...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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execution_context (v2) is the reference implementation of C++ proposal P099R1: A low-level API for stackful context switching. |
Note | |
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execution_context (v2) resides in the inlined sub-namespace
|
Note | |
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Segmented stacks (segmented-stacks=on), e.g. on demand growing stacks, are not supported by execution_context (v2). |
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. The address of the control structure is stored
in the first frame of context' stack (e.g. it can not directly accessed from
within execution_context). In contrast to execution_context
(v1) the ownership of the control structure is not shared (no member
variable to control structure in execution_context).
execution_context keeps internally a state that is moved
by a call of execution_context::operator() (*this
will be
invalidated), e.g. after a calling execution_context::operator(),
*this
can not be used for an additional context switch.
execution_context is only move-constructible and move-assignable.
The moved state is assigned to a new instance of execution_context. This object becomes the first argument of the context-function, if the context was resumed the first time, or the first element in a tuple returned by execution_context::operator() that has been called in the resumed context. In contrast to execution_context (v1), the context switch is faster because no global pointer etc. is involved.
Important | |
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Segmented stacks are not supported by execution_context (v2). |
On return the context-function of the current context has to specify an execution_context to which the execution control is transferred after termination of the current context.
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 context' stack is deallocated via the StackAllocator. The stack walking makes the destruction of execution_context slow and should be prevented if possible.
execution_context expects a context-function
with signature execution_context(execution_context
ctx, Args ... args)
. The
parameter ctx
represents the
context from which this context was resumed (e.g. that has called execution_context::operator()
on *this
)
and args
are the data passed
to execution_context::operator(). The return value represents
the execution_context that has to be resumed, after termiantion of this context.
Benefits of execution_context (v2) over execution_context (v1) are: faster context switch, type-safety of passed/returned arguments.
int n=35; ctx::execution_context<int> source( [n](ctx::execution_context<int> && sink,int) mutable { int a=0; int b=1; while(n-->0){ auto result=sink(a); sink=std::move(std::get<0>(result)); auto next=a+b; a=b; b=next; } return std::move(sink); }); for(int i=0;i<10;++i){ auto result=source(i); source=std::move(std::get<0>(result)); std::cout<<std::get<1>(result)<<" "; } output: 0 1 1 2 3 5 8 13 21 34
This simple example demonstrates the basic usage of execution_context
as a generator. The context sink
represents the main-context (function main()
running). sink
is generated
by the framework (first element of lambda's parameter list). Because the state
is invalidated (== changed) by each call of execution_context::operator(),
the new state of the execution_context, returned by execution_context::operator(),
needs to be assigned to sink
after each call.
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()).
Note that this example represents a generator thus the
value transferred into the lambda via source() is not
used. Using boost::optional<> as transferred type,
might also appropriate to express this fact.
The locale variables a
, b
and next
remain their values during each context switch (yield(a)).
This is possible due source
has its own stack and the stack is exchanged by each context switch.
With execution_context<void>
no
data will be transferred, only the context switch is executed.
boost::context::execution_context<void> ctx1([](boost::context::execution_context<void> && ctx2){ std::printf("inside ctx1\n"); return ctx2(); }); ctx1(); output: inside ctx1
ctx1()
resumes ctx1
, e.g. the lambda
passed at the constructor of ctx1
is entered. Argument ctx2
represents
the context that has been suspended with the invocation of ctx1()
. When the lambda returns ctx2
,
context ctx1
will be terminated
while the context represented by ctx2
is resumed, hence the control of execution returns from ctx1()
.
The arguments passed to execution_context::operator(), in one context, is passed as the last arguments of the context-function if the context is started for the first time. In all following invocations of execution_context::operator() the arguments passed to execution_context::operator(), in one context, is returned by execution_context::operator() in the other context.
boost::context::execution_context<int> ctx1([](boost::context::execution_context<int> && ctx2,int j){ std::printf("inside ctx1,j==%d\n",j); std::tie(ctx2,j)=ctx2(j+1); return std::move(ctx2); }); int i=1; std::tie(ctx1,i)=ctx1(i); std::printf("i==%d\n",i); output: inside ctx1,j==1 i==2
ctx1(i)
enters
the lambda in context ctx1
with argument j=1
. The expression ctx2(j+1)
resumes the
context represented by ctx2
and transfers back an integer of j+1
. On return
of ctx1(i)
, the variable
i
contains the value of j+1
.
If more than one argument has to be transferred, the signature of the context-function is simply extended.
boost::context::execution_context<int,int> ctx1([](boost::context::execution_context<int,int> && ctx2,int i,int j){ std::printf("inside ctx1,i==%d,j==%d\n",i,j); std::tie(ctx2,i,j)=ctx2(i+j,i-j); return std::move(ctx2); }); int i=2,j=1; std::tie(ctx1,i,j)=ctx1(i,j); std::printf("i==%d,j==%d\n",i,j); output: inside ctx1,i==2,j==1 i==3,j==1
For use-cases, that require to transfer data of different type in each direction, boost::variant<> could be used.
class X{ private: std::exception_ptr excptr_; boost::context::execution_context<boost::variant<int,std::string>> ctx_; public: X(): excptr_(), ctx_([=](boost::context::execution_context<boost::variant<int,std::string>> && ctx,boost::variant<int,std::string> data){ try { for (;;) { int i=boost::get<int>(data); data=boost::lexical_cast<std::string>(i); auto result=ctx(data); ctx=std::move(std::get<0>(result)); data=std::get<1>(result); } catch (std::bad_cast const&) { excptr_=std::current_exception(); } return std::move(ctx); }) {} std::string operator()(int i){ boost::variant<int,std::string> data=i; auto result=ctx_(data); ctx_=std::move(std::get<0>(result)); data=std::get<1>(result); if(excptr_){ std::rethrow_exception(excptr_); } return boost::get<std::string>(data); } }; X x; std::cout << x(7) << std::endl; output: 7
In the case of unidirectional transfer of data, boost::optional<> or a pointer are appropriate.
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 | |
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Do not jump from inside a catch block and then re-throw the exception in another execution context. |
Sometimes it is useful to execute a new function on top of a resumed context.
For this purpose execution_context::operator() with first
argument exec_ontop_arg
has
to be used. The function passed as argument must return a tuple of execution_context
and arguments.
boost::context::execution_context<int> f1(boost::context::execution_context<int> && ctx,int data) { std::cout << "f1: entered first time: " << data << std::endl; std::tie(ctx,data)=ctx(data+1); std::cout << "f1: entered second time: " << data << std::endl; std::tie(ctx,data)=ctx(data+1); std::cout << "f1: entered third time: " << data << std::endl; return std::move(ctx); } int f2(int data) { std::cout << "f2: entered: " << data << std::endl; return -1; } int data=0; boost::context::execution_context< int > ctx(f1); std::tie(ctx,data)=ctx(data+1); std::cout << "f1: returned first time: " << data << std::endl; std::tie(ctx,data)=ctx(data+1); std::cout << "f1: returned second time: " << data << std::endl; std::tie(ctx,data)=ctx(ctx::exec_ontop_arg,f2,data+1); output: f1: entered first time: 1 f1: returned first time: 2 f1: entered second time: 3 f1: returned second time: 4 f2: entered: 5 f1: entered third time: -1
The expression ctx(ctx::exec_ontop_arg,f2,data+1)
executes f2()
on top of context ctx
,
e.g. an additional stack frame is allocated on top of the context stack (in
front of f1()
).
f2()
returns argument -1
that will returned by the second invocation of ctx(data+1)
in f1()
.
~execution_context();
Destructs the associated stack if *this
is a valid context, e.g. execution_context::operator
bool() returns true
.
Nothing.
execution_context( execution_context && other) noexcept;
Moves underlying capture record to *this
.
Nothing.
execution_context & operator=( execution_context && other) noexcept;
Moves the state of other
to *this
using move semantics.
Nothing.
operator bool
()
explicit operator bool() const noexcept;
true
if *this
points to a capture record.
Nothing.
operator!
()
bool operator!() const noexcept;
true
if *this
does not point to a capture record.
Nothing.
operator()
()
std::tuple< execution_context< Args ... >, Args ... > operator()( Args ... args); // member of generic execution_context template execution_context< void > operator()(); // member of execution_context< void >
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 arguments, ... args
, are passed to the current context
to be returned by the most recent call to execution_context::operator()
in the same thread.
The tuple of execution_context and returned arguments passed to the most
recent call to execution_context::operator()
, if any and a execution_context representing
the context that has been suspended.
The returned execution_context indicates if the suspended context has
terminated (return from context-function) via bool
operator()
.
If the returned execution_context has terminated no data are transferred
in the returned tuple.
operator()
()
template< typename Fn > std::tuple< execution_context< Args ... >, Args ... > operator()( exec_ontop_arg_t, Fn && fn, Args ... args); // member of generic execution_context template< typename Fn > execution_context< void > operator()( exec_ontop_arg_t, Fn && fn); // member of execution_context< void >
Same as execution_context::operator(). Additionally,
function fn
is executed
in the context of *this
(e.g. the stack frame of fn
is allocated on stack of *this
).
The tuple of execution_context and returned arguments passed to the most
recent call to execution_context::operator()
, if any and a execution_context representing
the context that has been suspended .
The tuple of execution_context and returned arguments from fn
are passed as arguments to the context-function
of resumed context (if the context is entered the first time) or those
arguments are returned from execution_context::operator()
within the resumed context.
Function fn
needs to
return a tuple of arguments (see description).
The context calling this function must not be destroyed before the arguments,
that will be returned from fn
,
are preserved at least in the stack frame of the resumed context.
The returned execution_context indicates if the suspended context has
terminated (return from context-function) via bool
operator()
.
If the returned execution_context has terminated no data are transferred
in the returned tuple.
operator==
()
bool operator==( execution_context const& other) const noexcept;
true
if *this
and other
represent the same execution context, false
otherwise.
Nothing.
operator!=
()
bool operator!=( execution_context const& other) const noexcept;
! (other == * this)
Nothing.
operator<
()
bool operator<( execution_context const& other) const noexcept;
true
if *this != other
is true and the implementation-defined
total order of execution_context
values places *this
before other
, false otherwise.
Nothing.
operator>
()
bool operator>( execution_context const& other) const noexcept;
other <
* this
Nothing.
operator<=
()
bool operator<=( execution_context const& other) const noexcept;
! (other <
* this)
Nothing.
operator>=
()
bool operator>=( execution_context const& other) const noexcept;
! (*
this <
other)
Nothing.
operator<<()
template< typename charT, class traitsT > std::basic_ostream< charT, traitsT > & operator<<( std::basic_ostream< charT, traitsT > & os, execution_context const& other);
Writes the representation of other
to stream os
.
os