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gd_mplex_lookback(3)		    GETDATA		  gd_mplex_lookback(3)

NAME
       gd_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 ALSO
       dirfile(5), dirfile-format(5), gd_getdata(3).

Version 0.8.4			 3 April 2013		  gd_mplex_lookback(3)
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