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96 lines
3.4 KiB
Plaintext
96 lines
3.4 KiB
Plaintext
[/
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Copyright Oliver Kowalke 2009-2013.
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt
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]
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[#stack]
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[section:stack Stack allocation]
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A __fiber__ uses internally a __econtext__ which manages a set of registers and a stack.
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The memory used by the stack is allocated/deallocated via a __stack_allocator__
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which is required to model a __stack_allocator_concept__.
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A __stack_allocator__ can be passed to [link fiber_fiber `fiber::fiber()`] or
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to [ns_function_link fibers..async].
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[#stack_allocator_concept]
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[heading __stack_allocator_concept__]
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A __stack_allocator__ must satisfy the __stack_allocator_concept__ requirements
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shown in the following table, in which `a` is an object of a
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__stack_allocator__ type, `sctx` is a __stack_context__, and `size` is a `std::size_t`:
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[table
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[[expression][return type][notes]]
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[
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[`a(size)`]
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[]
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[creates a stack allocator]
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]
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[
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[`a.allocate()`]
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[__stack_context__]
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[creates a stack]
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]
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[
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[`a.deallocate( sctx)`]
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[`void`]
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[deallocates the stack created by `a.allocate()`]
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]
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]
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[important The implementation of `allocate()` might include logic to protect
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against exceeding the context's available stack size rather than leaving it as
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undefined behaviour.]
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[important Calling `deallocate()` with a __stack_context__ not obtained from
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`allocate()` results in undefined behaviour.]
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[note The memory for the stack is not required to be aligned; alignment takes
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place inside __econtext__.]
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See also [@http://www.boost.org/doc/libs/release/libs/context/doc/html/context/stack.html Boost.Context stack allocation].
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[class_heading protected_fixedsize_stack]
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__boost_fiber__ provides the class __pfixedsize_stack__ which models
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the __stack_allocator_concept__.
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It appends a guard page at the end of each stack to protect against exceeding
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the stack. If the guard page is accessed (read or write operation) a
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segmentation fault/access violation is generated by the operating system.
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[important Using __pfixedsize_stack__ is expensive. Launching a new fiber with
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a stack of this type incurs the overhead of setting the memory protection;
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once allocated, this stack is just as efficient to use as __fixedsize_stack__.]
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[note The appended `guard page` is [*not] mapped to physical memory, only
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virtual addresses are used.]
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[class_heading fixedsize_stack]
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__boost_fiber__ provides the class __fixedsize_stack__ which models
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the __stack_allocator_concept__.
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In contrast to __pfixedsize_stack__ it does not append a guard page at the
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end of each stack. The memory is simply managed by `std::malloc()` and
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`std::free()`.
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[class_heading segmented_stack]
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__boost_fiber__ supports usage of a __segmented_stack__, i.e.
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the stack grows on demand. The fiber is created with a minimal stack size
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which will be increased as required.
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Class __segmented_stack__ models the __stack_allocator_concept__.
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In contrast to __pfixedsize_stack__ and __fixedsize_stack__ it creates a
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stack which grows on demand.
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[note Segmented stacks are currently only supported by [*gcc] from version
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[*4.7] and [*clang] from version [*3.4] onwards. In order to use a
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__segmented_stack__, __boost_fiber__ must be built with
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property `segmented-stacks`, e.g. [*toolset=gcc segmented-stacks=on] at b2/bjam
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command line.]
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[endsect]
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