Boost C++ Libraries

Class execution_context encapsulates fcontext_t and related functions ( jump_fcontext() and make_fcontext()) as well as stack management. execution_context permits access to the current, active context via execution_context::current().

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

int main() {
    std::istringstream is("1+1");
    bool done=false;
    char c;

    // create handle to main execution context
    boost::context::execution_context main_ctx(
        boost::context::execution_context::current() );

    // executes parser in new execution context
    boost::context::execution_context parser_ctx(
        boost::context::fixedsize_stack(),
        [&main_ctx,&is,&c,&done](){
            // create parser with callback function
            Parser p(is,
                     [&main_ctx,&c](char ch){
                         c=ch;
                         // resume main execution context
                         main_ctx.resume();
                     });
            // start recursive parsing
            p.run();
            done=true;
            // return to main execution context
            main_ctx.resume();
        });

    // user-code pulls parsed data from parser
    // inverted control flow
    parser_ctx.resume();
    do {
        printf("Parsed: %c\n",c);
        parser_ctx.resume();
    } while( ! done);
}

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 interface of execution_context does not transfer data. This is not required because usually sharing data's address (pointer/reference) sufficient.

If the code executed by execution_context emits an exception, std::terminate() will be called.

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 strutures 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.

// stack-alloctor 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( preallocated( sp, size, sctx), salloc, entry_func) {
    }
    ...
};

exception handling

If the function executed inside a execution_context emitts ans exception, std::terminate() is called - std::exception_ptr can used to store exceptions thrown inside the other context.

parameter passing

Input and output parameters are transfered via a lambda capture list and references/pointers.

class X {
private:
    int * inp_;
    std::string outp_;
    std::exception_ptr excptr_;
    boost::context::execution_context caller_;
    boost::context::execution_context callee_;

public:
    X() :
        inp_( nullptr),
        outp_(),
        excptr_(),
        caller_( boost::context::execution_context::current() ),
        callee_( boost::context::fixedsize_stack(),
                 [=] () {
                    try {
                        int i = * inp_;
                        outp_ = boost::lexical_cast< std::string >( i);
                        caller_.resume();
                    } catch (...) {
                        excptr_ = std::current_exception();
                    }
                 })
    {}

    std::string operator()( int i) {
        inp_ = & i;
        callee_.resume();
        if ( excptr_) {
            std::rethrow_exception( excptr_);
        }
        return outp_;
    }
};

int main() {
    X x;
    std::cout << x( 7) << std::endl;
    std::cout << "done" << std::endl;
}

Class execution_context

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

    template< typename StackAlloc, typename Fn >
    execution_context( StackAlloc salloc, Fn && fn);

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

    template< typename StackAlloc, typename Fn >
    execution_context( preallocated palloc, StackAlloc salloc, Fn && fn);

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

    void resume() noexcept;

    explicit operator bool() const noexcept;

    bool operator!() const noexcept;
};

static execution_context current()

template< typename StackAlloc, typname Fn > execution_context( StackAlloc salloc, Fn && fn)

template< typename StackAlloc, typname Fn, typename ... Args > execution_context( StackAlloc salloc, Fn && fn, Args && ... args)

template< typename StackAlloc, typname Fn > execution_context( preallocated palloc, StackAlloc salloc, Fn && fn)

template< typename StackAlloc, typname Fn, typename ... Args > execution_context( preallocated palloc, StackAlloc salloc, Fn && fn, Args && ... args)

void resume()

explicit operator bool() const

bool operator!() const

Struct preallocated

struct preallocated {
    void        *   sp;
    std::size_t     size;
    stack_context   sctx;

    preallocated( void * sp, std:size_t size, stack_allocator sctx) noexcept;
};

preallocated( void * sp, std:size_t size, stack_allocator sctx)