Text units and text unit pointer lists

Many of the TRANSMIT and RECEIVE exits are passed the addresses of text unit pointer lists. Each entry in the text unit pointer list points to a copy of a text unit that contains information used to control the transmission -- information such as the record format, or the origin node name or user ID. Because the exits receive read-only copies of the text units, they cannot affect subsequent processing by changing the information in them.

Text units are sent in control records as part of the transmission. It is possible that a text unit could be split among two or more control records. For more information about the format of control records, see Format of transmitted data.

The following figure shows the format of text unit pointer lists and text units. Note that the last entry in the text unit pointer list has the high-order bit is set to one.

Figure 1. Format of text unit pointer lists and text units

Offset	Length	Description
+0	2	Text unit key. The key identifies the type of information contained in the text unit. Possible key values are given in Types of text units.
+2	2	Number. The number field contains the number of length-data pairs that follow. Most of the text units have only one length and one data field.
+4	2	Length. The first of perhaps many length fields. The length value includes only the length of the data field immediately following it, and not its own two-byte length.
+6	n	Data. The first of perhaps many data fields. The data field contains the parameter information being passed, for example, the target node name. The descriptions of the individual text units, which follow, describe the content of each.
+(8+n)		Second length-data entry if the number field indicates more than one entry is present.