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## Struct fcontext_t

Each instance of fcontext_t represents a context (CPU registers and stack space). Together with its related functions jump_fcontext() and make_fcontext() it provides a execution control transfer mechanism similar interface like ucontext_t. fcontext_t and its functions are located in boost::context and the functions are declared as extern "C".

| Warning
If fcontext_t is used in a multi threaded application, it can migrated between threads, but must not reference thread-local storage.

| Important
The low level API is the part to port to new platforms.

| Note
If fiber-local storage is used on Windows, the user is responsible for calling ::FlsAlloc(), ::FlsFree().

#### Executing a context

A new context supposed to execute a context-function (returning void and accepting void * as argument) will be created on top of the stack (at 16 byte boundary) by function make_fcontext().

    // context-function
    void f(intptr);

    // creates a new stack
    std::size_t size = 8192;
    void* sp(std::malloc(size));

    // context fc uses f() as context function
    // fcontext_t is placed on top of context stack
    // a pointer to fcontext_t is returned
    fcontext_t fc(make_fcontext(sp,size,f));

Calling jump_fcontext() invokes the context-function in a newly created context complete with registers, flags, stack and instruction pointers. When control should be returned to the original calling context, call jump_fcontext(). The current context information (registers, flags, and stack and instruction pointers) is saved and the original context information is restored. Calling jump_fcontext() again resumes execution in the second context after saving the new state of the original context.

    boost::context::fcontext_t fcm,fc1,fc2;

    void f1(void *)
    {
        std::cout<<"f1: entered"<<std::endl;
        std::cout<<"f1: call jump_fcontext( & fc1, fc2, 0)"<< std::endl;
        boost::context::jump_fcontext(&fc1,fc2,0);
        std::cout<<"f1: return"<<std::endl;
        boost::context::jump_fcontext(&fc1,fcm,0);
    }

    void f2(void *)
    {
        std::cout<<"f2: entered"<<std::endl;
        std::cout<<"f2: call jump_fcontext( & fc2, fc1, 0)"<<std::endl;
        boost::context::jump_fcontext(&fc2,fc1,0);
        BOOST_ASSERT(false&&!"f2: never returns");
    }

    std::size_t size(8192);
    void* sp1(std::malloc(size));
    void* sp2(std::malloc(size));

    fc1=boost::context::make_fcontext(sp1,size,f1);
    fc2=boost::context::make_fcontext(sp2,size,f2);

    std::cout<<"main: call jump_fcontext( & fcm, fc1, 0)"<<std::endl;
    boost::context::jump_fcontext(&fcm,fc1,0);

    output:
        main: call jump_fcontext( & fcm, fc1, 0)
        f1: entered
        f1: call jump_fcontext( & fc1, fc2, 0)
        f2: entered
        f2: call jump_fcontext( & fc2, fc1, 0)
        f1: return

First call of jump_fcontext() enters the context-function `f1()` by starting context fc1 (context fcm saves the registers of `main()`). For jumping between context's fc1 and fc2 `jump_fcontext()` is called. Because context fcm is chained to fc1, `main()` is entered (returning from jump_fcontext()) after context fc1 becomes complete (return from `f1()`).

| Warning
Calling jump_fcontext() to the same context from inside the same context results in undefined behaviour.

| Important
The size of the stack is required to be larger than the size of fcontext_t.

| Note
In contrast to threads, which are preemtive, fcontext_t switches are cooperative (programmer controls when switch will happen). The kernel is not involved in the context switches.

#### Transfer of data

The third argument passed to jump_fcontext(), in one context, is passed as the first argument of the context-function if the context is started for the first time. In all following invocations of jump_fcontext() the void * passed to jump_fcontext(), in one context, is returned by jump_fcontext() in the other context.

    boost::context::fcontext_t fcm,fc;

    typedef std::pair<int,int> pair_t;

    void f(void * param)
    {
        pair_t* p=(pair_t*)param;
        p=(pair_t*)boost::context::jump_fcontext(&fc,fcm,(void *)(p->first+p->second));
        boost::context::jump_fcontext(&fc,fcm,(void *)(p->first+p->second));
    }

    std::size_t size(8192);
    void* sp(std::malloc(size));

    pair_t p(std::make_pair(2,7));
    fc=boost::context::make_fcontext(sp,size,f);

    int res=(int)boost::context::jump_fcontext(&fcm,fc,(void *)&p);
    std::cout<<p.first<<" + "<<p.second<<" == "<<res<<std::endl;

    p=std::make_pair(5,6);
    res=(int)boost::context::jump_fcontext(&fcm,fc,(void *)&p);
    std::cout<<p.first<<" + "<<p.second<<" == "<<res<<std::endl;

    output:
        2 + 7 == 9
        5 + 6 == 11

#### Exceptions in context-function

If the context-function emits an exception, the behaviour is undefined.

| Important
context-function should wrap the code in a try/catch block.

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

#### Stack unwinding

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.

#### `fcontext_t` and related functions

    struct stack_t
    {
        void* sp;
        std::size_t size;
    };

    typedef <opaque pointer > fcontext_t;

    void * jump_fcontext(fcontext_t* ofc,fcontext_t nfc,void * vp);
    fcontext_t make_fcontext(void* sp,std::size_t size,void(*fn)(void *));

#### `sp`

Member:

  Pointer to the beginning of the stack (depending of the architecture the stack grows downwards or upwards).

#### `size`

Member:

  Size of the stack in bytes.

#### `fc_stack`

Member:

  Tracks the memory for the context's stack.

#### `void * jump_fcontext(fcontext_t* ofc,fcontext_t nfc,void * p)`

Effects:

  Stores the current context data (stack pointer, instruction pointer, and CPU registers) to `*ofc` and restores the context data from `nfc`, which implies jumping to execution context `nfc`. The void * argument, `p`, is passed to the current context to be returned by the most recent call to `jump_fcontext()` in the same thread.

Returns:

  The third pointer argument passed to the most recent call to `jump_fcontext()`, if any.

#### `fcontext_t make_fcontext(void* sp,std::size_t size,void(*fn)(void *))`

Precondition:

  Stack `sp` and function pointer `fn` are valid (depending on the architecture `sp` points to the top or bottom of the stack) and `size` > 0.

Effects:

  Creates an fcontext_t on top of the stack and prepares the stack to execute the context-function `fn`.

Returns:

  Returns a fcontext_t which is placed on the stack.