gd_mplex_lookback(3) GETDATA gd_mplex_lookback(3)NAMEgd_mplex_lookback — set the GetData lookback length for MPLEX fields.
SYNOPSIS
#include <getdata.h>
void gd_mplex_lookback(DIRFILE *dirfile, int lookback);
DESCRIPTION
The gd_mplex_lookback() function controls how far GetData searches
backwards for the initial value of a field when reading a MPLEX field
from the dirfile(5) database specified by dirfile.
If the first sample returned of a MPLEX field does not contain the val‐
ue 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, mea‐
sured in samples of the index field, which is defined as follows:
· if the MPLEX field in question specifies an optional period pos‐
itive 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.
SEE ALSOdirfile(5), dirfile-format(5), gd_getdata(3).
Version 0.8.4 3 April 2013 gd_mplex_lookback(3)