Next Up Previous Contents Index
Data Type Encoding

5.1 Data Type Encoding

5.1.1 Primitive Data Types

The four primitive types of the SANE standard are encoded as follows:

A byte is encoded as an 8 bit value. Since the transport protocol is assumed to be byte-orientd, the bit order is irrelevant.

A word is encoded as 4 bytes (32 bits). The bytes are ordered from most-significant to least-significant byte (big-endian byte-order).

A character is currently encoded as an 8-bit ISO LATIN-1 value. An extension to support wider character sets (16 or 32 bits) is planned for the future, but not supported at this point.

A string pointer is encoded as a SANE_Char array. The trailing NUL byte is considered part of the array and a NULL pointer is encoded as a zero-length array.
A handle is encoded like a word. The network backend needs to take care of converting these integer values to the opaque pointer values that are presented to the user of the network backend. Similarly, the SANE daemon needs to take care of converting the opaque pointer values it receives from its backends into 32-bit integers suitable for use for network encoding.

enumeration types:
Enumeration types are encoded like words.

5.1.2 Type Constructors

Closely following the type constructors of the C language, the SANE network protocol supports the following four constructors:

A pointer is encoded by a word that indicates whether the pointer is a NULL-pointer which is then followed by the value that the pointer points to (in the case of a non-NULL pointer; in the case of a NULL pointer, no bytes are encoded for the pointer value).

An array is encoded by a word that indicates the length of the array followed by the values of the elements in the array. The length may be zero in which case no bytes are encoded for the element values.

A structure is encoded by simply encoding the structure members in the order in which they appear in the corresponding C type declaration.

A union must always be accompanied by a tag value that indicates which of the union members is the currently the active one. For this reason, the union itself is encoded simply by encoding the value of the currently active member.

Note that for type constructors, the pointer, element, or member values themselves may have a constructed type. Thus, the above rules should be applied recursively until a sequence of primitive types has been found.

Also SANE had no need for encoding of circular structures. This greatly simplifies the network protocol.

Next Up Previous Contents Index