postinst is in libc6-dev-x32 2.23-0ubuntu10.
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 13 14 15 16 17 18 19 20 | #!/bin/sh
set -e
case $1 in
	(configure)
	# We want /usr/include/bits and /usr/include/gnu to become
	# symlinks, but only after succesfully unpacking libc6-dev-alt
	if ! test -h /usr/include/bits ; then
		rm -rf /usr/include/bits
		ln -sf x86_64-linux-gnu/bits /usr/include/
	fi
	if ! test -h /usr/include/gnu ; then
		rm -rf /usr/include/gnu
		ln -sf x86_64-linux-gnu/gnu /usr/include/
	fi
	;;
	(*)
	;;
esac
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