I am not really sure if I'm taking the right path here.
(See Edit below)
I have a C++ library called myapp that compiles into libmyapp.so. This library contains a file called swigtest.h.
The thing I want to do, is wrapping this file with SWIG to create an interface for python and using this interface to change or read specific values from myapp.
Let's say myapp is a console application that has some global variables like static int myGlobalVar = 5.
In myapp I have a function to read and to write the global variable:
int ReadGlobalVar() { return myGlobalVar; }
void SetGlobalVar(int value) { myGlobalVar = value; }
Here are my steps for compiling the module:
swig -c++ -python swigtest.i
g++ -fpic -c swigtest.cpp swigtest_wrap.cxx -L/pathToMyApp -lmyapp -I/usr/include/python3.6
g++ -Wall -Wextra -shared swigtest.o swigtest_wrap.o -o _swigtest.so -L/pathToMyApp -lmyapp
I open and load the python module as following:
LD_LIBRARY_PATH=/pathToMyApp python3
import swigtest
...
The problem I face know is, that python and my application are still separated from one another. If i modify the variable using the python module, it does not affect the c++ application and vice versa.
Is it even possible to link those two together or is SWIG only used to reuse C++ code in python?
I think the big problem here is that I have only a small understanding of how libraries are linked and created under C++.
I found a few tutorials on using SWIG with Python for example:http://books.gigatux.nl/mirror/pythonprogramming/0596000855_python2-CHP-19-SECT-8.html
and https://www.geeksforgeeks.org/wrapping-cc-python-using-swig-set-1/
===================
Edit:
I created another example application for this scenario:
1) files:
main.cpp
#include <stdio.h>
#include <unistd.h>
#include <iostream>
#include "Swigtest.h"
using namespace std;
int main()
{
Swigtest swigtest;
for(;;)
{
sleep(1);
cout << "Global Variable value:" << swigtest.ReadGlobalVar() << endl;
}
}
Swigtest.h
#ifndef SWIGTEST_H
#define SWIGTEST_H
static int myGlobalVar = 5;
class Swigtest
{
public:
Swigtest();
void SetGlobalVar(int var){ myGlobalVar = var; }
int ReadGlobalVar() { return myGlobalVar; }
};
#endif // SWIGTEST_H
Swigtest.cpp
#include "Swigtest.h"
Swigtest::Swigtest()
{
}
Swigtest.i
%module swigtest
%{
#include "Swigtest.h"
%}
%include "Swigtest.h"
2) compile application
g++ -fpic -c Swigtest.cpp
g++ -fpic -c main.cpp
g++ Swigtest.o main.o -o libmyapp.so
3) compile python module
swig -c++ -python Swigtest.i
g++ -fpic -c Swigtest.cpp Swigtest_wrap.cxx -L. -lmyapp -I/usr/include/python3.6
g++ -Wall -Wextra -shared Swigtest.o Swigtest_wrap.o -o _swigtest.so -L. -lmyapp
now I have my application in myapp.so and the python extension in _swigtest.so
4) executing test
executing application
I launch the application by executing it from the shell:
./libmyapp.so
Output:
Global Variable value:5
Global Variable value:5
Global Variable value:5
Global Variable value:5
Global Variable value:5
Global Variable value:5
Global Variable value:5
Global Variable value:5
Global Variable value:5
Global Variable value:5
Global Variable value:5
Global Variable value:5
Global Variable value:5
Global Variable value:5
Global Variable value:5
testing python module
in another terminal i open python (at the same path where libmyapp.sois placed) and import the module
LD_LIBRARY_PATH=. python3
output:
Python 3.6.9 (default, Nov 7 2019, 10:44:02)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import swigtest
>>> a=swigtest.Swigtest()
>>> a.ReadGlobalVar()
5
>>> a.SetGlobalVar(3)
>>> a.ReadGlobalVar()
3
>>> a.ReadGlobalVar()
3
>>>
in the meantime the c++ application I launched from the terminal is happily outputting its value of 5.
What do I want?
I want that the change in the value also affects the c++ application. Above I changed the value from 5 to 3 in the python module, but the c++ application was unaffected from it.
Is this possible with SWIG? or am I doing something wrong here.
In Python it changes because it changes in C++. But it changes the C++ value inside the Python process. Each process run in a completely separated context. So when you launch the C++ process separately it runs in completely other context, having its own set of variables, memory, registries. No matter what you do in a process will not affect the other process. No matter how many processes you launch, none will affect the content of the other. In order to send data from one process to another you need a way to communicate via Inter Process Communication (IPC). You send data into via files, sockets, pipes, RPC, messages.
Related
i am trying to wrap the C++ library PyrochloreAFM.hpp which itself uses the lib boost/randoom.hpp so that i can import it as a python module. The C++ part itself works fine and i can succesfully import and use all of this from my main.
#include "PyrochloreAFM.hpp"
int main (int argc, const char *argv[]) {
PyrochloreAFM pyrochloreAFM (¶meters, &statistics, &rng);
}
Now following a tutorial i set up my c++ wrapper:
// pybind11_wrapper.cpp
#include <pybind11/pybind11.h>
#include <PyrochloreAFM.hpp>
PYBIND11_MODULE(pybind11_example, m) {
m.doc() = "pybind11 example plugin"; // Optional module docstring
m.def("cpp_function", &PyrochloreAFM, "A function that multiplies two numbers");
}
and my tasks.py file
# tasks.py
import invoke
invoke.run(
"g++ -O3 -Wall -Werror -shared -std=c++11 -fPIC PyrochloreAFM.cpp "
"-o libpyro.so "
)
However now $invoke build-PyrochloreAFM or even $invoke --list seem to have lost the track of the standard C++ library.
In file included from PyrochloreAFM.cpp:1:
./Parameters.hpp:16:10: fatal error: 'boost/random.hpp' file not found
#include "boost/random.hpp" // tested with boost 1.53
^~~~~~~~~~~~~~~~~~
1 error generated.
This might be a simple PATH problem so i would be very glad for any tips!
Thank you, Andres!
"Of course" the tasks.py file which compiles the .cpp file through the python module invoke has to instruct the compiler g++ to use the libraries we desire. In my case it is the flag -I /opt/homebrew/Cellar/boost/1.80.0/include i also use in my makefile or directly in terminal.
I want to use SWIG to create Python bindings for C library, but I have some troubles with it. I used the following .c and .i files from tutorial.
example.c
/* Compute factorial of n */
int fact(int n) {
if (n <= 1)
return 1;
else
return n*fact(n-1);
}
/* Compute n mod m */
int my_mod(int n, int m) {
return(n % m);
}
double My_variable;
example.i
%module example
%{
extern double My_variable;
extern int fact(int);
extern int my_mod(int n, int m);
%}
extern double My_variable;
extern int fact(int);
extern int my_mod(int n, int m);
Commands that I used:
$ swig -python -py3 example.i
$ gcc -c -fpic example.c example_wrap.c -I/usr/include/python3.6
$ gcc -shared example.o example_wrap.o -o example.so
And when I try to import it in python3 I am getting this
>>> import example
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ImportError: dynamic module does not define module export function (PyInit_example)
Any help appreciated
SWIG generates a module with underscore prepended by default, so you'll get a module called _example which needs to be in _example.so.
This is explained in the SWIG documentation 34.2.3 Hand compiling a dynamic module:
While the preferred approach to building an extension module is to use the distutils, some people like to integrate building extensions with a larger build system, and thus may wish to compile their modules without the distutils. To do this, you need to compile your program using commands like this (shown for Linux):
$ swig -python example.i
$ gcc -O2 -fPIC -c example.c
$ gcc -O2 -fPIC -c example_wrap.c -I/usr/local/include/python2.5
$ gcc -shared example.o example_wrap.o -o _example.so
The exact commands for doing this vary from platform to platform. However, SWIG tries to guess the right options when it is installed. Therefore, you may want to start with one of the examples in the SWIG/Examples/python directory. If that doesn't work, you will need to read the man-pages for your compiler and linker to get the right set of options. You might also check the SWIG Wiki for additional information.
When linking the module, the name of the output file has to match the name of the module prefixed by an underscore. If the name of your module is example, then the name of the corresponding object file should be _example.so or _examplemodule.so. The name of the module is specified using the %module directive or the -module command line option.
Compatibility Note: In SWIG-1.3.13 and earlier releases, module names did not include the leading underscore. This is because modules were normally created as C-only extensions without the extra Python support file (instead, creating Python code was supported as an optional feature). This has been changed in SWIG-1.3.14 and is consistent with other Python extension modules. For example, the socket module actually consists of two files; socket.py and _socket.so. Many other built-in Python modules follow a similar convention.
I have successfully followed this example for how to connect C++ and python. It works fine when I use the given Makefile. When I try to use cmake instead, it does not go as well.
C++ Code:
#include <boost/python.hpp>
#include <iostream>
extern "C"
char const* greet()
{
return "hello, world";
}
BOOST_PYTHON_MODULE(hello_ext)
{
using namespace boost::python;
def("greet", greet);
}
int main(){
std::cout<<greet()<<std::endl;
return 0;
}
Makefile:
# location of the Python header files
PYTHON_VERSION = 27
PYTHON_DOT_VERSION = 2.7
PYTHON_INCLUDE = /usr/include/python$(PYTHON_DOT_VERSION)
# location of the Boost Python include files and library
BOOST_INC = /usr/include
BOOST_LIB = /usr/lib/x86_64-linux-gnu/
# compile mesh classes
TARGET = hello_ext
$(TARGET).so: $(TARGET).o
g++ -shared -Wl,--export-dynamic $(TARGET).o -L$(BOOST_LIB) -lboost_python-py$(PYTHON_VERSION) -L/usr/lib/python$(PYTHON_DOT_VERSION)/config-x86_64-linux-gnu -lpython$(PYTHON_DOT_VERSION) -o $(TARGET).so
$(TARGET).o: $(TARGET).cpp
g++ -I$(PYTHON_INCLUDE) -I$(BOOST_INC) -fPIC -c $(TARGET).cpp
When I compile this I get a .so file that can be included in the script
import sys
sys.path.append('/home/myname/Code/Trunk/TestBoostPython/build/')
import libhello_ext_lib as hello_ext
print(hello_ext.greet())
I really want to use cmake instead of a manually written Makefile so I wrote this:
cmake_minimum_required(VERSION 3.6)
PROJECT(hello_ext)
# Find Boost
find_package(Boost REQUIRED COMPONENTS python-py27)
set(PYTHON_DOT_VERSION 2.7)
set(PYTHON_INCLUDE /usr/include/python2.7)
set(PYTHON_LIBRARY /usr/lib/python2.7/config-x86_64-linux-gnu)
include_directories(${PYTHON_INCLUDE} ${Boost_INCLUDE_DIRS})
SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -lrt -O3")
SET(EXECUTABLE_OUTPUT_PATH ${PROJECT_SOURCE_DIR}/bin)
SET(LIBNAME hello_ext_lib)
add_library(${LIBNAME} SHARED src/hello_ext.cpp)
add_executable(${PROJECT_NAME} src/hello_ext.cpp)
TARGET_LINK_LIBRARIES(${PROJECT_NAME} ${Boost_LIBRARIES} -lpython2.7 -fPIC)
TARGET_LINK_LIBRARIES(${LIBNAME} ${Boost_LIBRARIES} -lpython2.7 -fPIC -shared)
Here I currently type the Python-paths by hand but I have also tried using fin_package(PythonLibs) without success.
The program compiles fine and executes when I run the executable file in ../bin/. However, when I run the python script I get always:
ImportError: dynamic module does not define init function (initlibhello_ext_lib)
I found this which says this can happen if the lib and the executable have different names. Which indeed is the case, but how can I obtain the .so with correct name?
I also tried to not compile the executable but only the library. That did also not work.
BOOST_PYTHON_MODULE(hello_ext) creates an init function "inithello_ext", which should correspond to a module "hello_ext". But you are trying to import "libhello_ext_lib".
Give the module the same name as the filename. E.g. BOOST_PYTHON_MODULE(libhello_ext_lib).
Hi everyone and thanks for trying to help me !
I encounter trouble when trying to import a python module generated by swig.
I have a basic library "example" containing few methods.
Next to it I have a main program dynamically linked to python.
This program imports the generated module and calls a function in it.
If my library example is a shared one, named _example.so, everything works perfectly, and I can import it in python.
But if my library is static, _example.a, and linked to the main program, then I will have the error "no module named _example was found" unless I add a call to SWIG_init() in the main function.
What exactly does SWIG_init() , and when should I use it ? It seems quite weird to me because it's never said in the documentation to do such a call.
I know that dealing with a .so shared library is better but I try to reproduce the behavior of what I have on a big project at work, so I really have to understand what happens when the module is static.
Here is my main file :
#include "Python.h"
#include <iostream>
#if PY_VERSION_HEX >= 0x03000000
# define SWIG_init PyInit__example
#else
# define SWIG_init init_example
#endif
#ifdef __cplusplus
extern "C"
#endif
#if PY_VERSION_HEX >= 0x03000000
PyObject*
#else
void
#endif
SWIG_init(void);
int main (int arc, char** argv)
{
Py_Initialize();
SWIG_init(); // needed only using the statically linked version of example ?
PyRun_SimpleString("print \"Hello world from Python !\"");
PyRun_SimpleString("import sys");
PyRun_SimpleString("sys.path.append(\"/path/to/my/module\")");
PyRun_SimpleString("import example");
PyRun_SimpleString("a = example.Example()");
PyRun_SimpleString("print a.fact(5)");
}
Here is how things are generated :
swig -c++ -python example.i
g++ -fpic -c example.cpp example_wrap.cxx -I/include/python2.7 -lstdc++
ar rvs libexample.a example.o example_wrap.o
// to generate dynamic instead of static : g++ -shared example.o example_wrap.o -o _example.so
g++ main.cpp -I/include/python2.7 libexample.a -lstdc++ -L/lib/python -lpython2.7 -o main
What you are calling is the init function of the native python module _example that is loaded by the SWIG generated python wrapper. For python 2 this function is named init_example, and for python 3 it is named PyInit__example.
Every python extension with C or C++ needs such a function, it basically initializes everything and registers the name of the module and all the methods available for it. In your case SWIG has generated this function for you.
The reason you have to call this function yourself when you compiled the library statically is simply that the python wrapper example imports the native module _example which is by the python convention a shared object, which you did not compile, and which is thus not found.
By calling SWIG_init, you "preload" the module, so python does not try to reimport it, so it works even though there is no shared object anywhere on the python module path.
If you have the shared object for your module, python will call this function for you after loading the shared object and you don't have to worry about this.
Simple C++ example class I want to talk to in a file called foo.cpp
#include <iostream>
Since ctypes can only talk to C functions, you need to provide those declaring them as extern "C"
extern "C" {
Foo* Foo_new(){ return new Foo(); }
void Foo_bar(Foo* foo){ foo->bar(); }
}
class Foo{
public:
void bar(){
std::cout << "Hello" << std::endl;
}
};
compile this to a shared library
g++ -c -fPIC foo.cpp -o foo.o
g++ -shared -Wl,-soname,libfoo.so -o libfoo.so foo.o
finally I have wrote python wrapper
from ctypes import cdll
lib = cdll.LoadLibrary('./libfoo.so')
class Foo(object):
def __init__(self):
self.obj = lib.Foo_new()
def bar(self):
lib.Foo_bar(self.obj)
f = Foo()
f.bar() #prints "Hello" on the screen
"My main intension is to compile C++ code in eclipse and call the C++ function from python in Linux". This works fine when I compiled C++ code in Linux and call the C++ method from python in Linux. But it doesn't work if I compile C++ code in eclipse and call the C++ method from python in Linux.
Error message:
symbol not found
I am new to the eclipse tool chain, But I am giving compiler option and linking option in as in this
g++ -c -fPIC foo.cpp -o foo.o
g++ -shared -Wl,-soname,libfoo.so -o libfoo.so foo.o
Snapshot of eclipse compiler option and linking option will be highly appreciated. Please help me in sorting out this issue. Thanks in advance
You need to create two projects in the Eclipse.
Makefile project with existing code. (File->New->Makefile project with existing code). In this project you must point to your foo.cpp file. Then in the project folder you must create file which name is "Makefile". Makefile must contain folowing lines:
all:
g++ -c -fPIC foo.cpp -o foo.o
g++ -shared -W1,-soname,libfoo.so -o libfoo.so foo.o
clean:
rm -f libfoo.so
Then You must create rules ("all" and "clean") for this project in the "Make Target" window. If you don't see this window You must do Window->Show view->Make Target. Thus you can create libfoo.so file using Eclipse when double-clicked on the "all" rule in the "Make target" view.
At this moment You can create PyDev project with foo.py file. If you don't know about PyDev you must go to this site. It is Eclipse plugin for python language. When you will have installed this plugin You will can to work with your python file under the Eclipse.
See some images.