Debugging util-linux programs
=============================

How to deal libtool
-------------------

There are considerations to be made when profiling or debugging some programs
found in the util-linux package. Because wrapper scripts are used for the
binaries to make sure all library dependencies are met, you cannot use tools
such as gdb or valgrind directly with them.

Let's take for example the mount command:

	$> cd /path/to/util-linux
	$> file mount/mount
	mount/mount: Bourne-Again shell script text executable

The binary itself is located in the mount/.libs/ directory:

	$> file mount/.libs/mount
	mount/.libs/mount: ELF 64-bit LSB executable, x86-64, version 1 \
	(SYSV), dynamically linked (uses shared libs) [...]

When this command is run, there's a library dependency error:

	$> mount/.libs/mount
	mount/.libs/mount: /lib/libblkid.so.1: version `BLKID_2.20' not found \
	(required by mount/.libs/mount)

To overcome this we need set the LD_LIBRARY_PATH variable to read the path of
the shared lib found in the sources, and not system-wide:

	$> export LD_LIBRARY_PATH=$PWD/libblkid/src/.libs/:$LD_LIBRARY_PATH

Now external debugging tools can be run on the binary.

Happy hacking!
Davidlohr Bueso, August 2011


The libmount & libblkid
-----------------------

Both of the libraries can be debugged by setting an environment variable
consistig a number. The number will be used as a bit mask, so the more 1 the
greater the debugging level. Seach for `DEBUG' from files

	libblkid/src/blkidP.h
	libmount/src/mountP.h

to see what the different bit mean. At the time of writing this the following
enabled full debug.

	export LIBBLKID_DEBUG=0xffff
	export LIBMOUNT_DEBUG=0xffff

The libblkid reads by default /etc/blkid.conf which can be overriden by the
environment variable BLKID_CONF. See manual libblkid/libblkid.3 for details
about the configuration file.

Block device information is normally kept in a cache file /etc/blkid.tab that
can be overridden by the environment variable BLKID_FILE.

To libmount uses three paths, which can be override by using environment
variables. Notice that these environment variables are ignored for non-root
users

	env variable		if not set defaults to
	LIBMOUNT_FSTAB		/etc/fstab
	LIBMOUNT_MTAB		/etc/mtab
	LIBMOUNT_UTAB		/run/mount/utab or /dev/.mount/utab
