If the first sample returned of a MPLEX field does not contain the value of the field, because the index field does not take on the specified multiplex index value, GetData must search the field backwards to find the last time the field was updated, in order to propagate that value into the initial portion of the returned data. (See gd_getdata(3) for further discussion on the data returned for a MPLEX field).

This function sets the amount of data GetData searches before giving up according to the value of lookback. If lookback is zero, this search is disabled completely, and the start of the returned data will be padded (with zero or IEEE-754-conformant not-a-number) unless the first sample returned happens to contain the value of the field. If lookback is the special symbol GD_LOOKBACK_ALL, GetData will exhaustively search all available data before the returned portion for an initial value.

Otherwise, lookback should be a positive number specifying the number of MPLEX periods to search. Each MPLEX field has its own period, measured in samples of the index field, which is defined as follows:

• if the MPLEX field in question specifies an optional period positive parameter (see the MPLEX definition in dirfile-format(5)), its period is that many samples;
• otherwise, if the MPLEX field's count_val is at least five, its period is one more sample than twice that value;
• otherwise, the period is ten samples.

So, given the following MPLEX definitions:

field1 MPLEX data_field index_field 7 12
field2 MPLEX data_field index_field 2 6
field3 MPLEX data_field index_field 7
field4 MPLEX data_field index_field 2

field1 has a period of 12 samples and field2 a period of 6 samples (both the value of their period parameters); field3 has a period of 15 samples (one more than twice it's count_val), and field4 has a period of 10 samples (the default value, since its count_val is less than five).

The initial value of the MPLEX lookback is ten periods. This value is provided in getdata.h as GD_DEFAULT_LOOKBACK.

This function always succeeds and returns no value.