kopia lustrzana https://gitlab.com/sane-project/website
90 wiersze
3.8 KiB
HTML
90 wiersze
3.8 KiB
HTML
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<title>Data Type Encoding</title>
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<h2><a name="s5.1">5.1 Data Type Encoding</a></h2>
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<p><h3><a name="s5.1.1">5.1.1 Primitive Data Types</a></h3>
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<p>The four primitive types of the SANE standard are encoded as follows:
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<dl>
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<p><dt><tt>SANE_Byte<a name="i117"></tt>:<dd> A byte is encoded as an 8 bit value.
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Since the transport protocol is assumed to be byte-orientd, the bit
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order is irrelevant.
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<p><dt><tt>SANE_Word<a name="i118"></tt>:<dd> A word is encoded as 4 bytes (32
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bits). The bytes are ordered from most-significant to
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least-significant byte (big-endian byte-order).
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<p><dt><tt>SANE_Char<a name="i119"></tt>:<dd> A character is currently encoded as an 8-bit
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ISO LATIN-1 value. An extension to support wider character sets (16 or 32
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bits) is planned for the future, but not supported at this point.
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<p><dt><tt>SANE_String<a name="i120"></tt>:<dd> A string pointer is encoded as a
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<tt>SANE_Char</tt> array. The trailing NUL byte is considered part
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of the array and a <tt>NULL</tt> pointer is encoded as a zero-length
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array.
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<dt><tt>SANE_Handle<a name="i121"></tt>:<dd> A handle is encoded like a word.
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The network backend needs to take care of converting these integer
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values to the opaque pointer values that are presented to the user
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of the network backend. Similarly, the SANE daemon needs to take
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care of converting the opaque pointer values it receives from its
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backends into 32-bit integers suitable for use for network encoding.
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<p><dt><em>enumeration types<a name="i122"></em>:<dd> Enumeration types are encoded
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like words.
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<p></dl>
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<p><h3><a name="s5.1.2">5.1.2 Type Constructors</a></h3>
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<p>Closely following the type constructors of the C language, the SANE network
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protocol supports the following four constructors:
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<dl>
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<p><dt><em>pointer<a name="i123"></em>:<dd> A pointer is encoded by a word that indicates
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whether the pointer is a NULL-pointer which is then followed by the
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value that the pointer points to (in the case of a non-NULL pointer;
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in the case of a NULL pointer, no bytes are encoded for the pointer
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value).
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<p><dt><em>array<a name="i124"></em>:<dd> An array is encoded by a word that indicates
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the length of the array followed by the values of the elements in
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the array. The length may be zero in which case no bytes are
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encoded for the element values.
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<p><dt><em>structure<a name="i125"></em>:<dd> A structure is encoded by simply encoding the
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structure members in the order in which they appear in the
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corresponding C type declaration.
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<p><dt><em>union<a name="i126"></em>:<dd> A union must always be accompanied by a tag
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value that indicates which of the union members is the currently the
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active one. For this reason, the union itself is encoded simply by
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encoding the value of the currently active member.
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<p></dl>
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<p>Note that for type constructors, the pointer, element, or member
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values themselves may have a constructed type. Thus, the above rules
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should be applied recursively until a sequence of primitive types has
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been found.
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<p>Also SANE had no need for encoding of circular structures. This
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greatly simplifies the network protocol.
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<p><p><hr>
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