__main__":
libname = pathlib.Path().absolute() / "build/libfoo.so"
# Declare argument types and result type that match the C call for ctypes to error check
# void foo_func(const char *binary, size_t binsz, size_t memsz, void *params, size_t paramssz, void *settings);
c_lib = ct.CDLL(str(libname)) # on windows, CDLL(libname) failed to accept WindwowsPath.
c_lib.foo_func.argtypes = ct.c_char_p, ct.c_size_t, ct.c_size_t, ct.c_void_p, ct.c_size_t, ct.c_void_p
c_lib.foo_func.restype = None
func_param = virt_buff(7, 0)
with open("build/fib.bin", mode='rb') as file:
binary = file.read()
# Indent under with, so file.tell() is in scope,
# Pass address of structure using ct.byref() and size with ct.sizeof()
# Use Python None instead of C NULL.
c_lib.foo_func(binary, file.tell(), 0x9000 + (file.tell() & ~0xFFF), ct.byref(func_param), ct.sizeof(func_param), None)

Your parameter is almost certainly supposed to a POINTER to char, and not just a single char:
c_lib.foo_func(ctypes.c_char_p(binary), ...

How to get a string from C++ to python when using ctypes and wchar_t?

I can:
Get an integer from C++ and use it in python
Send a python string (as a wchar_t) to C++ and do some logic with it
I cannot
Step 2 in opposite direction.
Here is my C++ code (compiled with clion and cygwin as a shared library using C++14).
#include <iostream>
wchar_t aa[2];
extern "C" {
int DoA()
{
return 10;
}
int DoB(wchar_t * in)
{
if (in[1] == 'a')
{
return 25;
}
return 30;
}
wchar_t * DoC()
{
aa[0] = 'a';
aa[1] = 'b';
return aa;
}
}
Here is my python 3.6.1 code that shows what I can and what I cannot do. So how should I get my string and do things with it in python? I expect to use the address with wstring_at to get the value, but it is not working.
from ctypes import *
import os.path
print('Hello')
itExist = os.path.exists('C:/Users/Daan/CLionProjects/stringproblem/cmake-build-release/cygstringproblem.dll')
print(itExist)
lib = cdll.LoadLibrary('C:/Users/Daan/CLionProjects/stringproblem/cmake-build-release/cygstringproblem.dll')
print('dll loaded')
A = lib.DoA()
print(A)
Bx = lib.DoB(c_wchar_p('aaa'))
print(Bx)
By = lib.DoB(c_wchar_p('bbb'))
print(By)
Ca = lib.DoC()
print(Ca)
print('Issue is coming')
Cb = wstring_at(Ca,2)
print(Cb)
Here is the output with error.
Hello
True
dll loaded
10
25
30
-1659080704
Issue is coming
Traceback (most recent call last):
File "ShowProblem.py", line 19, in <module>
Cb = wstring_at(Ca,2)
File "C:\Users\Daan\AppData\Local\Programs\Python\Python36\lib\ctypes\__init__.py", line 504, in wstring_at
return _wstring_at(ptr, size)
OSError: exception: access violation reading 0xFFFFFFFF9D1C7000

I reproduced your problem on Linux and corrected it by defining the return type from your DoC function:
from ctypes import *
print('Hello')
lib = cdll.LoadLibrary(PATH_TO_TOUR_LIB)
print('dll loaded')
# this line solved the issue for me
lib.DoC.restype = c_wchar_p
A = lib.DoA()
print(A)
Bx = lib.DoB(c_wchar_p('aaa'))
print(Bx)
By = lib.DoB(c_wchar_p('bbb'))
print(By)
Ca = lib.DoC()
print(Ca)
print('Issue is coming')
Cb = wstring_at(Ca,2)
print(Cb)
I also allocated the memory dynamically (some Python expert might comment on this, I guess that this causes a memory leak):
extern "C" {
int DoA()
{
return 10;
}
int DoB(wchar_t * in)
{
if (in[1] == 'a')
{
return 25;
}
return 30;
}
wchar_t * DoC()
{
wchar_t* aa = new wchar_t[2];
aa[0] = 'a';
aa[1] = 'b';
return aa;
}
}
Let me know if it works on Windows.

If you set the .argtypes and .restype of your wrapped functions, you can call them more naturally. To handle an output string, it will be thread safe if you allocate the buffer in Python instead of using a global variable, or just return a wide string constant. Here's an example coded for the Microsoft compiler:
test.c
#include <wchar.h>
#include <string.h>
__declspec(dllexport) int DoA(void) {
return 10;
}
__declspec(dllexport) int DoB(const wchar_t* in) {
if(wcslen(in) > 1 && in[1] == 'a') // Make sure not indexing past the end.
return 25;
return 30;
}
// This version good if variable data is returned.
// Need to pass a buffer of sufficient length.
__declspec(dllexport) int DoC(wchar_t* aa, size_t length) {
if(length < 3)
return 0;
aa[0] = 'a';
aa[1] = 'b';
aa[2] = '\0';
return 1;
}
// Safe to return a constant. No memory leak.
__declspec(dllexport) wchar_t* DoD(void) {
return L"abcdefg";
}
test.py
from ctypes import *
# Set up the arguments and return type
lib = CDLL('test')
lib.DoA.argtypes = None
lib.DoA.restype = c_int # default, but just to be thorough.
lib.DoB.argtypes = [c_wchar_p]
lib.DoB.restype = c_int
lib.DoC.argtypes = [c_wchar_p,c_size_t]
lib.DoC.restype = c_int
lib.DoD.argtypes = None
lib.DoD.restype = c_wchar_p
# Map to local namespace functions
DoA = lib.DoA
DoB = lib.DoB
DoD = lib.DoD
# Do some pre- and post-processing to hide the memory details.
def DoC():
tmp = create_unicode_buffer(3) # Writable array of wchar_t.
lib.DoC(tmp,sizeof(tmp))
return tmp.value # return a Python string instead of the ctypes array.
print(DoA())
print(DoB('aaa'))
print(DoB('bbb'))
print(DoC())
print(DoD())
Output:
10
25
30
ab
abcdefg

PyRun_File with recursive functions in Python C-API

I am working on embedding python interpreter in a larger system and one of the features is to run a python script on the fly.
Testing with the following code snippet works great, but with recursive function only the first call is executed then crashes
//C++ code
int main()
{
Py_Initialize();
PyObject* m_pMainModule = PyImport_AddModule("__main__");
PyObject* m_pGlobalDict = PyModule_GetDict(m_pMainModule);
PyObject* m_pLocalDict = PyDict_New();
PyObject* fd = PyFile_FromString("script.py", "r");
if (fd == NULL)
{
PyErr_SetString(PyExc_IOError, "File not found");
}
PyObject * s = PyRun_File(PyFile_AsFile(fd), "script.py", Py_file_input, m_pGlobalDict, m_pLocalDict);
Py_XDECREF(fd);
Py_XDECREF(s);
if (PyErr_Occurred())
{
std::string result;
PyObject* ptype;
PyObject* pvalue;
PyObject* ptraceback;
PyErr_Fetch(&ptype, &pvalue, &ptraceback);
PyErr_NormalizeException(&ptype, &pvalue, &ptraceback); // in order to convert pvalue from tuples to real objects
//Attach exception name first
PyObject* objectStr = PyObject_GetAttrString(ptype, "__name__");
result = PyString_AS_STRING(objectStr);
result = "Exception: " + result;;
Py_XDECREF(objectStr);
objectStr = PyObject_Str(pvalue);
if (objectStr != NULL) {
result = result + " was raised with error message : " + PyString_AS_STRING(objectStr);
Py_XDECREF(objectStr);
}
std::cout << result;
}
return 0;
}
Here is the python script I use
def fac(i):
print "Call to FAC(",i,") !"
if i <= 1:
return 1
else:
return i*fac(i-1)
print "Hello world"
print fac(4)
And here is the output
Hello world
Call to FAC( 4 ) !
Exception: NameError was raised with error message : global name 'fac' is not defined
While the expected output ( when run directly by invoking the script )
Hello world
Call to FAC( 4 ) !
Call to FAC( 3 ) !
Call to FAC( 2 ) !
Call to FAC( 1 ) !
24
Any ideas on how to proceed ?
Edit:
Platform: Windows 10 x64
Python: 2.7.14

Explanation:
In the following line, PyRun_File uses two different dictionaries; one for globals and one for locals.
PyObject * s = PyRun_File(PyFile_AsFile(fd), "script.py", Py_file_input, m_pGlobalDict, m_pLocalDict);
The strange part that fac function name is added to the local one of file.
I have no idea Why, But I really wanaa know.
So the recursive call fails as there is no fac inside locals of the function or the global.
Solution:
pass the same dictionary for both local and global
PyObject * s = PyRun_File(PyFile_AsFile(fd), "script.py", Py_file_input, m_pGlobalDict, m_pGlobalDict);

Python & Ctypes: Passing a struct to a function as a pointer to get back data

I've looked through other answers but can't seem to get this to work. I'm trying to call a function within a DLL for communicating with SMBus devices. This function takes a pointer to a struct, which has an array as one of it's fields. so...
In C:
typedef struct _SMB_REQUEST
{
unsigned char Address;
unsigned char Command;
unsigned char BlockLength;
unsigned char Data[SMB_MAX_DATA_SIZE];
} SMB_REQUEST;
I think I have to set values for the Address, Command and BlockLength while the DLL fills the Data array.
The function that requires this struct takes it as a pointer
SMBUS_API int SmBusReadByte( SMBUS_HANDLE handle, SMB_REQUEST *request );
So I've set up the struct in Python like so:
class SMB_REQUEST(ctypes.Structure):
_fields_ = [("Address", c_char),
("Command", c_char),
("BlockLength", c_char),
("Data", type(create_string_buffer(SMB_MAX_DATA_SIZE))]
*Note: I've also tried ctypes.c_char*SMB_MAX_DATA_SIZE for the data type*
To pass a pointer to a struct of this type to the function I have tried to initialise it first as follows:
data = create_string_buffer(SMB_MAX_DATA_SIZE)
smb_request = SMB_REQUEST('\x53', \x00', 1, data)
This responds with:
TypeError: expected string or Unicode object, c_char_Array_32 found
If I try leaving out the data array, like so:
smb_request = SMB_REQUEST('\x53', \x00', 1)
No, error.
However, then when I try to pass this to the function:
int_response = smbus_read_byte(smbus_handle, smb_request))
I get:
ArgumentError: argument 2: <type 'exceptions.TypeError'>: expected LP_SMB_REQUES
T instance instead of SMB_REQUEST
I've tried passing it as a pointer:
int_response = smbus_read_byte(smbus_handle, ctypes.POINTER(smb_request))
and I get:
----> 1
2
3
4
5
TypeError: must be a ctypes type
Here's how I've set up the art types:
smbus_read_byte.argtypes = (ctypes.c_void_p, ctypes.POINTER(SMB_REQUEST))
I've tried casting but still no go. Can anyone shed some light on this for me?
Update:
If I first initialise the struct like so:
smb_request = SMB_REQUEST('\xA6', '\x00', chr(1), 'a test string')
and then bass by reference:
int_response = smbus_receive_byte(smbus_handle, ctypes.byref(smb_request))
I get no error. However, the function returns -1 when it should return '0' for success and non-zero for a fail. Checking the value of smb_request.Data gives back 'a test string' so no change there.
Any suggestions as to what might be going on here would be greatly appreciated.
Thanks
UPDATE:
Since I've gotten a couple of enquiries about whether my handle is correct, here's how I'm using it. The header file for the DLL declares the following:
typedef void *SMBUS_HANDLE;
//
// This function call initializes the SMBus, opens the driver and
// allocates the resources associated with the SMBus.
// All SMBus API calls are valid
// after making this call except to re-open the SMBus.
//
SMBUS_API SMBUS_HANDLE OpenSmbus(void);
So here's how I'm doing this in python:
smbus_handle = c_void_p() # NOTE: I have also tried it without this line but same result
open_smbus = CDLL('smbus.dll').OpenSmbus
smbus_handle = open_smbus()
print 'SMBUS_API SMBUS_HANDLE OpenSmbus(void): ' + str(smbus_handle)
I call this before making the call to smbus_read_byte(). I have tried to set open_smbus.restype = c_void_p() but I get an error: TypeError: restype must be a type, a callable, or None

Here's a working example. It looks like you are passing the wrong type to the function.
Test DLL Code ("cl /W4 /LD x.c" on Windows)
#include <stdio.h>
#define SMBUS_API __declspec(dllexport)
#define SMB_MAX_DATA_SIZE 5
typedef void* SMBUS_HANDLE;
typedef struct _SMB_REQUEST
{
unsigned char Address;
unsigned char Command;
unsigned char BlockLength;
unsigned char Data[SMB_MAX_DATA_SIZE];
} SMB_REQUEST;
SMBUS_API int SmBusReadByte(SMBUS_HANDLE handle,SMB_REQUEST *request)
{
unsigned char i;
for(i = 0; i < request->BlockLength; i++)
request->Data[i] = i;
return request->BlockLength;
}
SMBUS_API SMBUS_HANDLE OpenSmbus(void)
{
return (void*)0x12345678;
}
Python code
from ctypes import *
SMB_MAX_DATA_SIZE = 5
ARRAY5 = c_ubyte * SMB_MAX_DATA_SIZE
class SMB_REQUEST(Structure):
_fields_ = [
("Address", c_ubyte),
("Command", c_ubyte),
("BlockLength", c_ubyte),
("Data", ARRAY5)]
smbus_read_byte = CDLL('x').SmBusReadByte
smbus_read_byte.argtypes = [c_void_p,POINTER(SMB_REQUEST)]
smbus_read_byte.restype = c_int
open_smbus = CDLL('x').OpenSmbus
open_smbus.argtypes = []
open_smbus.restype = c_void_p
handle = open_smbus()
print 'handle = %08Xh' % handle
smb_request = SMB_REQUEST(1,2,5)
print 'returned =',smbus_read_byte(handle,byref(smb_request))
print 'Address =',smb_request.Address
print 'Command =',smb_request.Command
print 'BlockLength =',smb_request.BlockLength
for i,b in enumerate(smb_request.Data):
print 'Data[%d] = %02Xh' % (i,b)
Output
handle = 12345678h
returned = 5
Address = 1
Command = 2
BlockLength = 5
Data[0] = 00h
Data[1] = 01h
Data[2] = 02h
Data[3] = 03h
Data[4] = 04h

You're almost there. You should use c_char * SMB_MAX_DATA_SIZE as the type for the definition of Data. This works for me on Mac OS X:
Shared library:
$ cat test.c
#include <stdio.h>
#define SMB_MAX_DATA_SIZE 16
typedef struct _SMB_REQUEST
{
unsigned char Address;
unsigned char Command;
unsigned char BlockLength;
unsigned char Data[SMB_MAX_DATA_SIZE];
} SMB_REQUEST;
int SmBusReadByte(void *handle, SMB_REQUEST *request)
{
printf("SmBusReadByte: handle=%p request=[%d %d %d %s]\n", handle,
request->Address, request->Command, request->BlockLength, request->Data);
return 13;
}
$ gcc test.c -fPIC -shared -o libtest.dylib
Python driver:
$ cat test.py
import ctypes
SMB_MAX_DATA_SIZE = 16
class SMB_REQUEST(ctypes.Structure):
_fields_ = [("Address", ctypes.c_ubyte),
("Command", ctypes.c_ubyte),
("BlockLength", ctypes.c_ubyte),
("Data", ctypes.c_char * SMB_MAX_DATA_SIZE)]
libtest = ctypes.cdll.LoadLibrary('libtest.dylib')
req = SMB_REQUEST(1, 2, 3, 'test')
result = libtest.SmBusReadByte(ctypes.c_voidp(0x12345678), ctypes.byref(req))
print 'result: %d' % result
$ python test.py
SmBusReadByte: handle=0x12345678 request=[1 2 3 test]
result: 13
UPDATE
You're having problems because you need to set the result type of open_smbus to void*. By default, ctypes assumes that functions return ints. You need to say this:
open_smbus.restype = ctypes.c_void_p
You were getting an error because you were using c_void_p() (note the extra parentheses). There's an important distinction between c_void_p and c_void_p(). The former is a type, and the latter is an instance of a type. c_void_p represents the C type void*, whereas c_void_p() represents an actual pointer instance (with a default value of 0).

Try changing
("Data", type(create_string_buffer(SMB_MAX_DATA_SIZE))
to
("Data", (c_char * SMB_MAX_DATA_SIZE)]