gd_mplex_lookback(3)

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9gd_mplex_lookback(3)                GETDATA               gd_mplex_lookback(3)
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NAME

14       gd_mplex_lookback — set the GetData lookback length for MPLEX fields.
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SYNOPSIS

18       #include <getdata.h>
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20       void gd_mplex_lookback(DIRFILE *dirfile, int lookback);
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DESCRIPTION

24       The  gd_mplex_lookback()  function  controls  how  far GetData searches
25       backwards for the initial value of a field when reading a  MPLEX  field
26       from the dirfile(5) database specified by dirfile.
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28       If the first sample returned of a MPLEX field does not contain the val‐
29       ue of the field, because the index field does not take on the specified
30       multiplex  index value, GetData must search the field backwards to find
31       the last time the field was updated, in order to propagate  that  value
32       into  the initial portion of the returned data.  (See gd_getdata(3) for
33       further discussion on the data returned for a MPLEX field).
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35       This function sets the amount of data GetData searches before giving up
36       according  to  the value of lookback.  If lookback is zero, this search
37       is disabled completely, and the start of  the  returned  data  will  be
38       padded (with zero or IEEE-754-conformant not-a-number) unless the first
39       sample returned happens to contain the value of the field.  If lookback
40       is the special symbol GD_LOOKBACK_ALL, GetData will exhaustively search
41       all available data before the returned portion for an initial value.
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43       If not one of the special values, lookback should be a positive integer
44       specifying the number of MPLEX periods to search.  Each MPLEX field has
45       its own period, measured in samples of the index field,  which  is  de‐
46       fined as follows:
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48       ·      if the MPLEX field in question specifies an optional period pos‐
49              itive parameter (see the MPLEX definition in dirfile-format(5)),
50              its period is that many samples;
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52       ·      otherwise,  if the MPLEX field's count_val is at least five, its
53              period is one more sample than twice that value;
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55       ·      otherwise, the period is ten samples.
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57       So, given the following MPLEX definitions:
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59              field1 MPLEX data_field index_field 7 12
60              field2 MPLEX data_field index_field 2 6
61              field3 MPLEX data_field index_field 7
62              field4 MPLEX data_field index_field 2
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64       field1 has a period of 12 samples and field2  a  period  of  6  samples
65       (both  the value of their period parameters); field3 has a period of 15
66       samples (one more than twice it's count_val), and field4 has  a  period
67       of  10  samples  (the  default  value, since its count_val is less than
68       five).
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70       The initial value of the MPLEX lookback is ten periods.  This value  is
71       provided in getdata.h as GD_DEFAULT_LOOKBACK.
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73       This function always succeeds and returns no value.
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HISTORY

77       The gd_mplex_lookback() function appeared in GetData-0.8.0.
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NAME

SYNOPSIS

DESCRIPTION

HISTORY

SEE ALSO