163 lines
5.2 KiB
Plaintext
163 lines
5.2 KiB
Plaintext
[/
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Copyright Oliver Kowalke 2009.
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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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[section:stack Stack allocation]
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A __coro__ uses internally a __ctx__ 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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[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.allocate( sctx, size)`]
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[`void`]
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[creates a stack of at least `size` bytes and stores both values in `sctx`]
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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 pointer not returned by `allocate()`
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results in undefined behaviour.]
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[note The stack is not required to be aligned; alignment takes place inside
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__coro__.]
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[note Depending on the architecture `allocate()` returns an address from the
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top of the stack (growing downwards) or the bottom of the stack (growing
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upwards).]
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[section:stack_allocator Class ['stack_allocator]]
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__boost_coroutine__ provides the class __coro_allocator__ 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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[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 stack_allocator
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{
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static bool is_stack_unbound();
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static std::size_t maximum_stacksize();
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static std::size_t default_stacksize();
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static std::size_t minimum_stacksize();
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void allocate( stack_context &, std::size_t size);
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void deallocate( stack_context &);
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}
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[heading `static bool is_stack_unbound()`]
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[variablelist
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[[Returns:] [Returns `true` if the environment defines no limit for the size of a stack.]]
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]
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[heading `static std::size_t maximum_stacksize()`]
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[variablelist
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[[Preconditions:] [`is_stack_unbound()` returns `false`.]]
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[[Returns:] [Returns the maximum size in bytes of stack defined by the environment.]]
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]
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[heading `static std::size_t default_stacksize()`]
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[variablelist
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[[Returns:] [Returns a default stack size, which may be platform specific.
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If the stack is unbound then the present implementation returns the maximum of
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`64 kB` and `minimum_stacksize()`.]]
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]
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[heading `static std::size_t minimum_stacksize()`]
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[variablelist
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[[Returns:] [Returns the minimum size in bytes of stack defined by the
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environment (Win32 4kB/Win64 8kB, defined by rlimit on POSIX).]]
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]
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[heading `void allocate( stack_context & sctx, std::size_t size)`]
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[variablelist
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[[Preconditions:] [`minimum_stacksize() > size` and
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`! is_stack_unbound() && ( maximum_stacksize() < size)`.]]
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[[Effects:] [Allocates memory of at least `size` Bytes and stores a pointer
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to the stack and its actual size in `sctx`.]]
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[[Returns:] [Returns pointer to the start address of the new stack. Depending
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on the architecture the stack grows downwards/upwards the returned address is
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the highest/lowest address of the stack.]]
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]
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[heading `void deallocate( stack_context & sctx)`]
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[variablelist
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[[Preconditions:] [`sctx.sp` is valid, `minimum_stacksize() > sctx.size` and
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`! is_stack_unbound() && ( maximum_stacksize() < size)`.]]
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[[Effects:] [Deallocates the stack space.]]
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]
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[endsect]
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[section:stack_context Class ['stack_context]]
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__boost_coroutine__ provides the class __stack_context__ which will contain
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the stack pointer and the size of the stack.
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In case of a __segmented_stack__ __stack_context__ contains some extra controll
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structures.
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struct stack_context
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{
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void * sp;
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std::size_t size;
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// might contain addition controll structures
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// for instance for segmented stacks
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}
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[heading `void * sp`]
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[variablelist
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[[Value:] [Pointer to the beginning of the stack.]]
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]
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[heading `std::size_t size`]
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[variablelist
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[[Value:] [Actual size of the stack.]]
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]
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[endsect]
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[section:segmented_stack Segmented stacks]
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__boost_coroutine__ supports usage of a __segmented_stack__, e. g. the size of
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the stack grows on demand. The coroutine is created with an minimal stack size and
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will be increased as required.
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Segmented stacks are currently only supported by [*gcc] from version [*4.7]
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onwards. In order to use a __segmented_stack__ __boost_coroutine__ must be build
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with [*toolset=gcc segmented-stacks=on] at b2/bjam command-line. Applications
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must be compiled with compiler-flags [*-fsplit-stack -DBOOST_USE_SEGMENTED_STACKS].
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[endsect]
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[endsect]
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