Category: POCO

Interesting Stuff about the POCO C++ Libraries

Debug Output

Crafting debug output from std::istream data can be dangerous!

Writing a blog post sometimes can be useful to get some face-palm kind of programming error out of one’s system.

Putting such an error into written words then serves a couple of purposes:

  • it helps oneself remembering
  • it helps others who read it not to do the same thing
  • it serves as error log for future reference

So here it comes:

In one project we use JSON to serialize objects in order to send them over HTTP (we use the very nice JSON Spirit library, btw).

For each object we have serialize/deserialize methods which do the heavy lifting. After having developed a new deserialize method I wanted to test it together with the HTTP request handling. Using curl for this I issued a command like this:

curl -X PUT http://localhost:30222/some/url -d @datafile

This command issues a PUT request to the given URL and uses data in ./datafile, which contains the JSON, as request data.

The request came through but the deserializer wouldn’t do its work. WTF? Let’s see what goes on – let’s put some debug output in:

MyObject MyObjectSerializer::deserialize(std::istream& jsonIn)
{
   // debug output starts here
   std::string stringToDeserialize;
   Poco::StreamCopier::copyToString(jsonIn, stringToDeserialize);
   std::cout << "The String: " << stringToDeserialize << std::endl;
   // debug output ends here

   json_spirit::Value value;
   json_spirit::read(jsonIn, value);
   ...
}

I’ll give you some time to spot the bug…. 3..2..1..got it? Please check Poco::StreamCopier documentation if you are not familiar with POCO libraries.
What’s particularly misleading is the “Copier” part of the name StreamCopier, because it does not exactly copy the bytes from the stream into the string – it moves them. This means that after the debug output code, the istream is empty.

Unfortunately, I did not immediately recognize the change in the error outputs of the JSON parser. This might have given me a hint to the real problem. Instead, during the next half hour I searched for errors in the JSON I was sending.

When I finally realized it …

SSL with POCO

A short introduction to using SSL support in POCO C++ libraries.

Admittedly, the topic of this post is very specific but I hope it will still be of some value for some people.The task for today is to setup SSL server and client with POCO framework classes. I will leave out the whole certificate managing issues and just assume that the right files are at hand.

The SSL related part of  the POCO libraries essentially wraps the OpenSSL library into a nice object-oriented interface. When you know OpenSSL, you can instantly relate to classes like Poco::Net::Context, or the …Handler classes (if you replace “handler” with “callback”).

“SSL” stands for Secure Socket Layer, so the first thing to discover is class Poco::Net::SecureServerSocket. As you would expect, this class is derived from Poco::Net::ServerSocket, extending it only with SSL related stuff. And sure enough, some constructors of Poco::Net::ServerSocket take a Poco::Net::ContextPtr as argument.

But why only some constructors? Since there is no setContext method, there must be some other mechanism in place by which SecureServerSockets get their SSL context.

Introducing Poco::Net::SSLManager. From the API docs:

SSLManager is a singleton for holding the default server/client Context and handling callbacks for certificate verification errors and private key passphrases.

Proper initialization of SSLManager is critical.

Aha! So all the constructors of SecureServerSocket that do not take Context pointers simply get it from the SSLManager singleton.

But how to initialize SSLManager?

1. The POCO Way:

If you developed your application with POCO from the ground up there probably exists a sub-class of Poco::Application, and all the configuration is handled by the built-in configuration classes.

With this in place, all you have to do is to add the proper ssl configuration elements:

openSSL.server.privateKeyFile = /path/to/key/file
openSSL.server.certificateFile = /path/to/certificate/file
openSSL.server.verificationMode = none
openSSL.server.verificationDepth = 9
openSSL.server.loadDefaultCAFile = false
openSSL.server.cypherList = ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH
openSSL.server.privateKeyPassphraseHandler.name = KeyFileHandler
openSSL.server.privateKeyPassphraseHandler.options.password = securePassword
openSSL.server.invalidCertificateHandler = AcceptCertificateHandler

2. Manually:

Depending on which side you are – client or server – you have to call SSLManager::initializeClient or  SSLManager::initializeServer. Both methods take three arguments:

  1. PrivateKeyPassphraseHandler pointer
  2. InvalidCertificateHandler pointer
  3. Context pointer

This is where it becomes a little bit tricky: If you try to instantiate a Context with a privateKey file in order to provide it as argument to the initialize… method, a PrivateKeyPassphraseHandler might be needed. This handler is fetched from the SSLManager singleton – which you are just about to initialize!.

This circular dependency between Context and SSLManager can be overcome e.g. if you call SSLManager::initializeServer first only with a PrivateKeyPassphraseHandler, a InvalidCertificateHandler and null Context pointer. Then instantiate the Context and call SSLManager::initializeServer again.

Now that SSL Manager is initialized we can use Secure… prefixed classes as we would used their non-SSL counterparts. As with SecureServerSocket, other Secure… classes are derieved from corresponding non-secure base classes.

Conclusion: Once you got around the initialization of SSLManager singelton, using SSL POCO classes is very easy and straight forward. Check it out!

Active Object with POCO’s Active Methods

POCO’s ActiveMethods require minimal additional code to implement the Active Object design pattern

Active Object is a well known design pattern for synchronizing access to an object and/or resource. The basic idea is to separate method invocation from method execution which is done in a dedicated thread.

Instead of using the objects interface directly, a client of an Active Object uses some kind of  proxy which enqueues a so-called Method Request for later execution. The proxy finishes immediately and returns to the client some sort of callback, or variable, by which the client can receive the result. These intermeditate result variables are also known as Futures.

As always, there are lots of ways to implement this pattern. For example, if you had an interface like this

class MyObject
{
  public:
    int doStuff(const std::string& param) =0;
    std::string doSomeOtherThing(int i) =0;
};

applying a straight forward implementation, you would first transform this into a proxy and method request classes:

class MyObjectProxy
{
  public:
    MyObjectProxy(MyObject* theObject);
    // proxy methods
    Future<int> doStuff(const std::string& param);
    Future<std::string> doSomeOtherThing(int i);
  private:
    MyObject * _myObject;
};

class MethodRequest_DoStuff :
  public AbstractMethodRequest
{
  public:
    MethodRequest_DoStuff(const std::string& param);
    // all method request classes must implement execute()
    virtual void execute(MyObject* theObject);

  private:
    const std::string _param;
};

… and so on (for more details see this basic paper by Douglas C. Schmidt, or read chapter Concurrency Patterns in POSA2).

It’s easy to see that this implementation produces a lot of boilerplate code. To solve this, you could either cook up some code generation, or look for some language support to reduce the amount of characters you have to type. In C++, some sort of template solution can be the way to go, or…

Introducing Active Methods

With class ActiveMethod together with support classes ActiveDispatcher and ActiveResult the POCO C++ libraries provide very simple and elegant building blocks for implementing  the Active Object pattern.

ActiveMethod:  this is the core piece. When called, an ActiveMethod executes in its own thread.

ActiveResult: this is what I referred to earlier as a Future. Instances of ActiveResult are used to pass the result of an ActiveMethod call back to the client.

ActiveDispatcher: if you only use ActiveMethods, every ActiveMethod thread can execute in parallel.  With ActiveDispatcher as base class, ActiveMethod calls are serialized, thus implementing real™ Active Object behaviour.

Here my earlier example using ActiveMethods:

class MyObject
{
  public:
    // ActiveMethods are initialized in the ctors
    // initializer list
    MyObject()
      : doStuff(this, &MyObject::doStuffImpl),
        doSomeOtherThing(this, &MyObject::doSomeOtherThingImpl)
    {}

    ActiveMethod<int, std::string, MyObject> doStuff;
    ActiveMethod<std::string, int, MyObject> doSomeOtherThing;
  private:
    int doStuffImpl(const std::string& param);
    std::string doSomeOtherThingImpl(int i);
};

This is used as follows:

MyObject myObject;
ActiveResult<std::string> result = myObject.doSomeOtherThing(42);
...
result.wait();
std::cout << result.data() << std::endl;

This solution requires minimal amounts of additional code to transform your lame and boring normal object into a full-fledged Active Object. The only downside is that Active Methods currently can only have one parameter. If you need more, you have to use tuples or parameter objects.

Have fun!

Shrink your dependency list with POCO

POCO is a nice set of C++ libraries which provides elegant solutions for day-to-day tasks.

When you write C++ applications of any sort you are very likely to need support libraries in addition to what comes with C++ (which is not much, btw). Of course, this holds true for any other language. But with Java and its rich JDK for example this need is not so imminent.

Starting at the very beginning, let’s see how fast the need for support arises.

int main(int argc, char** argv)
{
// parsing command line arguments
...

How to parse those command line arguments in a simple and easy way? How about a little help output when the program is called with -h or –help? Ok, we got boost::program_options for this.

Going further in your program you may want to have some sort of logging capability. Unfortunately, as of boost version 1.45 there is nothing to be found there. So you add a nice logging library.

And so on.

But wait! You don’t want to depend on too many 3rd party libraries because, among other things, they add deployment complexity.

Not even Qt, as one of the major players in the C++ framework world, provides solutions to both previous examples. As of version 4.7, no logging and not much support with command line arguments. And you end-up having to use QString, one of many non-std::string classes in C++ frameworks, which can get annoying at times (of course there are reasons why those exist).

I could go on with the list of smaller or larger concerns for which you either roll your own implementation or include yet another library in your project.

Instead I would like to point you to POCO, a nice set of C++ libraries which provide easy solutions for many basic and/or advanced day-to-day tasks. From their website:

Modern, powerful open source C++ class libraries and frameworks for building network- and internet-based applications that run on desktop, server and embedded systems

Besides very basic stuff like logging, date/time handling, threads, memory management, UTF-8, etc. they also provide lots of higher level classes for things like SMTP, POP3, SQL database access and HTTP. They even have a so called C++ Server Page Compiler which is basically something like JSP or Active Server Pages.

And they have no own string class! Yay! Instead they provide lots of functions classes and streams to do string manuipulation on good old std::string.

One thing I like most about POCO, though, is its clean, well-documented and apparently very high quality code. Although it is not overly functional or template-heavy, like you see it in in boost very often, it still provides elegant solutions.

Check it out and shrink your dependency list.