Boost C++ Libraries

call/cc (call with current continuation) is a universal control operator (well-known from the programming language Scheme) that captures the current continuation as a first-class object and pass it as an argument to another continuation.

A continuation (abstract concept of functional programming languages) represents the state of the cotnrol flow of a program at a given point in time. Continuations can be suspended and resumed later in order to change the control flow of a program.

Modern mico-processors are registers machines; the content of processor registers represent a continuation of the executed program at a given point in time. Operating systems simulate parallel execution of programs on a single processor by switching between programs (context switch) by preserving and restoring the continuation, e.g. the content of all registers.

callcc()

callcc() is the C++ equivalent to Scheme's call/cc operator. It captures the current continuation (the rest of the computation; code after callcc()) and trickers a context switch. The context switch is achived by preserving certain registers (including instruction and stack pointer), defined by the calling convention of the ABI, of the current continuation and restoring those registers of the resumed continuation. The control flow of the resumed continuation continues. The current continuation is suspended and passed as argument to the resumed continuation.

callcc() expects a context-function with signature 'continuation(continuation && c, Arg ...arg)'. The parameter c represents the current continuation from which this continuation was resumed (e.g. that has called callcc()) and arg are the data passed to callcc().

On return the context-function of the current continuation has to specify an continuation to which the execution control is transferred after termination of the current continuation.

If an instance with valid state goes out of scope and the context-function has not yet returned, the stack is traversed in order to access the control structure (address stored at the first stack frame) and continuation's stack is deallocated via the StackAllocator.

continuation

continuation represents a contiunation; it contains the content of preserved registers and manages the associated stack (allocation/deallocation). continuation s a one-shot continuation - it can be used only once, after applied to callcc() it is invalidated.

continuation is only move-constructible and move-assignable.

As a first-class object continuation can be applied to and returned from a function, assigned to a variable or stored in a container.

A contiunation is continued by calling resume()/resume_with().

Usage

namespace ctx=boost::context;
ctx::continuation source=ctx::callcc(
    [](ctx::continuation && sink){
        int a=0;
        int b=1;
        for(;;){
            sink=sink.resume(a);
            auto next=a+b;
            a=b;
            b=next;
        }
        return std::move(sink);
    });
for (int j=0;j<10;++j) {
    std::cout << source.get_data<int>() << " ";
    source=source.resume();
}

output:
    0 1 1 2 3 5 8 13 21 34 55

This simple example demonstrates the basic usage of call/cc as a generator. The continuation sink represents the main-continuation (function main()). sink is captured (current-contiunation) and passed as parameter to the lambda.

Because the state is invalidated (one-shot continuation) by each call of callcc(), the new state of the continuation, returned by callcc(), needs to be assigned to sink after each call.

The lambda that calculates the Fibonacci numbers is executed inside the continuation represented by source. Calculated Fibonacci numbers are transferred between the two continuations' as argument of callcc() (and returned by source.get_data<int>()). Note that this example represents a generator thus no value is transferred into the lambda via callcc().

The locale variables a, b and next remain their values during each context switch. This is possible due source has its own stack and the stack is exchanged by each context switch.

Parameter passing

Calling callcc() without arguments means, that no data will be transferred, only the context switch is executed.

The arguments passed to callcc(), in one continuation, are accessible via c.get_data<>() in the other continuation.

namespace ctx=boost::context;
int i=1;
ctx::continuation c1=callcc([](ctx::continuation && c2){
            int j=c2.get_data<int>();
            std::printf("inside c1,j==%d\n",j);
            return c2.resume(j+1);
        },
        i);
i=c1.get_data<int>();
std::printf("i==%d\n",i);

output:
    inside c1,j==1
    i==2

callcc(<lambda>,i) enters the lambda in continuation represented by c1 with argument i=1. The expression c2.resume(j+1) resumes the continuation c2 and transfers back an integer of j+1. On return of callcc(<lambda>, i), the variable i gets the value of j+1 assigned (i=c1.get_data<int>()).

More than one argument can be transferred too.

namespace ctx=boost::context;
int i=2,j=1;
ctx::continuation c1=callcc([](ctx::continuation && c2){
            int i, j;
            std::tie(i,j)=c2.get_data<int,int>();
            std::printf("inside c1,i==%d,j==%d\n",i,j);
            return c2.resume(i+j,i-j);
        },
        i, j);
std::tie(i,j)=c1.get_data<int,int>();
std::printf("i==%d,j==%d\n",i,j);

output:
    inside c1,i==2,j==1
    i==3,j==1

For use-cases, that require to transfer data of different type:

namespace ctx=boost::context;
ctx::continuation f1(ctx::continuation && c) {
    int i=3;
    c=c.resume(i);
    std::string s{ "abc" };
    c=c.resume(s);
    i=7;s="xyz";
    c=c.resume(i,s);
    c=c.resume();
    return std::move(c);
}

ctx::continuation c=ctx::callcc(f1);
int i=c.get_data<int>();
std::cout << "f1: returned : " << i << std::endl;
c=c.resume();
std::string s=c.get_data<std::string>();
std::cout << "f1: returned : " << s << std::endl;
c=c.resume();
std::tie(i,s)=c.get_data< int, std::string >();
std::cout << "f1: returned : " << i << ", " << s << std::endl;
c=c.resume();
std::cout << std::boolalpha;
std::cout << "f1: returned data : " << ctx::data_available( c) << std::endl;

output:
    f1: returned : 3
    f1: returned : abc
    f1: returned : 7, xyz
    f1: returned data : false

Exception handling

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

Executing function on top of a continuation

Sometimes it is useful to execute a new function on top of a resumed continuation. For this purpose callcc() with second argument exec_ontop_arg has to be used. The function passed as argument must accept a rvalue reference to continuation and return, depending on functions argument list, a tuple of its argument types, a single return type or void.

namespace ctx=boost::context;
ctx::continuation f1(ctx::continuation && c) {
    int data=c.get_data<int>();
    std::cout << "f1: entered first time: " << data  << std::endl;
    c=c.resume(data+1);
    data=c.get_data<int>();
    std::cout << "f1: entered second time: " << data  << std::endl;
    c=c.resume(data+1);
    data=c.get_data<int>();
    std::cout << "f1: entered third time: " << data << std::endl;
    return std::move(c);
};

int f2(ctx::continuation && c){
    int data=c.get_data<int>(c);
    std::cout << "f2: entered: " << data << std::endl;
    return data;
};

int data = 0;
ctx::continuation c=ctx::callcc(f1,data+1);
data=c.get_data<int>();
std::cout << "f1: returned first time: " << data << std::endl;
c=c.resume(data+1);
data=c.get_data<int>();
std::cout << "f1: returned second time: " << data << std::endl;
c=c.resume_with(f2,-1);
std::cout << "f1: returned third time" << std::endl;

output:
    f1: entered first time: 1
    f1: returned first time: 2
    f1: entered second time: 3
    f1: returned second time: 4
    f2: entered: -1
    f1: entered third time: -1
    f1: returned third time

The expression c.resume_with(f2,-1) executes f2() on top of contiunation c, e.g. an additional stack frame is allocated on top of the stack (in front of f1()). f2() returns argument -1 that will returned by the second invocation of c.resume(data+1) in f1().

Another option is to execute a function on top of the continuation that throws an exception.

namespace ctx=boost::context;
struct my_exception : public std::runtime_error {
    ctx::continuation    c;
    my_exception(ctx::continuation && c_,std::string const& what) :
        std::runtime_error{ what },
        c{ std::move( c_) } {
    }
};

ctx::continuation c=ctx::callcc([](ctx::continuation && c) {
    for (;;) {
        try {
            std::cout << "entered" << std::endl;
            c=c.resume();
        } catch (my_exception & ex) {
            std::cerr << "my_exception: " << ex.what() << std::endl;
            return std::move(ex.c);
        }
    }
    return std::move(c);
});
c = c.resume_with(
       [](ctx::continuation && c){
           throw my_exception(std::move(c),"abc");
       });

output:
    entered
    my_exception: abc

In this exception my_exception is throw from a function invoked ontop of continuation c and catched inside the for-loop.

Stack unwinding

On construction of continuation a stack is allocated. If the context-function returns the stack will be destructed. If the context-function has not yet returned and the destructor of an valid continuation instance (e.g. continuation::operator bool() returns true) is called, the stack will be destructed too.

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 continuation is created.

namespace ctx=boost::context;
// 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  {
    // captured continuation
    ctx::continuation   c;

    template< typename StackAllocator >
    my_control_structure(void * sp,std::size_t size,stack_context sctx,StackAllocator salloc) :
        // create captured continuation
        c{} {
        c=ctx::callcc(std::allocator_arg,preallocated(sp,size,sctx),salloc,entry_func);
    }
    ...
};

Inverting the control flow

namespace ctx=boost::context;
/*
 * grammar:
 *   P ---> E '\0'
 *   E ---> T {('+'|'-') T}
 *   T ---> S {('*'|'/') S}
 *   S ---> digit | '(' E ')'
 */
class Parser{
   char next;
   std::istream& is;
   std::function<void(char)> cb;

   char pull(){
        return std::char_traits<char>::to_char_type(is.get());
   }

   void scan(){
       do{
           next=pull();
       }
       while(isspace(next));
   }

public:
   Parser(std::istream& is_,std::function<void(char)> cb_) :
      next(), is(is_), cb(cb_)
    {}

   void run() {
      scan();
      E();
   }

private:
   void E(){
      T();
      while (next=='+'||next=='-'){
         cb(next);
         scan();
         T();
      }
   }

   void T(){
      S();
      while (next=='*'||next=='/'){
         cb(next);
         scan();
         S();
      }
   }

   void S(){
      if (isdigit(next)){
         cb(next);
         scan();
      }
      else if(next=='('){
         cb(next);
         scan();
         E();
         if (next==')'){
             cb(next);
             scan();
         }else{
             throw std::runtime_error("parsing failed");
         }
      }
      else{
         throw std::runtime_error("parsing failed");
      }
   }
};

std::istringstream is("1+1");
// execute parser in new continuation
ctx::continuation source;
// user-code pulls parsed data from parser
// invert control flow
source=ctx::callcc(
        [&is](ctx::continuation && sink){
        // create parser with callback function
        Parser p(is,
                 [&sink](char c){
                       // resume main continuation
                       sink=sink.resume(c);
                 });
            // start recursive parsing
            p.run();
            // resume main continuation
            return std::move(sink);
        });
while(ctx::data_available(source)){
    char c=source.get_data<char>();
    printf("Parsed: %c\n",c);
    source=source.resume();
}

output:
    Parsed: 1
    Parsed: +
    Parsed: 1

In this example a recursive descent parser uses a callback to emit a newly passed symbol. Using call/cc 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 continuations.

Class continuation

#include <boost/context/continuation.hpp>

class continuation {
public:
    continuation() noexcept = default;

    ~continuation();

    continuation(continuation && other) noexcept;

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

    continuation(continuation const& other) noexcept = delete;
    continuation & operator=(continuation const& other) noexcept = delete;

    template<typename ...Arg>
    continuation resume(Arg ...arg);

    template<typename Fn,typename ...Arg>
    continuation resume_with(Fn && fn,Arg ...arg);

    bool data_available() noexcept;

    template<typename ...Arg>
    <unspecified> get_data();

    explicit operator bool() const noexcept;

    bool operator!() const noexcept;

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

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

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

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

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

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

    template<typename charT,class traitsT>
    friend std::basic_ostream<charT,traitsT> &
    operator<<(std::basic_ostream<charT,traitsT> & os,continuation const& other) {

    void swap(continuation & other) noexcept;
};

Constructor

continuation() noexcept;

Destructor

~continuation();

Move constructor

continuation(continuation && other) noexcept;

Move assignment operator

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

Member function operator()()

template<typename ...Arg>
continuation resume(Arg ...arg);

template<typename Fn,typename ...Arg>
continuation resume_with(Fn && fn,Arg ...arg);

Member function data_available()

bool data_available() noexcept;

Member function get_available()

template<typename ...Arg>
<unspecified> get_data();

Member function operator bool()

explicit operator bool() const noexcept;

Member function operator!()

bool operator!() const noexcept;

Member function operator==()

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

Member function operator!=()

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

Member function operator<()

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

Member function operator>()

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

Member function operator<=()

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

Member function operator>=()

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

Non-member function operator<<()

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

Call with current contiunation

#include <boost/context/continuation.hpp>

template<typename Fn,typename ...Arg>
continuation callcc(Fn && fn,Arg ...arg);

template<typename StackAlloc,typename Fn,typename ...Arg>
continuation callcc(std::allocator_arg_t,StackAlloc salloc,Fn && fn,Arg ...arg);

template<typename StackAlloc,typename Fn,typename ...Arg>
continuation callcc(std::allocator_arg_t,preallocated palloc,StackAlloc salloc,Fn && fn,Arg ...arg);