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### Class stack_allocator

Boost.Coroutine provides the class boost::coroutines::stack_allocator which models the stack-allocator concept. It appends a guard page at the end of each stack to protect against exceeding the stack. If the guard page is accessed (read or write operation) a segmentation fault/access violation is generated by the operating system.

| Note
The appended `guard page` is not mapped to physical memory, only virtual addresses are used.

    class stack_allocator
    {
        static bool is_stack_unbound();

        static std::size_t maximum_stacksize();

        static std::size_t default_stacksize();

        static std::size_t minimum_stacksize();

        void allocate( stack_context &, std::size_t size);

        void deallocate( stack_context &);
    }

##### `static bool is_stack_unbound()`

Returns:

  Returns `true` if the environment defines no limit for the size of a stack.

##### `static std::size_t maximum_stacksize()`

Preconditions:

  `is_stack_unbound()` returns `false`.

Returns:

  Returns the maximum size in bytes of stack defined by the environment.

##### `static std::size_t default_stacksize()`

Returns:

  Returns a default stack size, which may be platform specific. If the stack is unbound then the present implementation returns the maximum of `64 kB` and `minimum_stacksize()`.

##### `static std::size_t minimum_stacksize()`

Returns:

  Returns the minimum size in bytes of stack defined by the environment (Win32 4kB/Win64 8kB, defined by rlimit on POSIX).

##### `void allocate( stack_context & sctx, std::size_t size)`

Preconditions:

  `minimum_stacksize() > size` and `! is_stack_unbound() && ( maximum_stacksize() < size)`.

Effects:

  Allocates memory of at least `size` Bytes and stores a pointer to the stack and its actual size in `sctx`.

Returns:

  Returns pointer to the start address of the new stack. Depending on the architecture the stack grows downwards/upwards the returned address is the highest/lowest address of the stack.

##### `void deallocate( stack_context & sctx)`

Preconditions:

  `sctx.sp` is valid, `minimum_stacksize() > sctx.size` and `! is_stack_unbound() && ( maximum_stacksize() < size)`.

Effects:

  Deallocates the stack space.