[def __on_exit__ [globalref boost::process::on_exit on_exit]]
[def __on_success__ [globalref boost::process::extend::on_success ex::on_success]]
[def __child__ [classref boost::process::child child]]
[def __handler__ [classref boost::process::extend::handler handler]]
[def __on_success__ [memberref boost::process::extend::handler::on_success on_success]]
[def __posix_executor__ [classref boost::process::extend::posix_executor ex::posix_executor]]
[def __windows_executor__ [classref boost::process::extend::windows_executor ex::windows_executor]]
[def __io_context__  [@http://www.boost.org/doc/libs/release/doc/html/boost_asio/reference/io_context.html boost::asio::io_context]]
[def __require_io_context__ [classref boost::process::extend::require_io_context ex::require_io_context]]
[def __async_handler__ [classref boost::process::extend::async_handler ex::async_handler]]
[def __get_io_context__ [funcref boost::process::extend::get_io_context ex::get_io_context]]

[section:extend Extensions]
To extend the library, the header [headerref boost/process/extend.hpp extend] is provided.

It only provides the explicit style for custom properties, but no implicit style.

What this means is, that a custom initializer can be implemented, a reference which can be passed to one of the launching functions.
If a class inherits [classref boost::process::extend::handler] it will be regarded as an initializer and thus directly put into the sequence 
the executor gets passed.

[section:structure Structure]

The executor calls different handlers of the initializers during the process launch. 
The basic structure consists of three functions, as given below:

* [globalref boost::process::extend::on_setup on_setup]
* [globalref boost::process::extend::on_error on_error]
* [globalref boost::process::extend::on_success on_success]

'''
<imagedata fileref="boost_process/windows_exec.svg"/>
'''

Additionally posix provides three more handlers, listed below:

* [globalref boost::process::extend::on_fork_error on_fork_error]
* [globalref boost::process::extend::on_exec_setup on_exec_setup]
* [globalref boost::process::extend::on_exec_error on_exec_error] 

For more information see the reference of [classref boost::process::extend::posix_executor posix_executor]. 

[endsect]
[section:simple Simple extensions]

The simplest extension just takes a single handler, which can be done in a functional style.
So let's start with a simple hello-world example, while we use a C++14 generic lambda.

```
using namespace boost::process;
namespace ex = bp::extend;

__child__ c("foo", __on_success__=[](auto & exec) {std::cout << "hello world" << std::endl;});
```

Considering that lambdas can also capture values, data can easily be shared between handlers. 

To see which members the executor has, refer to [classref boost::process::extend::windows_executor windows_executor]
and [classref boost::process::extend::posix_executor posix_executor].

[note Combined with __on_exit__ this can also handle the process exit.]

[caution The posix handler symbols are not defined on windows.]

[endsect]

[section:handler Handler Types]

Since the previous example is in a functional style, it is not very reusable. 
To solve that problem, the [classref boost::process::extend::handler handler] has an alias in the `boost::process::extend` namespace, to be inherited.
So let's implement the hello world example in a class.

```
struct hello_world : __handler__
{
    template<typename Executor>
    void __on_success__(Executor & exec) const
    {
        std::cout << "hello world" << std::endl;
    }
};

//in our function
__child__ c("foo", hello_world()); 
```

[note The implementation is done via overloading, not overriding.]

Every handler not implemented defaults to [classref boost::process::extend::handler handler], where an empty handler is defined for each event.

[endsect]



[section:async Asynchronous Functionality]

Since `boost.process` provides an interface for [@http://www.boost.org/doc/libs/release/libs/asio/ boost.asio], 
this functionality is also available for extensions. If the class needs the __io_context__ for some reason, the following code will do that.

```
struct async_foo : __handler__, __require_io_context__
{
    template<typename Executor>
    void on_setup(Executor & exec)
    {
        __io_context__ & ios = __get_io_context__(exec.seq); //gives us a reference and a compiler error if not present.
        //do something with ios
    }
};
```
[note Inheriting [globalref boost::process::extend::require_io_context require_io_context] is necessary, so [funcref boost::process::system system] provides one.]

Additionally the handler can provide a function that is invoked when the child process exits. This is done through __async_handler__.

[note [globalref boost::process::extend::async_handler async_handler] implies [globalref boost::process::extend::require_io_context require_io_context] .]

```
struct async_bar : __handler, __async_handler__
{
    template<typename Executor>
    std::function<void(int, const std::error_code&)> on_exit_handler(Executor & exec)
    {
        auto handler_ = this->handler;
        return [handler_](int exit_code, const std::error_code & ec)
               {
                   std::cout << "hello world, I exited with " << exit_code << std::endl; 
               };
    
    }
}; 
```


[caution `on_exit_handler` does not default and is always required when [classref boost::process::extend::async_handler async_handler] is inherited. ]

[caution `on_exit_handler` uses `boost::asio::signal_set` to listen for SIGCHLD on posix. The application must not also register a signal handler for SIGCHLD using functions such as `signal()` or `sigaction()` (but using `boost::asio::signal_set` is fine). ]

[endsect]

[section:error Error handling]

If an error occurs in the initializers it shall be told to the executor and not handled directly. This is because
the behaviour can be changed through arguments passed to the launching function. Hence the executor
has the function `set_error`, which takes an [@http://en.cppreference.com/w/cpp/error/error_code std::error_code] and a string.
Depending on the configuration of the executor, this may either throw, set an internal `error_code`, or do nothing.

So let's take a simple example, where we set a randomly chosen `error_code`.

```
auto set_error = [](auto & exec)
        {
            std::error_code ec{42, std::system_category()};
            exec.set_error(ec, "a fake error");
            
        };
__child__ c("foo", on_setup=set_error);
```

Since we do not specify the error-handling mode in this example, this will throw [classref boost::process::process_error process_error]. 

[endsect]

[section:exec_over Executor Overloading]

Now that we have a custom initializer, let's consider how we can handle differences between different executors.
The distinction is between posix and windows and `char` and `wchar_t` on windows. 
One solution is to use the [@http://www.boost.org/doc/libs/master/boost/system/api_config.hpp BOOST_WINDOWS_API and BOOST_POSIX_API] macros,
which are automatically available as soon as any process-header is included.

Another variant are the type aliases __posix_executor__ and __windows_executor__, where the executor, not on the current system is a forward-declaration.
This works fine, because the function will never get invoked. So let's implement another example, which prints the executable name __on_success__.

```
struct hello_exe : __handler__
{
    template<typename Sequence>
    void __on_success__(__posix_executor__<Sequence> & exec)
    {
        std::cout << "posix-exe: " << exec.exe << std::endl;
    }
    
    template<typename Sequence>
    void __on_success__(__windows_executor__<char, Sequence> & exec)
    {
        //note: exe might be a nullptr on windows.
        if (exec.exe != nullptr)
            std::cout << "windows-exe: " << exec.exe << std::endl;
        else
            std::cout << "windows didn't use exe" << std::endl;
    }
    
    template<typename Sequence>
    void __on_success__(__windows_executor__<wchar_t, Sequence> & exec)
    {
        //note: exe might be a nullptr on windows.
        if (exec.exe != nullptr)
            std::wcout << L"windows-exe: " << exec.exe << std::endl;
        else
            std::cout << "windows didn't use exe" << std::endl;
    }
    
};
```

So given our example, the definitions with the non-native executor are still a template so that they will not be evaluated if not used. Hence this provides a
way to implement system-specific code without using the preprocessor.

[note If you only write a partial implementation, e.g. only for __posix_executor__, the other variants will default to __handler__].

[endsect]

[endsect]
