How can I use a DLL file from Python?

For ease of use, ctypes is the way to go.

The following example of ctypes is from actual code I've written (in Python 2.5). This has been, by far, the easiest way I've found for doing what you ask.

import ctypes

# Load DLL into memory.

hllDll = ctypes.WinDLL ("c:\\PComm\\ehlapi32.dll")

# Set up prototype and parameters for the desired function call.
# HLLAPI

hllApiProto = ctypes.WINFUNCTYPE (
    ctypes.c_int,      # Return type.
    ctypes.c_void_p,   # Parameters 1 ...
    ctypes.c_void_p,
    ctypes.c_void_p,
    ctypes.c_void_p)   # ... thru 4.
hllApiParams = (1, "p1", 0), (1, "p2", 0), (1, "p3",0), (1, "p4",0),

# Actually map the call ("HLLAPI(...)") to a Python name.

hllApi = hllApiProto (("HLLAPI", hllDll), hllApiParams)

# This is how you can actually call the DLL function.
# Set up the variables and call the Python name with them.

p1 = ctypes.c_int (1)
p2 = ctypes.c_char_p (sessionVar)
p3 = ctypes.c_int (1)
p4 = ctypes.c_int (0)
hllApi (ctypes.byref (p1), p2, ctypes.byref (p3), ctypes.byref (p4))

The ctypes stuff has all the C-type data types (int, char, short, void*, and so on) and can pass by value or reference. It can also return specific data types although my example doesn't do that (the HLL API returns values by modifying a variable passed by reference).

In terms of the specific example shown above, IBM's EHLLAPI is a fairly consistent interface.

All calls pass four void pointers (EHLLAPI sends the return code back through the fourth parameter, a pointer to an int so, while I specify int as the return type, I can safely ignore it) as per IBM's documentation here. In other words, the C variant of the function would be:

int hllApi (void *p1, void *p2, void *p3, void *p4)

This makes for a single, simple ctypes function able to do anything the EHLLAPI library provides, but it's likely that other libraries will need a separate ctypes function set up per library function.

The return value from WINFUNCTYPE is a function prototype but you still have to set up more parameter information (over and above the types). Each tuple in hllApiParams has a parameter "direction" (1 = input, 2 = output and so on), a parameter name and a default value - see the ctypes doco for details

Once you have the prototype and parameter information, you can create a Python "callable" hllApi with which to call the function. You simply create the needed variable (p1 through p4 in my case) and call the function with them.


This page has a very simple example of calling functions from a DLL file.

Paraphrasing the details here for completeness:

It's very easy to call a DLL function in Python. I have a self-made DLL file with two functions: add and sub which take two arguments.

add(a, b) returns addition of two numbers
sub(a, b) returns substraction of two numbers

The name of the DLL file will be "demo.dll"

Program:

from ctypes import*
# give location of dll
mydll = cdll.LoadLibrary("C:\\demo.dll")
result1= mydll.add(10,1)
result2= mydll.sub(10,1)
print "Addition value:"+result1
print "Substraction:"+result2

Output:

Addition value:11
Substraction:9


Building a DLL and linking it under Python using ctypes

I present a fully worked example on how building a shared library and using it under Python by means of ctypes. I consider the Windows case and deal with DLLs. Two steps are needed:

  1. Build the DLL using Visual Studio's compiler either from the command line or from the IDE;
  2. Link the DLL under Python using ctypes.

The shared library

The shared library I consider is the following and is contained in the testDLL.cpp file. The only function testDLL just receives an int and prints it.

#include <stdio.h>
​
extern "C" {
​
__declspec(dllexport)
​
void testDLL(const int i) {
    printf("%d\n", i);
}
​
} // extern "C"

Building the DLL from the command line

To build a DLL with Visual Studio from the command line run

"C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\Tools\vsdevcmd"

to set the include path and then run

cl.exe /D_USRDLL /D_WINDLL testDLL.cpp /MT /link /DLL /OUT:testDLL.dll

to build the DLL.

Building the DLL from the IDE

Alternatively, the DLL can be build using Visual Studio as follows:

  1. File -> New -> Project;
  2. Installed -> Templates -> Visual C++ -> Windows -> Win32 -> Win32Project;
  3. Next;
  4. Application type -> DLL;
  5. Additional options -> Empty project (select);
  6. Additional options -> Precompiled header (unselect);
  7. Project -> Properties -> Configuration Manager -> Active solution platform: x64;
  8. Project -> Properties -> Configuration Manager -> Active solution configuration: Release.

Linking the DLL under Python

Under Python, do the following

import os
import sys
from ctypes import *

lib = cdll.LoadLibrary('testDLL.dll')

lib.testDLL(3)

ctypes can be used to access dlls, here's a tutorial:

http://docs.python.org/library/ctypes.html#module-ctypes

Tags:

Python

Dll

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