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content/blog/pythonswig.md

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+---
+title: "C++ within Python with SWIG"
+date: 2020-10-27T23:49:54-04:00
+draft: false
+tags: []
+---
+
+For performance reasons, it can be useful to write functions in C/C++ which can then be called within Python. This will be an introductory post, in where we will call a simple C++ function (with a dependency) within Python using [SWIG](http://swig.org/).
+
+First we need to install SWIG:
+
+```bash
+sudo apt install swig
+```
+
+We're going to use [GNU MP](https://gmplib.org/) in order to have arbitrary precision arithmetic for our factorial function. 
+
+```bash
+sudo apt install libgmp-dev
+```
+
+## Source Setup
+
+Normally people use headers for larger C++ programs, though we're going to create one just so we can see how to include it later in SWIG. Let's called this file `factorial.hpp`
+
+```c++
+#ifndef FACTORIAL_H
+#define FACTORIAL_H
+std::string fact(unsigned int n);
+#endif
+```
+
+In order to get it the large number from C++ to Python. We are going to use `std::string` as the return of our `fact` function.
+
+Here is the source `factorial.cpp`
+
+```c++
+#include <gmpxx.h>
+#include "factorial.hpp"
+
+std::string fact(unsigned int n) {
+        if (n == 0) {
+                n = 1;
+        }
+        mpz_class result(n);
+        while (n > 1) {
+                n--;
+                result *= n;
+        }
+        return result.get_str(10); // Base 10
+}
+
+```
+
+Now that we have our C++ code, we need to create a swig template file called `factorial.i`
+
+```
+ %module factorial
+ %{
+ #include "factorial.hpp"
+ %}
+ 
+ %include <std_string.i>
+ %include "factorial.hpp"
+ 
+```
+
+Since we're returning a `std::string` we need to tell SWIG what that is. We do this through the `<std_string.i>` include.
+
+We can now ask SWIG to write the C++ code that will interface with Python. This will create the files  `factorial_wrap.cxx` and `factorial.py`.
+
+```bash
+swig -c++ -python factorial.i
+```
+
+## Compilation and Linkage
+
+Let's compile our C++ code.
+
+```bash
+g++ -O2 -fPIC -c factorial.cpp 
+```
+
+| Flag  | Description                                                  |
+| ----- | ------------------------------------------------------------ |
+| -O2   | Perform nearly all supported optimizations that don't involve a space-speed tradeoff. |
+| -fPIC | Create Position-Independent Code                             |
+| -c    | Don't link at this time                                      |
+
+To compile `factorial_wrap.cxx` we need to include the directory where `Python.h` lives. You can find this by issuing the command `locate Python.h`. Below is where it is located on my system.
+
+```bash
+g++ -O2 -fPIC -c factorial_wrap.cxx -I/home/user/.pyenv/versions/3.8.2/include/python3.8/
+```
+
+Finally let's create the needed shared object file by linking `factorial.o`, `factorial_wrap.o`, and the GNU MP libraries.
+
+```bash
+g++ -O2 -fPIC -shared factorial.o factorial_wrap.o -lgmpxx -lgmp -o _factorial.so
+```
+
+It is important that our final output is called `_` + module_name.so
+
+We should at this time be able to open up `python` and import our function.
+
+```python
+import factorial
+factorial.fact(5)
+```
+
+If you run into any errors, the [SWIG Documentation](http://www.swig.org/Doc3.0/Python.html#Python_nn3) is quite helpful.
+
+In order to not have to type out the compiling and linking commands every time, here is a  Makefile
+
+```makefile
+CC=g++
+CFLAGS=-O2 -fPIC -Wall
+PYTHON_PATH=/home/user/.pyenv/versions/3.8.2/include/python3.8/
+
+all: _factorial.so
+
+_factorial.so: factorial.o factorial_wrap.o
+	$(CC) $(CFLAGS) -shared factorial.o factorial_wrap.o -lgmpxx -lgmp -o _factorial.so
+
+factorial_wrap.o: factorial_wrap.cxx
+	$(CC) $(CFLAGS) -c factorial_wrap.cxx -I$(PYTHON_PATH)
+
+factorial.o: factorial.cpp
+	$(CC) $(CFLAGS) -c factorial.cpp
+
+factorial_wrap.cxx: factorial.i
+	swig -c++ -python factorial.i
+
+clean:
+	rm *.o *.so factorial_wrap.cxx factorial.py
+```
+
+Then you can call `make clean` to clean up everything and `make` to run all the individual compilation steps we did before.