Develop Reference

Ctypes calling Go Dll with arguments (C string)

How to pass a string as argument from Python to a Go Dll using ctypes:
Go-code:
package main
import "C"
import "fmt"
//export GetInt
func GetInt() int32 {
return 42
}
//export GetString
func GetString() {
fmt.Println("Foo")
}
//export PrintHello
func PrintHello(name string) {
// if name == "hello" { ... }
fmt.Printf("From DLL: Hello, %s!", name)
}
func main() {
// Need a main function to make CGO compile package as C shared library
}
Compiled on MacOs using: GOOS=windows GOARCH=386 CGO_ENABLED=1 CC=i686-w64-mingw32-gcc go build -buildmode=c-shared -o ./dist/perf_nlp.dll
Python code:
import ctypes
def getString():
nlp = ctypes.windll.LoadLibrary("H:/perf_nlp.dll")
dllFunc = nlp.GetString
dllFunc.restype = ctypes.c_char_p
return dllFunc()
def getInt():
nlp = ctypes.windll.LoadLibrary("H:/perf_nlp.dll")
dllFunc = nlp.GetInt
dllFunc.restype = int
return dllFunc()
def readString():
nlp = ctypes.windll.LoadLibrary("H:/perf_nlp.dll")
dllFunc = nlp.ReadString
dllFunc.argtypes = [ctypes.c_char_p]
dllFunc.restype = ctypes.c_char_p
return dllFunc(b'Foo')
print(getInt())
print(getString())
print(readString()). # Fails
Out:
42
Foo
None
unexpected fault address 0x871000
fatal error: fault
[signal 0xc0000005 code=0x0 addr=0x871000 pc=0x623e501f]
goroutine 17 [running, locked to thread]:
runtime.throw(0x6245b592, 0x5)
/Users/foobar/.goenv/versions/1.14.15/src/runtime/panic.go:1116 +0x64 fp=0x1242fda4 sp=0x1242fd90 pc=0x623be404
runtime.sigpanic()
/Users/foobar/.goenv/versions/1.14.15/src/runtime/signal_windows.go:249 +0x1ed fp=0x1242fdb8 sp=0x1242fda4 pc=0x623ceb8d
runtime.memmove(0x12500011, 0x800588, 0x32efe4)
/Users/foobar/.goenv/versions/1.14.15/src/runtime/memmove_386.s:89 +0x7f fp=0x1242fdbc sp=0x1242fdb8 pc=0x623e501f
fmt.(*buffer).writeString(...)
/Users/foobar/.goenv/versions/1.14.15/src/fmt/print.go:82
fmt.(*fmt).padString(0x124a60b0, 0x800588, 0x32efe4)
/Users/foobar/.goenv/versions/1.14.15/src/fmt/format.go:110 +0x6c fp=0x1242fdfc sp=0x1242fdbc pc=0x6241576c
fmt.(*fmt).fmtS(0x124a60b0, 0x800588, 0x32efe4)
/Users/foobar/.goenv/versions/1.14.15/src/fmt/format.go:359 +0x4d fp=0x1242fe14 sp=0x1242fdfc pc=0x6241664d
fmt.(*pp).fmtString(0x124a6090, 0x800588, 0x32efe4, 0x73)
/Users/foobar/.goenv/versions/1.14.15/src/fmt/print.go:450 +0x188 fp=0x1242fe38 sp=0x1242fe14 pc=0x62418f58
fmt.(*pp).printArg(0x124a6090, 0x62447c80, 0x1248c110, 0x73)
/Users/foobar/.goenv/versions/1.14.15/src/fmt/print.go:698 +0x776 fp=0x1242fe80 sp=0x1242fe38 pc=0x6241ad56
fmt.(*pp).doPrintf(0x124a6090, 0x6245e0a5, 0x14, 0x1242ff48, 0x1, 0x1)
/Users/foobar/.goenv/versions/1.14.15/src/fmt/print.go:1030 +0x12b fp=0x1242fef0 sp=0x1242fe80 pc=0x6241d81b
fmt.Fprintf(0x62476550, 0x1248c0d8, 0x6245e0a5, 0x14, 0x1242ff48, 0x1, 0x1, 0x623e2fe7, 0x0, 0x12488030)
/Users/foobar/.goenv/versions/1.14.15/src/fmt/print.go:204 +0x52 fp=0x1242ff20 sp=0x1242fef0 pc=0x62417bd2
fmt.Printf(...)
/Users/foobar/.goenv/versions/1.14.15/src/fmt/print.go:213
main.ReadString(...)
/Users/foobar/__projects__/heine/db_dll/perf_nlp.go:19
main._cgoexpwrap_c3579cea1e16_ReadString(0x800588, 0x32efe4)
_cgo_gotypes.go:71 +0x8d fp=0x1242ff54 sp=0x1242ff20 pc=0x6241e9bd
runtime.call16(0x0, 0x32ef1c, 0x32ef68, 0x8, 0x0)
/Users/foobar/.goenv/versions/1.14.15/src/runtime/asm_386.s:565 +0x30 fp=0x1242ff68 sp=0x1242ff54 pc=0x623e3020
runtime.cgocallbackg1(0x0)
/Users/foobar/.goenv/versions/1.14.15/src/runtime/cgocall.go:332 +0x149 fp=0x1242ffb0 sp=0x1242ff68 pc=0x62393f59
runtime.cgocallbackg(0x0)
/Users/foobar/.goenv/versions/1.14.15/src/runtime/cgocall.go:207 +0xb5 fp=0x1242ffe0 sp=0x1242ffb0 pc=0x62393d85
runtime.cgocallback_gofunc(0x0, 0x0, 0x0, 0x0)
/Users/foobar/.goenv/versions/1.14.15/src/runtime/asm_386.s:806 +0x7e fp=0x1242fff0 sp=0x1242ffe0 pc=0x623e419e
runtime.goexit()
/Users/foobar/.goenv/versions/1.14.15/src/runtime/asm_386.s:1337 +0x1 fp=0x1242fff4 sp=0x1242fff0 pc=0x623e4651
Working solution:
//export ReadString
func ReadString(name *C.char) *C.char {
res := "";
goName := C.GoString(name);
if goName == "Foo" {
res = "From DLL: Hello, Foo"
}else{
res = "From DLL: Hello!"
}
return C.CString(res)
}
Python:
def readString():
nlp = ctypes.windll.LoadLibrary("H:/perf_nlp.dll")
dllFunc = nlp.ReadString
dllFunc.argtypes = [ctypes.c_char_p]
dllFunc.restype = ctypes.c_char_p
return dllFunc(b'cFoo')

A Go string and a C string are entirely unrelated (except in that both are called strings, which is a lie for at least one of them).
Here Python is sending a C string because you've told it to, but Go expects a Go string, which has a completely diffrent layout so it blows up. And if it didn't blow up at the callsite it'd probably blow up when the GC tries to handle the string, which it can't, because it's not a Go string.
You want to look at the magical "C" pseudo-package: you need to take in a *C.char and copy that to a Go string using C.GoString before you can pass it to anything expecting a go String. Or something along those lines, my experience with cgo (especially calling into it) is limited to avoiding this as a bad idea.
Regardless you probably want to at the very least read the cgo documentation in full, FFI is tricky at the best of time, and FFI between two managed languages much more so.

python ctypes and C++ pointers

C++ funtion:
DLLENTRY int VTS_API
SetParamValue( const char *paramName, void *paramValue ) //will return zero(0) in the case of an error
{
rtwCAPI_ModelMappingInfo* mmi = &(rtmGetDataMapInfo(PSA_StandAlone_M).mmi);
int idx = getParamIdx( paramName );
if (idx<0) {
return 0; //Error
}
int retval = capi_ModifyModelParameter( mmi, idx, paramValue );
if (retval == 1 ) {
ParamUpdateConst();
}
return retval;
}
and my Python code:
import os
from ctypes import *
print(os.getcwd())
os.chdir(r"C:\MY_SECRET_DIR")
print(os.getcwd())
PSAdll=cdll.LoadLibrary(os.getcwd()+"\PSA_StandAlone_1.dll")
setParam=PSAdll.SetParamValue
setParam.restype=c_int
setParam.argtypes=[c_char_p, c_void_p]
z=setParam(b"LDOGenSpdSetpoint", int(20) )
returns
z=setParam(b"LDO_PSA_GenSpdSetpoint01", int(20) )
WindowsError: exception: access violation reading 0x00000020
Any idea what can help?
I already tried POINTER and byref(), but I am getting the same output

SetParamValue expects a pointer (void* or c_void_p) as the 2nd argument, but you're passing an int. The function will interpret it as a pointer (memory address), and when attempting to dereference it (to get its content), it will segfault (Access Violation), as the process doesn't have permissions on that address.
To fix the problem, pass the proper arguments to the function:
z = setParam(b"LDOGenSpdSetpoint", pointer(c_int(20)))
You can find more details on [Python 3]: ctypes - A foreign function library for Python.

Can Python read from a Windows Powershell namedpipe?

I have the following named pipe created in Windows Powershell.
# .NET 3.5 is required to use the System.IO.Pipes namespace
[reflection.Assembly]::LoadWithPartialName("system.core") | Out-Null
$pipeName = "pipename"
$pipeDir = [System.IO.Pipes.PipeDirection]::InOut
$pipe = New-Object system.IO.Pipes.NamedPipeServerStream( $pipeName, $pipeDir )
Now, what i need is some Python code snippet to read from the above named pipe created. Can Python do that ?
Thanks in advance !

Courtesy :http://jonathonreinhart.blogspot.com/2012/12/named-pipes-between-c-and-python.html
Here's the C# Code
using System;
using System.IO;
using System.IO.Pipes;
using System.Text;
class PipeServer
{
static void Main()
{
var server = new NamedPipeServerStream("NPtest");
Console.WriteLine("Waiting for connection...");
server.WaitForConnection();
Console.WriteLine("Connected.");
var br = new BinaryReader(server);
var bw = new BinaryWriter(server);
while (true)
{
try
{
var len = (int)br.ReadUInt32(); // Read string length
var str = new string(br.ReadChars(len)); // Read string
Console.WriteLine("Read: \"{0}\"", str);
//str = new string(str.Reverse().ToArray()); // Aravind's edit: since Reverse() is not working, might require some import. Felt it as irrelevant
var buf = Encoding.ASCII.GetBytes(str); // Get ASCII byte array
bw.Write((uint)buf.Length); // Write string length
bw.Write(buf); // Write string
Console.WriteLine("Wrote: \"{0}\"", str);
}
catch (EndOfStreamException)
{
break; // When client disconnects
}
}
}
}
And here's the Python code:
import time
import struct
f = open(r'\\.\pipe\NPtest', 'r+b', 0)
i = 1
while True:
s = 'Message[{0}]'.format(i)
i += 1
f.write(struct.pack('I', len(s)) + s) # Write str length and str
f.seek(0) # EDIT: This is also necessary
print 'Wrote:', s
n = struct.unpack('I', f.read(4))[0] # Read str length
s = f.read(n) # Read str
f.seek(0) # Important!!!
print 'Read:', s
time.sleep(2)
Convert the C# code into a .ps1 file.

Python Wrapper to a C Callback

Trying to create a python callback which needs to be invoked while calling the C callback
from a dll in Windows environment. Please review the code below to understand the issue.
from ctypes import *
#---------qsort Callback-------------#
IntArray5 = c_int * 5
ia = IntArray5(5,1,7,33,99)
libc = cdll.msvcrt
qsort = libc.qsort
qsort.restype = None
CMPFUNC = CFUNCTYPE(c_int,POINTER(c_int),POINTER(c_int) )
test = 0
def py_cmp_func(a,b):
#print 'py_cmp_func:',a[0],b[0]
global test
test = 10000
return a[0]-b[0]
cmp_func = CMPFUNC(py_cmp_func)
qsort(ia, len(ia), sizeof(c_int), cmp_func)
print "global test=",test
for item in ia : print item
#----------Load DLL & Connect ------------#
gobiDLL = WinDLL("C:\LMS\QCWWAN2k.dll")
print 'Output of connect : ',gobiDLL.QCWWANConnect()
#----------SetByteTotalsCallback----------#
tx = POINTER(c_ulonglong)
rx = POINTER(c_ulonglong)
proto_callback = WINFUNCTYPE(c_void_p,tx,rx)
gtx = grx = 0 # Used to copy the response in the py_callback
def py_callback(t,r):
sleep(10)
print 'python callback ...'
print "tx=",t,"rx=",r
global gtx,grx
gtx = 5000 # gtx = t
grx = 2000 # grx = r
#return 0
callback = proto_callback(py_callback)
gobiDLL.SetByteTotalsCallback.restype = c_ulong
gobiDLL.SetByteTotalsCallback.argtypes = [proto_callback,c_byte]
print "SetByteTotalsCallback = ",gobiDLL.SetByteTotalsCallback(callback, c_byte(256))
print "gtx = ",gtx
print "grx = ",grx
The DLL Documents the Prototype and the callback for the SetByteTotalsCallback() method as shown below.
Prototype :
ULONG QCWWANAPI2K SetSessionStateCallback( tFNSessionState pCallback );
Callback :
void ByteTotalsCallback( ULONGLONG txTotalBytes, ULONGLONG rxTotalBytes );
OUTPUT :
>>>
global test= 10000
1
5
7
33
99
Output of connect : 0
SetByteTotalsCallback = 0
gtx = 0
grx = 0
>>>>
The current problem is that the whole program gets called properly,
but the python callback does not get called at all. The program exits with 0 status from
gobiDLL.SetByteTotalsCallback(callback, c_byte(256)) method, but the callback() method written
in python does not called during the call.
Could you please point out what could help enter the python callback ?
The other sample qsort() method passes the pointer to the python function pointer wonderfully.
At a loss to get the root cause of the issue here.
TIA,
Anthony

You can't. C/C++ functions can't access Python functions directly - that function prototype is probably expecting a pointer to C. Python will be passing it a pointer to its internal data structure for that particular function.
This is the time to build a C extension to python to wrap that DLL and expose it to Python. What you'd do is essentially have the C callback call the Python callback, since that can be done. To be clearer, what you want to achieve is:
| This side is C land i.e. "real" addresses
|
Python objects --> C extension -- register callback with --> DLL
| |
in the python | Calls callback
| |
interpreter <-------------- Callback in C extension <-------
|
The following is a very quick explanation for building a calling a python function from C. You'll need to build this code with the MSVC (or alternative tool) that was used to build your Python distribution; use depends.exe to find out which msvcXX.dll it is linked against.
Global state is generally considered bad, but for simplicity that's what I used:
static PyObject* pyfunc_event_handler = NULL;
static PyObject* pyfunc_event_args = NULL;
I then added a set handler function to make the process of setting the callback easier. However, you don't need to do that, you just need to
static PyObject* set_event_handler(PyObject *self, PyObject *args)
{
PyObject *result = NULL;
PyObject *temp;
The next line is the important one - I allow passing of two python objects (the O arguments to PyArg_ParseTuple. One object contains the function, the other its parameters.
if (PyArg_ParseTuple(args, "OO", &temp, &pyfunc_event_args)) {
if (!PyCallable_Check(temp)) {
PyErr_SetString(PyExc_TypeError, "parameter must be a function");
return NULL;
}
Sort out references. Python needs you to do this.
Py_XINCREF(temp); /* Add a reference to new func */
Py_XDECREF(pyfunc_event_handler); /* Dispose of previous callback */
pyfunc_event_handler = temp; /* Remember new callback */
/* Boilerplate to return "None" */
Py_INCREF(Py_None);
result = Py_None;
}
return result;
}
You can then call this elsewhere with:
PyObject* arglist = Py_BuildValue("(O)", pyfunc_event_args);
pyobjresult = PyObject_CallObject(pyfunc_event_handler, arglist);
Py_DECREF(arglist);
Don't forget the DECREF, you need Python to gc the arglist.
From python, using this is as simple as:
set_event_handler(func, some_tuple)
Where func has matching parameters like so:
def func(obj):
/* handle obj */
Things you probably want to read up on:
LoadLibrary (load DLL from C).
GetProcAddress (find a function to call).
Extending Python with C or C++ from the Python docs.

C to Python via SWIG: can't get void** parameters to hold their value

I have a C interface that looks like this (simplified):
extern bool Operation(void ** ppData);
extern float GetFieldValue(void* pData);
extern void Cleanup(p);
which is used as follows:
void * p = NULL;
float theAnswer = 0.0f;
if (Operation(&p))
{
theAnswer = GetFieldValue(p);
Cleanup(p);
}
You'll note that Operation() allocates the buffer p, that GetFieldValue queries p, and that Cleanup frees p. I don't have any control over the C interface -- that code is widely used elsewhere.
I'd like to call this code from Python via SWIG, but I was unable to find any good examples of how to pass a pointer to a pointer -- and retrieve its value.
I think the correct way to do this is by use of typemaps, so I defined an interface that would automatically dereference p for me on the C side:
%typemap(in) void** {
$1 = (void**)&($input);
}
However, I was unable to get the following python code to work:
import test
p = None
theAnswer = 0.0f
if test.Operation(p):
theAnswer = test.GetFieldValue(p)
test.Cleanup(p)
After calling test.Operation(), p always kept its initial value of None.
Any help with figuring out the correct way to do this in SWIG would be much appreciated. Otherwise, I'm likely to just write a C++ wrapper around the C code that stops Python from having to deal with the pointer. And then wrap that wrapper with SWIG. Somebody stop me!
Edit:
Thanks to Jorenko, I now have the following SWIG interface:
% module Test
%typemap (in,numinputs=0) void** (void *temp)
{
$1 = &temp;
}
%typemap (argout) void**
{
PyObject *obj = PyCObject_FromVoidPtr(*$1, Cleanup);
$result = PyTuple_Pack(2, $result, obj);
}
%{
extern bool Operation(void ** ppData);
extern float GetFieldValue(void *p);
extern void Cleanup(void *p);
%}
%inline
%{
float gfv(void *p){ return GetFieldValue(p);}
%}
%typemap (in) void*
{
if (PyCObject_Check($input))
{
$1 = PyCObject_AsVoidPtr($input);
}
}
The python code that uses this SWIG interface is as follows:
import test
success, p = test.Operation()
if success:
f = test.GetFieldValue(p) # This doesn't work
f = test.gvp(p) # This works!
test.Cleanup(p)
Oddly, in the python code, test.GetFieldValue(p) returns gibberish, but test.gfv(p) returns the correct value. I've inserting debugging code into the typemap for void*, and both have the same value of p! The call Any ideas about that?
Update: I've decided to use ctypes. MUCH easier.

I agree with theller, you should use ctypes instead. It's always easier than thinking about typemaps.
But, if you're dead set on using swig, what you need to do is make a typemap for void** that RETURNS the newly allocated void*:
%typemap (in,numinputs=0) void** (void *temp)
{
$1 = &temp;
}
%typemap (argout) void**
{
PyObject *obj = PyCObject_FromVoidPtr(*$1);
$result = PyTuple_Pack(2, $result, obj);
}
Then your python looks like:
import test
success, p = test.Operation()
theAnswer = 0.0f
if success:
theAnswer = test.GetFieldValue(p)
test.Cleanup(p)
Edit:
I'd expect swig to handle a simple by-value void* arg gracefully on its own, but just in case, here's swig code to wrap the void* for GetFieldValue() and Cleanup():
%typemap (in) void*
{
$1 = PyCObject_AsVoidPtr($input);
}

Would you be willing to use ctypes? Here is sample code that should work (although it is untested):
from ctypes import *
test = cdll("mydll")
test.Operation.restype = c_bool
test.Operation.argtypes = [POINTER(c_void_p)]
test.GetFieldValue.restype = c_float
test.GetFieldValue.argtypes = [c_void_p]
test.Cleanup.restype = None
test.Cleanup.argtypes = [c_void_p]
if __name__ == "__main__":
p = c_void_p()
if test.Operation(byref(p)):
theAnswer = test.GetFieldValue(p)
test.Cleanup(p)