Tuesday, January 19, 2016

Using Boost C++ Library from C

This post is related to another post: Building C++ Application with Boost Library and Autotools in Linux. If you haven't know how to use Boost C++library in an autotools project, please read that post.

The purpose of this post is to explain what you need to do to use use Boost from your C language code. The following are the steps required to use Boost from C:
  1. Decide which part of Boost that you require in your C application.
  2. Wrap that part of Boost as "convenience" C library.
  3. Link your C application code to the "convenience" library. 
The steps above is easier said than done. Don't worry, I have provided a sample project over at github: https://github.com/pinczakko/boost_spsc_queue_c_wrapper. Just download the code and try to make sense of it.
- The code assumes that the platform in which it runs has a working pthread implementation.
- The code is not production quality code. Use it at your own risk.
The sample code basically wraps Boost SPSC (single producer single consumer) lockfree queue into a convenience C library and the sample application links to the convenience library.

Anyway, the most important part of the code that you need to understand is the part that "returns" a C++ class as an "opaque" C structure. Probably, it's a quite alien concept. But, I assure you that this alien concept is the core of C<-->C++ interoperability. Below is the relevant code snippets:

//---- START spsc_interface.hpp file -----------------
#include "spsc_wrapper.h"
#endif //SPSC_WRAPPER_H 

class spsc_interface {
    explicit spsc_interface();
    explicit spsc_interface(const spsc_interface &);
    ~spsc_interface() {};

//---- END spsc_interface.hpp file -----------------

//---- START spsc_wrapper.h file -----------------
#ifdef __cplusplus
extern "C" {
 typedef struct spsc_interface spsc_interface;

 spsc_interface *create_spsc_interface();

//---- END spsc_wrapper.h file -----------------

//---- START spsc_wrapper.cc file -----------------

#include "spsc_interface.hpp"
#include "spsc_wrapper.h"

#ifdef __cplusplus
extern "C" {

spsc_interface * create_spsc_interface()
    return new spsc_interface();

#ifdef __cplusplus
//---- END spsc_wrapper.cc file -----------------

As you see above, the wrapper function create_spsc_interface() returns an opaque object of type struct spsc_interface. In fact, this opaque object is a C++ class in disguise as you can see in the spsc_wrapper.cc snippet above. If you ask: How's that possible? Well, the answer lies in the linking process. Let's see the relevant snippets from src/Makefile.am:

wrapper_test_SOURCES = wrapper_test.c 
### Below is just a trick to force using C++ linker
nodist_EXTRA_wrapper_test_SOURCES = dummy.cc 

As you can see, we're not using a C linker to link the entire project, but we use a C++ linker (by tricking autotools ;) . A C++ linker "understand" the idiom used in spsc_wrapper.cc above.
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