Scott Hanselman

13 hours debugging a segmentation fault in .NET Core on Raspberry Pi and the solution was...

July 18, 2017 Comment on this post [63] Posted in Bugs | DotNetCore
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Debugging is a satisfying and special kind of hell. You really have to live it to understand it. When you're deep into it you never know when it'll be done. When you do finally escape it's almost always a DOH! moment.

I spent an entire day debugging an issue and the solution ended up being a checkbox.

NOTE: If you get a third of the way through this blog post and already figured it out, well, poop on you. Where were you after lunch WHEN I NEEDED YOU?

I wanted to use a Raspberry Pi in a tech talk I'm doing tomorrow at a conference. I was going to show .NET Core 2.0 and ASP.NET running on a Raspberry Pi so I figured I'd start with Hello World. How hard could it be?

You'll write and build a .NET app on Windows or Mac, then publish it to the Raspberry Pi. I'm using a preview build of the .NET Core 2.0 command line and SDK (CLI) I got from here.

C:\raspberrypi> dotnet new console
C:\raspberrypi> dotnet run
Hello World!
C:\raspberrypi> dotnet publish -r linux-arm
Microsoft Build Engine version for .NET Core

raspberrypi1 -> C:\raspberrypi\bin\Debug\netcoreapp2.0\linux-arm\raspberrypi.dll
raspberrypi1 -> C:\raspberrypi\bin\Debug\netcoreapp2.0\linux-arm\publish\

Notice the simplified publish. You'll get a folder for linux-arm in this example, but could also publish osx-x64, etc. You'll want to take the files from the publish folder (not the folder above it) and move them to the Raspberry Pi. This is a self-contained application that targets ARM on Linux so after the prerequisites that's all you need.

I grabbed a mini-SD card, headed over to and downloaded the latest Raspbian image. I used - a lovely image burner for Windows, Mac, or Linux - and wrote the image to the SD Card. I booted up and got ready to install some prereqs. I'm only 15 min in at this point. Setting up a Raspberry Pi 2 or Raspberry Pi 3 is VERY smooth these days.

Here's the prereqs for .NET Core 2 on Ubuntu or Debian/Raspbian. Install them from the terminal, natch.

sudo apt-get install libc6 libcurl3 libgcc1 libgssapi-krb5-2 libicu-dev liblttng-ust0 libssl-dev libstdc++6 libunwind8 libuuid1 zlib1g

I also added an FTP server and ran vncserver, so I'd have a few ways to talk to the Raspberry Pi. Yes, I could also SSH in but I have a spare monitor, and with that monitor plus VNC I didn't see a need.

sudo apt-get pure-ftpd

Then I fire up Filezilla - my preferred FTP client - and FTP the publish output folder from my dotnet publish above. I put the files in a folder off my ~\Desktop.

FTPing files

Then from a terminal I

pi@raspberrypi:~/Desktop/helloworld $ chmod +x raspberrypi

(or whatever the name of your published "exe" is. It'll be the name of your source folder/project with no extension. As this is a self-contained published app, again, all the .NET Core runtime stuff is in the same folder with the app.

pi@raspberrypi:~/Desktop/helloworld $ ./raspberrypi 
Segmentation fault

The crash was instant...not a pause and a crash, but it showed up as soon as I pressed enter. Shoot.

I ran "strace ./raspberrypi" and got this output. I figured maybe I missed one of the prerequisite libraries, and I just needed to see which one and apt-get it. I can see the error, but that's a historical Debian-ism and more of a warning than a fatal.

strace on a bad exe in Linux

I used to be able to read straces 20 years ago but much like my Spanish, my skills are only good at Chipotle. I can see it just getting started loading libraries, seeking around in them, checking file status,  mapping files to memory, setting memory protection, then it all falls apart. Perhaps we tried to do something inappropriate with some memory that just got protected? We are dereferencing a null pointer.

Maybe you can read this and you already know what is going to happen! I did not.

I run it under gdb:

pi@raspberrypi:~/Desktop/WTFISTHISCRAP $ gdb ./raspberrypi 
GNU gdb (Raspbian 7.7.1+dfsg-5+rpi1) 7.7.1
Copyright (C) 2014 Free Software Foundation, Inc.
This GDB was configured as "arm-linux-gnueabihf".
"/home/pi/Desktop/helloworldWRONG/./raspberrypi1": not in executable format: File truncated

Ok, sick files?

I called Peter Marcu from the .NET team and we chatted about how he got it working and compared notes.

I was using a Raspberry Pi 2, he a Pi 3. Ok, I'll try a 3. 30 minutes later, new SD card, new burn, new boot, pre-reqs, build, FTP, run, SAME RESULT - segfault.


Maybe corruption? Here's a thread about Corrupted Files on Raspbian Jesse 2017-07-05! That's the version I have. OK, I'll try the build of Raspbian from a week before.

30 minutes later, burn another SD card, new boot, pre-reqs, build, FTP, run, SAME RESULT - segfault.



Maybe a bad nuget.config? No.

Bad daily .NET build? No.


Ok, I'll try Ubuntu Mate for Raspberry Pi. TOTALLY different OS.

30 minutes later, burn another SD card, new boot, pre-reqs, build, FTP, run, SAME RESULT - segfault.

What's the common thread here? Ok, I'll try from another Windows machine.

SAME RESULT - segfault.

I call Peter back and we figure it's gotta be prereqs...but the strace doesn't show we're even trying to load any interesting libraries. We fail FAST.

Ok, let's get serious.

We both have Raspberry Pi 3s. Check.

What kind of SD card does he have? Sandisk? Ok,  I'll use Sandisk. But disk corruption makes no sense at that level...because the OS booted!

What did he burn with? He used Win32diskimager and I used Etcher. Fine, I'll bite.

30 minutes later, burn another SD card, new boot, pre-reqs, build, FTP, run, SAME RESULT - segfault.

He sends me HIS build of a HelloWorld and I FTP it over to the Pi. SAME RESULT - segfault.

Peter is freaking out. I'm deeply unhappy and considering quitting my job. My kids are going to sleep because it's late.

I ask him what he's FTPing with, and he says WinSCP. I use FileZilla, ok, I'll try WinSCP.

WinSCP's New Session dialog starts here:

SFTP is Default

I say, WAIT. Are you using SFTP or FTP? Peter says he's using SFTP so I turn on SSH on the Raspberry Pi and SFTP into it with WinSCP and copy over my Hello World.


Hello World on a Raspberry Pi


I make a folder called Good and a folder called BAD. I copy with FileZilla to BAD and with WinSCP to GOOD. Then I run a compare. Maybe some part of .NET Core got corrupted? Maybe a supporting native library?

pi@raspberrypi:~/Desktop $ diff --brief -r helloworld/ helloworldWRONG/
Files helloworld/raspberrypi1 and helloworldWRONG/raspberrypi1 differ

Wait, WHAT? The executable are different? One is 67,684 bytes and the bad one is 69,632 bytes.

Time for a  visual compare.

All the ODs are gone

At this point I saw it IMMEDIATELY.

0D is CR (13) and 0A is LF (10). I know this because I'm old and I've written printer drivers for printers that had both carriages and lines to feed. Why do YOU know this? Likely because you've transferred files between Unix and Windows once or thrice, perhaps with FTP or Git.

All the CRs are gone. From my binary file.


I went straight to settings in FileZilla:

Treat files without extensions as ASCII files

See it?

Treat files without extensions as ASCII files

That's the default in FileZilla. To change files that are just chilling, minding their own business, as ASCII, and then just randomly strip out carriage returns. What could go wrong? And it doesn't even look for CR LF pairs! No, it just looks for CRs and strips them. Classy.

In retrospect I should have used known this, but it wasn't even the switch to SFTP, it was the switch to an FTP program with different defaults.

This bug/issue whatever burned my whole Monday. But, it'll never burn another Monday, Dear Reader, because I've seen it before now.


Why does experience matter? It means I've failed a lot in the past and it's super useful if I remember those bugs because then next time this happens it'll only burn a few minutes rather than a day.

Go forth and fail a lot, my loves.

Oh, and FTP sucks.

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About Scott

Scott Hanselman is a former professor, former Chief Architect in finance, now speaker, consultant, father, diabetic, and Microsoft employee. He is a failed stand-up comic, a cornrower, and a book author.

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July 18, 2017 11:05
I'm sorry man, I had it the moment you said FTP :D
July 18, 2017 11:08
The binary transfer option wasted one of my days in the past i remember. I was uploading some ioncube obfuscaded php code to server and it was getting corrupted over and over again. And after hours, binary transfer saved the day. Now every time i install filezilla, i make sure all transfers are binary by default.
July 18, 2017 11:08
Rico - ;)
July 18, 2017 11:13
Bash on windows. Zip sftp. Done.
I tend to find when interacting with and Linux system I get less errors using bash on windows do do things
July 18, 2017 11:17
When I saw the screenshot of FileZilla, the stars aligned for me and I muttered 'poor Scott', only because I've had this issue before.

Another hint for anyone new to doing development work across devices, md5sum (or something similar) the files on either side to ensure they've arrived intact.
July 18, 2017 11:35
I looked up the checkbox in question and found this:

Wow, eight years of ignoring this!
July 18, 2017 11:39
I would have been there with you at nn:nn am in the early hours sorting that.

I would also have been annoyed with Filezilla for having that as a default and fired off an email to them to say, if in doubt, don't mess with it!

I used to spend days debugging horrific spaghetti code written by others (or myself earlier in my career). I hated it. Multi-threading was a whole new rollercoaster of debugging.

It's part of life though. We soldier on. ;)
July 18, 2017 11:46
I remember this issue. From 1995 :-)
July 18, 2017 12:06
Hah, been there done that. This problem and the network cards set to auto-negotiate (very slow network speeds) issues have gotten me here and there over the ~20 years I have been in IT more than once.
July 18, 2017 12:41
In the good old days, used to fileshare certain bits between 2 "industry tech giants" on a somewhat regular basis using ftp (yeah I am old too). Learned very quickly to automatically set binary mode default before xfering after being burned a couple of times. Sorry man.
July 18, 2017 12:52
Thanks for sharing that Scott. Though painful at the time these eventually become memories that make us smile :-)
July 18, 2017 13:33
I can't remember how often I told people who where using Filezilla to switch to WinSCP when they had problems with FTP - maybe it was the same problem for them and I never cared enough and just said "Use WinSCP, it just works" ;-)...

...and I generally have the feeling that the longest debugging-sessions tend to be solved by VERY simple fixes - like just (un-)checking a checkbox x-P.
July 18, 2017 13:51
Sorry for the pain. Brilliantly written and hopefully no-one else will ever have to suffer that.
July 18, 2017 14:18
Filezilla </3

Any ideas about services to find out issues like this in FTP? (checksum, etc)
July 18, 2017 14:19

I use MobaXTerm which is ssh and WinScp combined, it's actually quite nice I would check it out. I'm not related to them in anyway btw...
July 18, 2017 15:52
Amazing, would never got it.
July 18, 2017 16:14
How i hate crlf to lf issues, had a similar tail of woe with a config file i was editing in notepad++, it is for these reasons i virtually never use ftp for anything any more, if its not tracked in git then its scp for me, if you use superputty the built in scp tools do a decent enough job if you dont have winscp to hand...
July 18, 2017 16:22
It's always worth taking the time to set up passwordless SSH to the Pi from your main machine. Then copying the files is as simple as:

$ scp -r bin/Debug/netcoreapp2.0/linux-arm/publish pi@

Added bonus: you don't need to run an FTP server on the Pi.
July 18, 2017 16:29
Well that took me back. 20 years ago I was working on porting an app from Unix to Windows in C++ and we had a similar corruption problem that just made no sense - it was the exact same code on both platforms. For some random reason I found myself looking at the binary files much as you did, comparing the Unix-generated file to the Windows-generated one and noticed 0D0A vs 0D - problem solved. I'd completely forgotten about it until now - thanks for the memory! :)
July 18, 2017 16:34
Been there. I feel your pain, Scott.
July 18, 2017 17:01
Next time you're waist deep in some horrendous issue, open it up to your disciples in your Twitterverse. Clearly you've many followers - many have been through similar nightmares - the collective always knows the answer!
July 18, 2017 17:01
Nothing new, but still a very annoying default.

By the way, why the hell are you running Windows in the first place? It's not free (as in freedom), and is not even a decent operating system (such as Mac OS), so why even bother? Putting aside Microsoft's imperialism and many illegal practices to make Windows the only OS on this planet, does this system really have ANYTHING worth it?
July 18, 2017 17:02
Wasn't there some guy on dailywtf that said his company had been doing backups over FTP for years and years and when they finally needed a restore, they discovered they were all transferred with ASCII mode? ... None of them worked. So close, and yet so far away.

Which is why I always insist on doing a full restore every six months to make sure it works.
July 18, 2017 17:02
Another +1 for the "been there, done that" camp.

scp for the win!
July 18, 2017 17:30
I got burned by this (or very closely related anyway) as a student by the ftp-client on Solaris that always defaulted to "text"-mode. First thing I do when using *any* FTP-client is check if it is in binary mode. I don't trust any "auto"-setting when it comes to FTP. :-)
July 18, 2017 17:46
Excellent write-up. As usual!
July 18, 2017 17:55
The first few paragraphs I thought to myself: "Please don't let the error be the idiotic CR stripping!" I think this default behavior of mostly Open-Source Linux things interchanging stuf with Windows is a cultural thing of hardcore Linux users. They think a CRNL is inferior to only having NL und MUST be wrong because it is mainly used by Windows. Sometimes even GIT clients are configured in a way that they brake whole repositories by checking out without CRs and checking in whole code bases with only NLs.

The whole concept of changing files on transfer by default is very flawed in my mind. Every developer who does this should get banned from making such tools or get capital punishment.
July 18, 2017 18:44
CR, backslash, and Max_Path. All need to die quickly
July 18, 2017 19:16
What a terrible, terrible default setting in FileZilla. I'm sure you're not the first person to encounter this.

I like the UI, but it seems like FileZilla's maintainer(s) are just unwilling to fix problems that don't suit their tastes (like the plain text password file for so many years).
July 18, 2017 19:50
@Stelzhammer Wolfgang

You're absolutely right. A data transfer tool should not inspect a file and randomly change its content. Why change the content? WHY?

It's pissed me off to no avail in the 90s, and my blood boils when thinking such problem still exists.

EOL encoding is a problem for editors, not file transfer programs.
July 18, 2017 20:11
I figured it out at "bad executable file format, file truncated"... But only because earlier this morning I panicked when git told me it replaced CR/LFs with CRs in a file I believed to be binary - but was text indeed :-)

July 18, 2017 20:38
The very second that you said a "checkbox" the binary/text issue came to mind.
I so hate that the Unix derivatives have never changed to something that makes a little more sense.

This is, by far, not the only time that time has been wasted by this insidious little issue.
July 18, 2017 21:03
Been there lots od times, if something doesnt work and there is FTP involved the first thing I check is transfer type, before thinking of other stuff.
And lots od times was od course transfer type.
July 18, 2017 21:53
We've all been bitten by the EOL monster before. It's such an unintuitive thing to have to check when debugging.
July 18, 2017 21:54
Thanks for the writeup. That's exasperating.

I've been burned too many times by file corruption, so I would have reached for md5sum fairly early to confirm that there was no difference between the original on my dev machine and the copy on the Pi.

If I were writing a tool with an "Auto" mode for ASCII vs Binary, I'd do a better job of detecting binary files than FileZilla apparently does. Pretty much the only bytes less than 0x20 that are reasonable in ASCII text are 0x09, 0x0A, and 0x0D. And if the bytes are greater than 0x7E, I'd look for valid UTF-8 sequences. (It's 2017: lots of "ASCII" files are UTF-8 now.) Any EXE would fail this heuristic. Or to put it another way, if the output of strings is significantly different from the original, it's not an ASCII file.

Not to mention, many binary files have well known magic numbers that can be used fairly reliably for file type detection.

Finally, it's almost always the case that when you want to strip CRs they're immediately followed by LFs (\r\n → \n) and isolated CRs are rare and suspicious.

I'm ambivalent about its very existence, but Git's autocrlf behavior seems to work well most of the time.
July 18, 2017 21:55
Stuff like this makes concerned for the "glass back" learners of the future. The story of why these settings exists could be its own book (with a chapter named Dropping the BOM ;).

I used to think the ASCII option was useless because I could control line endings from my text editor. Then I downloaded some text files from OpenVMS in binary mode. There were no line endings at all. Turns out OpenVMS stores the line endings as record lengths in the filesystem metadata. Honestly, I'm only in my thirties.
July 18, 2017 22:10
I had this as soon as you said ftp. I was suspicious when you eschewed ssh in favour of vncviewer. Who does that? Clearly someone not steeped in the ancient lore of the network before the GUI and the www.

It is irritating that ftp defaults to ascii transaltion. It was always the first thing one did - switch to binary mode - before a file transfer. Back in the day when I searched for files using Archie.
July 18, 2017 23:16
Sorry you had to go through that. Thanks for taking the time to document it for everyone else.
July 19, 2017 2:30
I was wondering which of your (albeit rusty) strace-reading skills are useful at Chipotle?
July 19, 2017 3:46
Awesome post.
July 19, 2017 4:22
@Spongman: That's two points. Well played.
July 19, 2017 6:51
Welcome to Linux! I had the same thought as Rico, and as soon as I read about transfers, I remembered being bitten by similar issues in the past.

(I know you have prior experience with Linux, but everyday is a potential new adventure with it).
July 19, 2017 9:36
Yup, been there, done that. Don't know if I'd have remembered without your "checkbox" spoiler at the beginning.

What isn't clear to me is why the default shouldn't always be binary?
July 19, 2017 9:53
When I saw FTP my first thought was that it was not using binary file transfer. Kind of odd that it is ASCII by default.
July 19, 2017 10:41
Great article, mad debugging skills, Scott!
July 19, 2017 12:15
How often in the last years I had the same thoughts during debugging a problem:

"I'm deeply unhappy and considering quitting my job. "

I feel with you.
Best Regards
Matthias :-)
July 19, 2017 12:39

Don't remember the last time I typed these but they're etched into my memory from my sysadmin days in the world of ms-dos
July 19, 2017 13:06
Experiences like this tends to get a permanent spot in longtime memory :)
July 19, 2017 13:08
This also explains why estimating is difficult.
"Hey, Scott, how long will it take you to make a Hello World program on a raspberry PI ?"
July 19, 2017 13:12
This is first reason why i left FileZilla long time ago.

Second is even more disgusting. They store passwords plain text in files. And some malware authors knows this so they make special modules for their "tools". I know two cases when sites was infected over and over w/o security holes in site. Once they change FTP passwords, hacks stops. And your guess was right - that laptop with passwords was infected.
July 19, 2017 14:49
That's why I always rar/zip files with a password and encrypted header when transfering binaries between systems.
There are too many caveats to be dealing with. One day it's a program like filezilla modifying your datastream. Another day it's gmail doing stuff and another day it's Windows or the browser locking dll's

The encrypted header is because then the medium you use cannot look into the archive and see all them 'dangerous' files.
Plus, you also get a CRC check for free for unfinished/broken transfers.
July 19, 2017 16:04
One thought that might have got you there quicker is to apply a more binary search approach to diagnosing things. In your case you knew yours wasn't working and your colleagues was.

So getting the working file from your colleague would have been a reasonable start because that would exclude about half of the differences that there could be between your two setups (the building of the file vs. your Pi environment).

I know it's easy to say in retrospect though.
July 20, 2017 0:06
Changed codesquid's FileZilla Wiki profile to reflect his true feelings about the matter.

If I could shoot this guy, I would.
July 20, 2017 15:53
Good rule of thumb: never use FTP. It's not secure and it can mangle files. SFTP, scp or rsync. FTP is always a yellow flag.
July 20, 2017 16:27
Good old days... Downloading some .wmv video from very slow FTP for a whole day only to find it doesn't play at all, just because it was transferred in text mode.
July 21, 2017 5:17
This literally made me reminisce back to my php 1 days. I used to ftp the sales info to a box. One day ascii upload caught me. I caught this as soon as you said FTP. I read the whole thing in anticipation. Sorry it took so long.
July 21, 2017 23:26
Thank you on behalf of all of us who have considered alternate careers when faced with such a challenging, yet simple debugging problem. I very recently went through the entire Kubler-Ross stages over a Kendo rowTemplate.
July 22, 2017 12:40
Sorry to say I got it the 1/3 of a way through, but Linux has been my thing alongside Windows for over 20 years. I was probably available at lunch, look me up on Skype next time.
July 22, 2017 15:40
I used to do this once every couple of years. It's usually due to unfamiliar servers that are set up to different than my default. I mostly solved this by using a PortableApps Version and leaving binary as a default.
July 22, 2017 16:59
Hi Scott,

I too faced similar issue with CVS version control.

It had a unique requirement of specifying the file type (ASCII/binary) when we add them (cvs add).

We didn't know that and did some mass checkin of GIF files using custom made tool.

After a few days we started getting complaints about some grabled images in UI and we couldn't figure out why they appear perfectly fine on our machines (windows) where we checked in from.

I compared the MD5 HASH of two copies (the one I had checked out and the one packaged in build) and found they are different, which seriously raised doubts on our build process, but the things were perfect in build server logs (Jenkins running on Linux).

Finally comparing both images using HxD viewer, it hit me the hex patterns were changing after a few bytes, and the problem was 0D 0A byte sequence in normal file converted to 0A.

The renderer logic of GIF seems to be forgiving enough to render the image as much as possible even though it is corrupt.

Anyways everyone has their own share of debugging hell !

July 23, 2017 20:09
There is an excellent extension for Visual Studio Code I use for SFTP access to my remote machine when working on shell scripts and such. For source files, I simply add my settings.json to the .vscode folder at the root of my project:

"files.eol": "\n",
"": "C:\\Windows\\sysnative\\bash.exe"

I bet you could wire up a build process in VSCode to run dotnet publish and then use the SFTP extension (or your own, external SFTP/FTP client if it has a CLI tool) to upload the result, making the whole process a simple keyboard shortcut away from pushing your updates to test.

The reason I use bash as my shell for the project is to open an SSH session to the target machine; now to actually run the executable to test it, I just have to hit Ctrl+` to access my remote shell.
July 24, 2017 1:48
Scott, I want to say thank you for this article! I was going a bit mad trying to figure it out! I'm using a Pi3 with FileZilla.
July 26, 2017 21:43
very good

Comments are closed.

Disclaimer: The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.