Ad Astra Per Aspera – Software, science and random thoughts about IT

GDB – A lot more than you knew

This talk covers a lot of interesting things about GDB.

How to find the IP address of Linux guests

There’s a lot of examples in the internet showing how you can obtain the IP address of a libvirt guest virtual machine. Most of the examples show how to do that when the network is using NAT forwarding (aka “Virtual Networks”).

However, how to do that if your setup is using Bridged Networking and your guests receive an IP address from an external DHCP server?

One idea is to install the read the DHCP leases file straight from the guest FS. You can do that by using the libguestfs-tools to cat the file within the image. For RHEL 6 (and CentOS 6 and similar distributions) the process is similar to this:

virt-cat <domain-name> /var/lib/dhclient/dhclient-eth1.leases | tail -17 | grep "fixed-address" | sed 's/;//' | awk '{ print $2 }'

1	virt-cat <domain-name> /var/lib/dhclient/dhclient-eth1.leases \| tail -17 \| grep "fixed-address" \| sed 's/;//' \| awk '{ print $2 }'

For RHEL 7 (as well as CentOS 7 and similar) the process is slightly different due to the way NetworkManager stores the leases file:

leases_file=$(virt-ls <domain-name> /var/lib/NetworkManager/ | grep lease)
virt-cat <domain-name> /var/lib/NetworkManager/$leases_file | tail -17 | grep "fixed-address" | sed 's/;//' | awk '{ print $2 }'

1 2	leases_file=$(virt-ls <domain-name> /var/lib/NetworkManager/ \| grep lease) virt-cat <domain-name> /var/lib/NetworkManager/$leases_file \| tail -17 \| grep "fixed-address" \| sed 's/;//' \| awk '{ print $2 }'

In this Gist there’s a modified script I used to update the IP address of all guests in one shot.

Do you want colors on your man pages?

Then, try this.

Malloc Internals

The Red Hat Developers Blog recently published this article about malloc internals which might be pretty interesting to those working with C.

MQTT Cheat Sheet

The MQTT (Message Queue Telemetry Transport) messaging protocol is a lightweight protocol well suited for the Internet of Things. It is a simple protocol and an ISO standard. It comes with an extensible reference documentation.

To help me learn about it and simplify my work flow, I created this cheat sheet (warning: PDF) which highlights the most relevant details about the protocol. You can also see it online here.

Messaging performance: tips, tricks and tools

Today I gave a talk entitled “Messaging Performance: tips, tricks and tools” at the DevConf.cz 2017 in Brno. For those interested in the subject, I have made the slides available here.

The talk itself should also be available soon on the DevConf.cz channel on youtube.

Otavio’s must-watch linux.conf.au talks

I compiled a list of 8 talks from linux.conf.au 2017 that interested me. You can watch it here or just click play on the embedded video below:

EOY 2016 links

Some links about interesting things I’ve read or watched by the end of 2016. Further comments are provided on each item of the list.

Single file libraries: open source libraries with minimal dependencies.
The sorry state of programming world as the end of 2016 AD: food for though regarding how most of development is done nowadays.
Find my Phone: what happens when your phone is stolen.
PayPal Cuts Costs 10x With Open Source CI: self descriptive.
Learning to Read x86 Assembly: self descriptive.
Json-c tutorial: how to manipulate json in C.
The 101 of ELF Binaries on Linux: Understanding and Analysis: self descriptive.
How the Linux kernel knows it’s running in a Virtual Machine: self descriptive.
Stack Overflow: The Architecture – 2016 Edition: self descriptive.
AMQP 1.0 in Azure Service Bus and Event Hubs protocol guide: a detailed explanation abou AMQP 1.0.
10 Tips for C and C++ Performance Improvement Code Optimization: simple optimization tips.
Light Weight Protocol: Critical Implications: a talk about MQTT and security.
Modern C: a book about modern C programming.

Google Sanitizers and Fedora 25

Here’s a tip if you are using Fedora and trying to use one of the Google Sanitizers with clang and is having problems doing so. One of the common problems is not being able to find libclang_rt.asan-x86_64.a:

/usr/bin/ld: cannot find /usr/bin/../lib64/clang/3.8.0/lib/linux/libclang_rt.asan-x86_64.a: No such file or directory

1	/usr/bin/ld: cannot find /usr/bin/../lib64/clang/3.8.0/lib/linux/libclang_rt.asan-x86_64.a: No such file or directory

This can be resolved by simply installing the compiler-rt package:

dnf install -y compiler-rt

1	dnf install -y compiler-rt

Restoring click method behavior on Fedora 25 and libinput

Here’s a quick tip if you need to enable the Software Button Area behavior on an Apple macbook running Fedora 25:

xinput set-prop bcm5974 "libinput Click Method Enabled" 1 0

1	xinput set-prop bcm5974 "libinput Click Method Enabled" 1 0

Some comments about this:

You have to run it after every reboot
There’s, probably, a better way to do this. Per documentation, it should be possible to set it up permanently by creating a file on /etc/X11/xorg.conf.d however I did not manage to create a file that works so far.

References:

Libinput documentation: Clickpad Software Buttons
Libinput FAQ
Libinput Man Page