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40 lines
1.6 KiB
40 lines
1.6 KiB
9 years ago
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define: DUK_USE_EXTSTR_INTERN_CHECK
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introduced: 1.1.0
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requires:
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- DUK_USE_HSTRING_EXTDATA
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default: false
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tags:
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- memory
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- experimental
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description: >
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Provide a hook for checking if data for a certain string can be used from
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external memory (outside of Duktape heap, e.g. memory mapped flash).
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The hook is called during string interning with the following semantics:
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The string data with no NUL termination resides at "ptr" and has "len"
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bytes. The "udata" argument is the heap userdata which may be ignored
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if not needed. If the hook returns NULL, Duktape interns the string
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normally, i.e. string data is allocated from Duktape heap. Otherwise the
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hook return value must point to a memory area which contains
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"len" bytes from "ptr" followed by a NUL byte which is NOT PRESENT
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in the input data. Data behind the returned pointer may not change after
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the hook returns.
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The hook may be called several times for the same input string. This
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happens when a string is interned, garbage collected, and then interned
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again.
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The DUK_USE_EXTSTR_FREE() hook allows application code to detect when
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an external string is about to be freed.
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In most cases the hook should reject strings whose "len" is less than 4
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because there is no RAM advantage in moving so short strings into external
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memory. The ordinary "duk_hstring" header followed by the data (and a
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NUL byte) has the same size as "duk_hstring_external" header which hosts
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a pointer instead of string data.
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Also enable DUK_USE_HSTRING_EXTDATA to use this feature.
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See doc/low-memory.rst for more discussion how to use this feature option
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in practice.
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