C to Python Function

Question

hondros 25 Junior Poster

16 Years Ago

I have the following function (from windows.h) which I am trying to use the Python C API to allow python to call it. This is the function:

BOOL ReadConsoleOutputCharacter(
  HANDLE hConsoleOutput, (Type Handle to Change (use STD_OUTPUT))
  LPTSTR lpCharacter, (char pointer to store Characters read)
  DWORD nLength, (length to read)
  COORD dwReadCoord, (Type COORD to start reading at)
  LPDWORD lpNumberOfCharsRead (length actually read)

This is a function that uses it correctly:

#include <windows.h>
#include <iostream>
using namespace std;

int main()
{
    HANDLE hOut;
    char Letters[5];
    COORD Where;
    DWORD NumRead;
    int i;

    hOut = GetStdHandle(STD_OUTPUT_HANDLE);

    cout << "A line of little consequence." << endl;

    Where.X = 4;
    ReadConsoleOutputCharacter(hOut,
                               Letters,
                               5,
                               Where,
                               &NumRead);
    cout << "5 letters starting from (4,0) ";
    cout << endl;
	cin.get(); //To see output

    return 0;
}

This is the function that I have written to try to allow python to access it:

static PyObject *
WindowsH_ReadCharacters(PyObject *self, PyObject *args)
{
	int length;
	length = 1;
	int x;
	int y;
	
	if (!PyArg_ParseTuple(args, "iii", &length, &x, &y))
		return PyInt_FromLong(1);

	char read[60000]; //I know this is a large number, I'm not sure how much the user will need.
	char *_string;
	COORD position;
	position.X = x;
	position.Y = y;

	HANDLE hOut;
	hOut = GetStdHandle(STD_OUTPUT_HANDLE);

	ReadConsoleOutputCharacter(
		hOut,
		read,
		length,
		position,
		NULL);

	for(x = 0; x <= length; x++)
	{
		_string[x] = read[x];
	}



	return Py_BuildValue("s", _string);
}

Can anybody help me? Python crashes as soon as I call this. If anyone has a way that could make it better, please share, it'll help =]
And if a mod thinks this will do better in C/C++, please move it there

c python

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Latest Post 16 Years Ago Latest Post by hondros

Dani AI

Generated 9 Months Ago

The crash and the short “\x…” output both come from a few concrete bugs in the extension code: writing from/to uninitialized pointers, not checking the API return value (so you never know how many chars were actually read), not null-terminating or using length-aware Python builders, and the Unicode/macro mismatch on Windows (ReadConsoleOutputCharacter may map to the W version when UNICODE is defined). was on the right track; ’s tip about the Tools/modulator helper is also useful for building wrappers, but the wrapper logic itself must be robust.

Checklist to fix the function

Validate Python args and return NULL on parse failure (don’t return arbitrary PyInts).
Cap and validate length (avoid huge allocations).
Allocate a buffer and use the API’s out-parameter to get the actual number of chars read.
Call the ANSI or wide API explicitly: use ReadConsoleOutputCharacterA if you want bytes, or ReadConsoleOutputCharacterW and Unicode APIs if building a Unicode/UTF-16 path.
Build the Python string with a length-aware call (Py_BuildValue("s#", ...) or PyString_FromStringAndSize / PyUnicode_FromWideChar) so no reliance on a C NUL terminator.
Free allocated memory and raise a Windows error on failure (PyErr_SetFromWindowsErr/GetLastError).

Example corrected pattern (adapt for your Python version and whether you need bytes or Unicode):

static PyObject *
WindowsH_ReadCharacters(PyObject *self, PyObject *args)
{
    int length, x, y;
    if (!PyArg_ParseTuple(args, "iii", &length, &x, &y))
        return NULL;

    if (length <= 0) {
        PyErr_SetString(PyExc_ValueError, "length must be > 0");
        return NULL;
    }
    if (length > 65536) length = 65536; /* safety cap */

    COORD pos = { (SHORT)x, (SHORT)y };
    DWORD actuallyRead = 0;
    char *buf = (char*)PyMem_Malloc((size_t)length);
    if (!buf) return PyErr_NoMemory();

    HANDLE hOut = GetStdHandle(STD_OUTPUT_HANDLE);
    if (!ReadConsoleOutputCharacterA(hOut, buf, (DWORD)length, pos, &actuallyRead)) {
        PyMem_Free(buf);
        PyErr_SetFromWindowsErr(GetLastError());
        return NULL;
    }

    PyObject *res = Py_BuildValue("s#", buf, (int)actuallyRead);
    PyMem_Free(buf);
    return res;
}

Troubleshooting notes

If the console is redirected (pipes/files), ReadConsoleOutputCharacter will fail; check the handle with GetFileType/GetConsoleMode or use GetConsoleScreenBufferInfo to validate the coordinates.
If your build defines UNICODE or you want proper wide-character output to Python3, call the W variant and return a PyUnicode object instead.

All 8 Replies

TrustyTony 888 ex-Moderator

16 Years Ago

Use

[/B] for C part
[code=C]
#include <windows.h>
#include <iostream>
using namespace std;

int main()
{
HANDLE hOut;
char Letters[5];
COORD Where;
DWORD NumRead;
int i;

hOut = GetStdHandle(STD_OUTPUT_HANDLE);

cout << "A line of little consequence." << endl;

Where.X = 4;
ReadConsoleOutputCharacter(hOut,
Letters,
5,
Where,
&NumRead);
cout << "5 letters starting from (4,0) ";
cout << endl;
cin.get(); //To see output

return 0;
}

This is the function that I have written to try to allow python to access it:

static PyObject *
WindowsH_ReadCharacters(PyObject *self, PyObject *args)
{
int length;
length = 1;
int x;
int y;

if (!PyArg_ParseTuple(args, "iii", &length, &x, &y))
return PyInt_FromLong(1);

char read[60000]; //I know this is a large number, I'm not sure how much the user will need.
char *_string;
COORD position;
position.X = x;
position.Y = y;

HANDLE hOut;
hOut = GetStdHandle(STD_OUTPUT_HANDLE);

ReadConsoleOutputCharacter(
hOut,
read,
length,
position,
NULL);
for(x = 0; x <= length; x++)
{
_string[x] = read[x];
}


return Py_BuildValue("s", _string);
}

What is your Python code you wanted to access from C?

Edited 16 Years Ago by TrustyTony because: n/a

TrustyTony 888 ex-Moderator

16 Years Ago

Did you check up this thing mentioned in documentation:

A more substantial example module is included in the Python source distribution as Modules/xxmodule.c. This file may be used as a template or simply read as an example. The script included in the source distribution or Windows install provides a simple graphical user interface for declaring the functions and objects which a module should implement, and can generate a template which can be filled in. The script lives in the Tools/modulator/ directory; see the README file there for more information.

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hondros 25 Junior Poster · Answer 1 · 2010-07-02T04:56:17+00:00

Thanks, thought it was something else. I want to be able to call it like this:

ReadCharacters(amount, x, y) -> 
   Read the amount of characters at position x, y;
   return a string with the characters

That will call this C funtion

BOOL ReadConsoleOutputCharacter(
HANDLE hConsoleOutput, (Type Handle to Change (use STD_OUTPUT))
LPTSTR lpCharacter, (char pointer to store Characters read)
DWORD nLength, (length to read)
COORD dwReadCoord, (Type COORD to start reading at)
LPDWORD lpNumberOfCharsRead (length actually read)

I'm having a big trouble with this one. I have a ton of other functions from windowsC.h that work. I realize there is win32Console, but it doesn't include all of the functions I need. I just need a C function that will allow me to use that function in Python.

hondros 25 Junior Poster · Answer 2 · 2010-07-03T02:01:01+00:00

I seem to not be able to find those files. Either in source or the build. I have a new dilemma. I was able to get the function to run through python, however, it now only returns a three character string of \x bits. Here's the code:

static PyObject *
WindowsH_ReadCharacters(PyObject *self, PyObject *args)
{
	DWORD length;
	int x;
	int y;
	
	if (!PyArg_ParseTuple(args, "iii", &length, &x, &y))
		return PyInt_FromLong(1);
	char* string=NULL;
	string = new char[length];
	COORD position;
	position.X = x;
	position.Y = y;

	HANDLE hOut;
	hOut = GetStdHandle(STD_OUTPUT_HANDLE);

	ReadConsoleOutputCharacter(
		hOut,
		string,
		length,
		position,
		NULL);

	return PyString_FromString(string);
}

* Note that the <string> header is not included in the source.

TrustyTony 888 ex-Moderator Team Colleague Featured Poster · Answer 3 · 2010-07-03T02:55:57+00:00

I seem to not be able to find those files.

Modulator.py seems to exist in net at least in source files:

hondros 25 Junior Poster · Answer 4 · 2010-07-03T02:59:34+00:00

Right. I downloaded that package and I can't seem to find it.

TrustyTony 888 ex-Moderator Team Colleague Featured Poster · Answer 5 · 2010-07-03T03:03:31+00:00

Right. I downloaded that package and I can't seem to find it.

drwxrwxr-x       0 2010-03-18 23:17 Python-2.6.5/Tools/modulator/
-rw-rw-r--    1360 2003-10-24 22:09 Python-2.6.5/Tools/modulator/EXAMPLE.py
-rwxrwxr-x    4824 2004-07-18 08:16 Python-2.6.5/Tools/modulator/genmodule.py
-rwxrwxr-x   14438 2004-07-18 08:16 Python-2.6.5/Tools/modulator/modulator.py
-rw-rw-r--     980 1999-02-16 18:19 Python-2.6.5/Tools/modulator/README
-rw-rw-r--    1477 2003-10-24 22:09 Python-2.6.5/Tools/modulator/ScrolledListbox.py
drwxrwxr-x       0 2010-03-18 23:17 Python-2.6.5/Tools/modulator/Templates/
-rw-rw-r--       0 2000-09-02 01:29 Python-2.6.5/Tools/modulator/Templates/copyright
-rw-rw-r--     113 1995-06-20 20:45 Python-2.6.5/Tools/modulator/Templates/mod
.................... etc ................................

hondros 25 Junior Poster · Answer 6 · 2010-07-03T03:13:48+00:00

Oh okay, I was looking in the wrong place xD
I'll take a look, thank you

C to Python Function

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