[/
          Copyright Oliver Kowalke 2009-2013.
 Distributed under the Boost Software License, Version 1.0.
    (See accompanying file LICENSE_1_0.txt or copy at
          http://www.boost.org/LICENSE_1_0.txt
]

[#stack]
[section:stack Stack allocation]

A __fiber__ uses internally a __econtext__ which manages a set of registers and a stack.
The memory used by the stack is allocated/deallocated via a __stack_allocator__
which is required to model a __stack_allocator_concept__.

A __stack_allocator__ can be passed to [link fiber_fiber `fiber::fiber()`] or
to [ns_function_link fibers..async].

[#stack_allocator_concept]
[heading __stack_allocator_concept__]
A __stack_allocator__ must satisfy the __stack_allocator_concept__ requirements
shown in the following table, in which `a` is an object of a
__stack_allocator__ type, `sctx` is a __stack_context__, and `size` is a `std::size_t`:

[table
    [[expression][return type][notes]]
    [
        [`a(size)`]
        []
        [creates a stack allocator]
    ]
    [
        [`a.allocate()`]
        [__stack_context__]
        [creates a stack]
    ]
    [
        [`a.deallocate( sctx)`]
        [`void`]
        [deallocates the stack created by `a.allocate()`]
    ]
]

[important The implementation of `allocate()` might include logic to protect
against exceeding the context's available stack size rather than leaving it as
undefined behaviour.]

[important Calling `deallocate()` with a __stack_context__ not obtained from
`allocate()` results in undefined behaviour.]

[note The memory for the stack is not required to be aligned; alignment takes
place inside __econtext__.]

See also [@http://www.boost.org/doc/libs/release/libs/context/doc/html/context/stack.html Boost.Context stack allocation].

[class_heading protected_fixedsize_stack]

__boost_fiber__ provides the class __pfixedsize_stack__ 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.

[important Using __pfixedsize_stack__ is expensive. Launching a new fiber with
a stack of this type incurs the overhead of setting the memory protection;
once allocated, this stack is just as efficient to use as __fixedsize_stack__.]

[note The appended `guard page` is [*not] mapped to physical memory, only
virtual addresses are used.]


[class_heading fixedsize_stack]

__boost_fiber__ provides the class __fixedsize_stack__ which models
the __stack_allocator_concept__.
In contrast to __pfixedsize_stack__ it does not append a guard page at the
end of each stack. The memory is simply managed by `std::malloc()` and
`std::free()`.


[class_heading segmented_stack]

__boost_fiber__ supports usage of a __segmented_stack__, i.e.
the stack grows on demand. The fiber is created with a minimal stack size
which will be increased as required.
Class __segmented_stack__ models the __stack_allocator_concept__.
In contrast to __pfixedsize_stack__ and __fixedsize_stack__ it creates a
stack which grows on demand.

[note Segmented stacks are currently only supported by [*gcc] from version
[*4.7] and [*clang] from version [*3.4] onwards. In order to use a
__segmented_stack__, __boost_fiber__ must be built with
property `segmented-stacks`, e.g. [*toolset=gcc segmented-stacks=on] at b2/bjam
command line.]

[endsect]