---
name: "gd_mplex_lookback"
section: "3"
platform: "linux"
package: "libgetdata-doc"
version: "0.11.0-15"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/gd_mplex_lookback"
---

# gd\_mplex\_lookback(3)

set the GetData lookback length for MPLEX fields.

## 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)](/dirfile/5.md) database specified by *dirfile*.

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)](/gd_getdata/3.md) 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.

If not one of the special values, *lookback* should be a positive integer 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)](/dirfile-format/5.md)), 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.

## HISTORY

The **gd\_mplex\_lookback**() function appeared in GetData-0.8.0.

## SEE ALSO

[**gd\_getdata**(3)](/gd_getdata/3.md), [**gd\_open**(3)](/gd_open/3.md), [dirfile(5)](/dirfile/5.md), [dirfile-format(5)](/dirfile-format/5.md)