postinst is in libc-bin 2.15-0ubuntu10.18.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12  | #!/bin/sh
set -e
if [ "x$1" = xtriggered ]
then
	LDCONFIG_NOTRIGGER=y
	export LDCONFIG_NOTRIGGER
	echo ldconfig deferred processing now taking place
	ldconfig || ldconfig --verbose
	exit 0
fi
 |