Class execution_context (version 1)

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## Class execution_context (version 1)

| Note
This class is only enabled if property segmented-stacks=on (enables segmented stacks) or compiler flag BOOST_EXECUTION_CONTEXT=1 is specified at b2-commandline.

Class execution_context encapsulates context switching and manages the associated context' stack (allocation/deallocation).

execution_context allocates the context stack (using its StackAllocator argument) and creates a control structure on top of it. This structure is responsible for managing context' stack. Instances of execution_context, associated with a specific context, share the ownership of the control structure. If the last reference goes out of scope, the control structure is destroyed and the stack gets deallocated via the StackAllocator.

execution_context is copy-constructible, move-constructible, copy-assignable and move-assignable.

execution_context maintains a static (thread-local) pointer, accessed by execution_context::current(), pointing to the active context. On each context switch the pointer is updated. The usage of this global pointer makes the context switch a little bit slower (due access of thread local storage) but has some advantages. It allows to access the control structure of the current active context from arbitrary code paths required in order to support segmented stacks, which require to call certain maintenance functions (like __splitstack_getcontext() etc.) before each context switch (each context switch exchanges the stack).

execution_context expects a function/functor with signature `void(void* vp)` (`vp` is the data passed at the first invocation of `ecv1::operator()()`).

#### usage of execution_context

    int n=35;
    boost::context::execution_context sink(boost::context::execution_context::current());
    boost::context::execution_context source(
        [n,&sink](void*)mutable{
            int a=0;
            int b=1;
            while(n-->0){
                sink(&a);
                auto next=a+b;
                a=b;
                b=next;
            }
        });
    for(int i=0;i<10;++i){
        std::cout<<*(int*)source()<<" ";
    }

    output:
        0 1 1 2 3 5 8 13 21 34

This simple example demonstrates the basic usage of execution_context. The context `sink`, returned by execution_context::current(), represents the main-context (function main() running) and is one of the captured parameters in the lambda expression. The lambda that calculates the Fibonacci numbers is executed inside the context represented by `source`. Calculated Fibonacci numbers are transferred between the two context' via expression sink(&a) (and returned by source()).

The locale variables `a`, `b` and `next` remain their values during each context switch (yield(a)). This is possible because `ctx` owns a stack (exchanged by context switch).

#### inverting the control flow

    /*
     * grammar:
     *   P ---> E '\0'
     *   E ---> T {('+'|'-') T}
     *   T ---> S {('*'|'/') S}
     *   S ---> digit | '(' E ')'
     */
    class Parser{
        // implementation omitted; see examples directory
    };

    std::istringstream is("1+1");
    bool done=false;
    std::exception_ptr except;

    // create handle to main execution context
    auto main_ctx(boost::context::execution_context::current());
    // execute parser in new execution context
    boost::context::execution_context source(
            [&sink,&is,&done,&except](void*){
            // create parser with callback function
            Parser p(is,
                     [&sink](char ch){
                            // resume main execution context
                            sink(&ch);
                    });
                try {
                    // start recursive parsing
                    p.run();
                } catch (...) {
                    // store other exceptions in exception-pointer
                    except = std::current_exception();
                }
                // set termination flag
                done=true;
                // resume main execution context
                sink();
            });

    // user-code pulls parsed data from parser
    // invert control flow
    void* vp = source();
    if (except) {
        std::rethrow_exception(except);
    }
    while( ! done) {
        printf("Parsed: %c\n",* static_cast<char*>(vp));
        vp = source();
        if (except) {
            std::rethrow_exception(except);
        }
    }

    output:
        Parsed: 1
        Parsed: +
        Parsed: 1

In this example a recursive descent parser uses a callback to emit a newly passed symbol. Using execution_context the control flow can be inverted, e.g. the user-code pulls parsed symbols from the parser - instead to get pushed from the parser (via callback).

The data (character) is transferred between the two execution_context.

If the code executed by execution_context emits an exception, the application is terminated. std::exception_ptr can be used to transfer exceptions between different execution contexts.

Sometimes it is necessary to unwind the stack of an unfinished context to destroy local stack variables so they can release allocated resources (RAII pattern). The user is responsible for this task.

#### allocating control structures on top of stack

Allocating control structures on top of the stack requires to allocated the stack_context and create the control structure with placement new before execution_context is created.

| Note
The user is responsible for destructing the control structure at the top of the stack.

    // stack-allocator used for (de-)allocating stack
    fixedsize_stack salloc( 4048);
    // allocate stack space
    stack_context sctx( salloc.allocate() );
    // reserve space for control structure on top of the stack
    void * sp = static_cast< char * >( sctx.sp) - sizeof( my_control_structure);
    std::size_t size = sctx.size - sizeof( my_control_structure);
    // placement new creates control structure on reserved space
    my_control_structure * cs = new ( sp) my_control_structure( sp, size, sctx, salloc);
    ...
    // destructing the control structure
    cs->~my_control_structure();
    ...
    struct my_control_structure  {
        // execution context
        execution_context ectx;

        template< typename StackAllocator >
        my_control_structure( void * sp, std::size_t size, stack_context sctx, StackAllocator salloc) :
            // create execution context
            ectx( std::allocator_arg, preallocated( sp, size, sctx), salloc, entry_func) {
        }
        ...
    };

#### exception handling

If the function executed inside a execution_context emits ans exception, the application is terminated by calling std::terminate(). std::exception_ptr can be used to transfer exceptions between different execution contexts.

| Important
Do not jump from inside a catch block and then re-throw the exception in another execution context.

#### parameter passing

The void pointer argument passed to execution_context::operator(), in one context, is passed as the last argument of the context-function if the context is started for the first time. In all following invocations of execution_context::operator() the void pointer passed to execution_context::operator(), in one context, is returned by execution_context::operator() in the other context.

    class X {
    private:
        std::exception_ptr excptr_;
        boost::context::execution_context caller_;
        boost::context::execution_context callee_;

    public:
        X() :
            excptr_(),
            caller_( boost::context::execution_context::current() ),
            callee_( [=] (void * vp) {
                        try {
                            int i = * static_cast< int * >( vp);
                            std::string str = boost::lexical_cast<std::string>(i);
                            caller_( & str);
                        } catch (std::bad_cast const&) {
                            excptr_=std::current_exception();
                        }
                     })
        {}

        std::string operator()( int i) {
            void * ret = callee_( & i);
            if(excptr_){
                std::rethrow_exception(excptr_);
            }
            return * static_cast< std::string * >( ret);
        }
    };

    X x;
    std::cout << x( 7) << std::endl;

    output:
        7

#### Class `execution_context`

    class execution_context {
    public:
        static execution_context current() noexcept;

        template< typename Fn, typename ... Args >
        execution_context( Fn && fn, Args && ... args);

        template< typename StackAlloc, typename Fn, typename ... Args >
        execution_context( std::allocator_arg_t, StackAlloc salloc, Fn && fn, Args && ... args);

        template< typename StackAlloc, typename Fn, typename ... Args >
        execution_context( std::allocator_arg_t, preallocated palloc, StackAlloc salloc, Fn && fn, Args && ... args);

        execution_context( execution_context const& other) noexcept;
        execution_context( execution_context && other) noexcept;

        execution_context & operator=( execution_context const& other) noexcept;
        execution_context & operator=( execution_context && other) noexcept;

        explicit operator bool() const noexcept;
        bool operator!() const noexcept;

        void * operator()( void * vp = nullptr);

        template< typename Fn >
        void * operator()( exec_ontop_arg_t, Fn && fn, void * vp = nullptr);

        bool operator==( execution_context const& other) const noexcept;

        bool operator!=( execution_context const& other) const noexcept;

        bool operator<( execution_context const& other) const noexcept;

        bool operator>( execution_context const& other) const noexcept;

        bool operator<=( execution_context const& other) const noexcept;

        bool operator>=( execution_context const& other) const noexcept;

        template< typename charT, class traitsT >
        friend std::basic_ostream< charT, traitsT > &
        operator<<( std::basic_ostream< charT, traitsT > & os, execution_context const& other);
    };

##### Static member function `current`()

    static execution_context current() noexcept;

Returns:

  Returns an instance of excution_context pointing to the active execution context.

Throws:

  Nothing.

##### Constructor

    template< typename Fn, typename ... Args >
    execution_context( Fn && fn, Args && ... args);

    template< typename StackAlloc, typename Fn, typename ... Args >
    execution_context( std::allocator_arg_t, StackAlloc salloc, Fn && fn, Args && ... args);

    template< typename StackAlloc, typename Fn, typename ... Args >
    execution_context( std::allocator_arg_t, preallocated palloc, StackAlloc salloc, Fn && fn, Args && ... args);

Effects:

  Creates a new execution context and prepares the context to execute `fn`. `fixedsize_stack` is used as default stack allocator (stack size == fixedsize_stack::traits::default_size()). The constructor with argument type `preallocated`, is used to create a user defined data (for instance additional control structures) on top of the stack.

##### Copy constructor

    execution_context( execution_context const& other) noexcept;

Effects:

  Copies `other`, e.g. underlying control structure is shared with `*this`.

Throws:

  Nothing.

##### Move constructor

    execution_context( execution_context && other) noexcept;

Effects:

  Moves underlying control structure to `*this`.

Throws:

  Nothing.

##### Copy assignment operator

    execution_context & operator=( execution_context const& other) noexcept;

Effects:

  Copies the state of `other` to `*this`, control structure is shared.

Throws:

  Nothing.

##### Move assignment operator

    execution_context & operator=( execution_context && other) noexcept;

Effects:

  Moves the control structure of `other` to `*this` using move semantics.

Throws:

  Nothing.

##### Member function `operator bool`()

    explicit operator bool() const noexcept;

Returns:

  `true` if `*this` points to a control structure.

Throws:

  Nothing.

##### Member function `operator!`()

    bool operator!() const noexcept;

Returns:

  `true` if `*this` does not point to a control structure.

Throws:

  Nothing.

##### Member function `operator()`()

    void * operator()( void * vp = nullptr) noexcept;

Effects:

  Stores internally the current context data (stack pointer, instruction pointer, and CPU registers) of the current active context and restores the context data from `*this`, which implies jumping to `*this`'s context. The void pointer argument, `vp`, is passed to the current context to be returned by the most recent call to `execution_context::operator()` in the same thread. `fn` is executed with arguments `args` on top of the stack of `this`.

Note:

  The behaviour is undefined if `operator()()` is called while execution_context::current() returns `*this` (e.g. resuming an already running context). If the top-level context function returns, `std::exit()` is called.

Returns:

  The void pointer argument passed to the most recent call to execution_context::operator(), if any.

##### Member function `operator(exec_ontop_arg_t)`()

    template< typename Fn >
    void * operator()( exec_ontop_arg_t, Fn && fn, void * vp = nullptr);

Effects:

  Same as execution_context::operator(). Additionally, function `fn` is executed with arguments `vp` in the context of `*this` (e.g. the stack frame of `fn` is allocated on stack of `*this`).

Returns:

  The void pointer argument passed to the most recent call to execution_context::operator(), if any.

##### Member function `operator==`()

    bool operator==( execution_context const& other) const noexcept;

Returns:

  `true` if `*this` and `other` represent the same execution context, `false` otherwise.

Throws:

  Nothing.

##### Member function `operator!=`()

    bool operator!=( execution_context const& other) const noexcept;

Returns:

  `! (other == * this)`

Throws:

  Nothing.

##### Member function `operator<`()

    bool operator<( execution_context const& other) const noexcept;

Returns:

  `true` if `*this != other` is true and the implementation-defined total order of `execution_context` values places `*this` before `other`, false otherwise.

Throws:

  Nothing.

##### Member function `operator>`()

    bool operator>( execution_context const& other) const noexcept;

Returns:

  `other < * this`

Throws:

  Nothing.

##### Member function `operator<=`()

    bool operator<=( execution_context const& other) const noexcept;

Returns:

  `! (other < * this)`

Throws:

  Nothing.

##### Member function `operator>=`()

    bool operator>=( execution_context const& other) const noexcept;

Returns:

  `! (* this < other)`

Throws:

  Nothing.

##### Non-member function `operator<<()`

    template< typename charT, class traitsT >
    std::basic_ostream< charT, traitsT > &
    operator<<( std::basic_ostream< charT, traitsT > & os, execution_context const& other);

Efects:

  Writes the representation of `other` to stream `os`.

Returns:

  `os`