Buffer Element Format

This topic lists the common fields that always occur at the start of a buffer element. The dataru field contains information specific to the particular message; see Message Formats for details of individual message formats.

PTRBFELT        hdreptr–>elteptr;
INTEGER         hdreptr–>startd;
INTEGER         hdreptr–>endd;
CHAR            hdreptr–>trpad;
CHAR[SNANBEDA]  hdreptr–>dataru;
};
 

Members

hdreptr–>elteptr

Pointer to next buffer element in the chain; NULL if this element is the last or only element in the chain.

hdreptr–>startd

Start of valid data in this element. The index into dataru of the first byte of valid data.

hdreptr–>endd

End of valid data in this element. The index into dataru of the last byte of valid data.

hdreptr–>trpad

Pad byte (reserved).

hdreptr–>dataru

An array of characters that contains the data for this element. Note that the valid data might not occupy the whole of the element; startd and endd give the indexes into this array of the start and end of the valid data. The constant SNANBEDA is defined in SNA_DLC.H as 268.

The following information will help you to interpret the message formats:

• Fields that occupy 2 bytes are represented with the arithmetically most significant byte in the lowest byte address, irrespective of the normal orientation used by the processor on which the software executes. That is, the 2-byte value 0x1234 has the byte 0x12 in the lowest byte address. The exceptions to this are the startd and endd fields in elements, which are always stored in low-byte, high-byte orientation (the normal orientation of an Intel processor).
• The offsets indicated by the startd and endd fields are expressed in terms of the first byte of dataru being offset 1; the first byte of valid data is at dataru(startd–1). For example, if startd is 11 and endd is 18, then dataru begins with 10 bytes that are not valid data, followed by 8 bytes of valid data.

In the example message format illustrated in Overview of Message Formats, each element has a startd of 13, indicating 12 bytes of padding before the start of the valid data. This leaves room for 256 bytes of data, and hence the element data (300 bytes long in this example) requires two elements.