Mounts

w8d0

Announcements

• Welcome to “Systems in C”
  • Back to containers.
  • Week off from C
  • Two lectures with exercises

Today

• The file system
• Linux and even… WINDOWS development????
  • Has science gone too far
• Symbolic Linkages
• Containers but good

Backstory

Motivation

Motivation

Motivation

In brief

• I am often on a Linux partition when a non-technical PM attempts to reach me through MS Suite
  • I can run MS Suite on Linux which, uh, defeats to purpose of being on Linux

In brief

• I am often on a Windows partition when I get questions like:
  • Can I fix this compatibility issue with whitespace line termination?
  • Will this run on Win and *nix?
  • Can you take a quick md5sum of this file?
• Often I just need to run a quick script or executable on files in a Windows file system using Linux syscalls.

Separate Problem

1. vim mywork.c
2. podman build -t tester . ## COPY mywork.c
3. podman run tester python3 tester.py
4. I get negative one billion points
5. vim mywork.c
6. podman run tester python3 tester.py still gives negative on billion because the file systems are desynced.

Mounts

Bind Mounts

When you use a bind mount, a file or directory on the host machine is mounted from the host into a container. By contrast, when you use a volume, a new directory is created within Docker’s storage directory on the host machine, and Docker manages that directory’s contents.

Let’s step through the process

• I make a container image that I’m relatively happy with.
  • Save this in a containerfile
  • Build it and have it live under the local podman images container repository.
  • Or… push to GHCR? Hmmm?
    • Soon™️

Container image

• I rarely use utilities other than these few.

Containerfile

FROM ubuntu 

RUN apt update && apt install gcc vim python3 astyle -y

Astyle

• astyle might be new to you
• It completely rules
• It’s old, new one is Clang-Tidy
• Requires LLVM back-end which is…
• For the Rust class
• Also coc usually via npm and (gags) neovim

Sample

$ cat octal.c
#include <stdio.h>
int main(){printf("%o\n",10);/* hm */return/* blah */ 0;}
$ gcc octal.c
$ ./a.out
12
$ astyle octal.c
Formatted  /home/user/tmp/octal.c
$ cat octal.c
#include <stdio.h>
int main() {
    printf("%o\n",10);/* hm */return/* blah */ 0;
}
$ gcc octal.c
$ ./a.out
12

Hightlight

• vim octal.c

before

#include <stdio.h>
int main(){printf("%o\n",10);/* hm */return/* blah */ 0;}

• astyle octal.c

astyle

#include <stdio.h>
int main() {
    printf("%o\n",10);/* hm */return/* blah */ 0;
}

• Learn more astyle -h (I’ve never read it).

Style Guides

• The people that taught me C wrote this
• The iconic living legend Linux kernel hackers, who are never wrong, wrote this
• I’m not up-to-date on Google C but Google C++ is here
  • Our “header” instruction is from their “The #define Guard” heading.

Anyways

• To my knowledge this is all I’ve used this term outside of one-offs for testing.

gcc \
vim \
python3 \
astyle

• Though can probably manage gcc only after today.

Let’s step through the process

• I make a container image that I’m relatively happy with.
  • vim Containerfile
  • podman build -t sysinc .
• Now, just spin up a transient container when I need anything.

Run options

• Two competing idealogues
  • I am on docker documentation not podman
  • podman I think assumes you know what you are doing (I don’t)
• Mount

docker run --mount type=bind,src=<host-path>,dst=<container-path>

• Volume

docker run --volume <host-path>:<container-path>

Quote Docker

In general, --mount is preferred. The main difference is that the --mount flag is more explicit and supports all the available options.

• Volume is basically being gradually depreciated.
• Volumes are in general podman/docker managed and…
  • We don’t trust code other people wrote.
  • Or code we wrote.

Sample

• https://github.com/cd-c89/sysinc/blob/main/podman/podman.sh

podman run --mount type=bind,src=$PWD/..,target=/mnt -d -t sysinc /bin/bash

Breakdown

Arg	Meaning
podman	container command
run	start a container from some image
–mount	let container see files that live outside container

Breakdown

Arg	Meaning
type=bind	the files will be the same in a host folder and container folder
src=	This is the folder on the host
$PWD/..	This is the file above the current working directory

Try it

• $PWD is a shell built-in like $?

$ echo $PWD
/home/user
$ pwd
/home/user

• Can use it in scripts.

$ ls $PWD/..
user
$ ls ..
user

Sample

• Make the .. directory on host be visible to container.

podman run --mount type=bind,src=$PWD/..,target=/mnt -d -t sysinc /bin/bash

Breakdown

Arg	Meaning
target=	This is the folder on the container
/mnt	That is the mnt (mount) directory under root /
-d	Start the container detached

• I couldn’t get this to work attached (on Windows) but didn’t try very hard.
  • On Linux everything is easy (always)

Breakdown

Arg	Meaning
-t	What should the name (tag) be of Container image?
sysinc	The tag I used during podman build
/bin/bash	The executable to run within the container

Sample

podman run --mount type=bind,src=$PWD/..,target=/mnt -d -t sysinc /bin/bash

1. Start a shell
2. In a container of the image based on the Containerfile
3. Where /mnt refers to files on the host machine

See it

user@DESKTOP-THMS2PJ:~/dev/sysinc/repos/sysinc/podman$ /bin/bash podman.sh
c25d7d0db5d60efca7c1140dd9b8217a27e511caa4f5a5fdf2fc21e90ad45f17
user@DESKTOP-THMS2PJ:~/dev/sysinc/repos/sysinc/podman$ ls ..
4096_t  README.md  enigma  macros  ops_ui  podman  rsainc  shainc
user@DESKTOP-THMS2PJ:~/dev/sysinc/repos/sysinc/podman$ podman exec -it c2 /bin/bash
root@c25d7d0db5d6:/## ls /mnt
4096_t  README.md  enigma  macros  ops_ui  podman  rsainc  shainc
root@c25d7d0db5d6:/#

Execution

More fun

• I have gcc inside that container.
• I can compile inside that container.
• Let’s step through an example.
• On my host, that is, not in container:

hi.c

#include <stdio.h>

int main() {
        printf("hi\n");
        return 0;
}

user@DESKTOP-THMS2PJ:~/dev/sysinc/repos/sysinc/podman$ cd ..
user@DESKTOP-THMS2PJ:~/dev/sysinc/repos/sysinc$ podman ps
CONTAINER ID  IMAGE                    COMMAND     CREATED        STATUS            PORTS       NAMES
c25d7d0db5d6  localhost/sysinc:latest  /bin/bash   7 minutes ago  Up 7 minutes ago              determined_shockley
user@DESKTOP-THMS2PJ:~/dev/sysinc/repos/sysinc$ vim hi.c
user@DESKTOP-THMS2PJ:~/dev/sysinc/repos/sysinc$ cat hi.c
#include <stdio.h>

int main() {
        printf("hi\n");
        return 0;
}
user@DESKTOP-THMS2PJ:~/dev/sysinc/repos/sysinc$ podman exec -it c2 cat /mnt/hi.c
#include <stdio.h>

int main() {
        printf("hi\n");
        return 0;
}

Compile

• I can compile within the Container
  • Just use exec (run command)
  • With gcc
  • With /mnt files

  user@DESKTOP-THMS2PJ:~/dev/sysinc/repos/sysinc$ podman ps
  CONTAINER ID  IMAGE                    COMMAND     CREATED        STATUS            PORTS       NAMES
  c25d7d0db5d6  localhost/sysinc:latest  /bin/bash   7 minutes ago  Up 7 minutes ago              determined_shockley
  podman exec -it c2 gcc /mnt/hi.c -o /mnt/hi.out

Run it

• Can run on the host device.
  • Well, making some pretty heavy assumptions.
  • This was ubuntu-to-ubuntu, after all

  user@DESKTOP-THMS2PJ:~/dev/sysinc/repos/sysinc$ ./hi.out
  hi

• Can run in the container

user@DESKTOP-THMS2PJ:~/dev/sysinc/repos/sysinc$ podman exec -it c2 ./mnt/hi.out
hi

Windows

Ubuntu on Windows

• An astute learning will notice this means it no longer matters which OS your text editor runs under
• And here, in this class, we all love MS Windows.

It’s Popular

Windows Commands

• Windows has two command lines.
  • Don’t blame me I voted for Free and Open Source Software (FOSS)
  • So does Windows, now (hence WSL)

1. cmd.exe from DOS, which runs .bat files
2. PowerShell, for .NET, which runs .ps1 files

Why two shells?

• How does Windows work (pick one):
  • It doesn’t.
  • I don’t know.

Scripts

• For reference, here is my .sh script to mount folders:

podman.sh

podman run --mount type=bind,src=$PWD/..,target=/mnt -d -t sysinc /bin/bash

• Here is how I run the same under the “Command” shell cmd.exe

windows.bat

podman run --mount type=bind,src=%cd%\..,target=/mnt -d -t sysinc /bin/bash

One difference

• podman commands are the same on all OSes
• All commands within the container are the same, as the container is the same
• The only difference is the src
  • On Windows:
    • Paths are backslash vs. forward slash delineated
    • %cd% not$PWD` gives the current directory.

    windows.bat

    podman run --mount type=bind,src=%cd%\..,target=/mnt -d -t sysinc /bin/bash

Windows Image

• Presumably I can refer to wsl images from the Windows host, I didn’t try.

C:\Users\cd-desk\Documents\dev\sysinc\podman>podman machine start
Starting machine "podman-machine-default"
API forwarding for Docker API clients is not available due to the following startup failures.
        could not start api proxy since expected pipe is not available: podman-machine-default

Podman clients are still able to connect.
Machine "podman-machine-default" started successfully

C:\Users\cd-desk\Documents\dev\sysinc\podman>podman build -t sysinc .

Launch Container

C:\Users\cd-desk\Documents\dev\sysinc\podman>.\window.bat

C:\Users\cd-desk\Documents\dev\sysinc\podman>podman run --mount type=bind,src=C:\Users\cd-desk\Documents\dev\sysinc\podman\..,target=/mnt -d -t sysinc /bin/bash
9a1b0fb2a8f87d3639149ee41fb2dcb822f5a7cbd971a39cdc77150a79774765

C:\Users\cd-desk\Documents\dev\sysinc\podman>

vim

• I of course also have vim installed on Windows, but I don’t know when I did that or how.

C:\Users\cd-desk\Documents\dev\sysinc>vim hi.c

C:\Users\cd-desk\Documents\dev\sysinc>type hi.c
#include <stdio.h>

int main() {
        printf("i wrote this on windows lol meow\n");
        return 0;
}

C:\Users\cd-desk\Documents\dev\sysinc>

Compile

C:\Users\cd-desk\Documents\dev\sysinc>podman exec -it 9a gcc /mnt/hi.c -o /mnt/hi.out

C:\Users\cd-desk\Documents\dev\sysinc>dir hi.out
 Volume in drive C has no label.
 Volume Serial Number is 5CEC-43F9

 Directory of C:\Users\cd-desk\Documents\dev\sysinc

03/12/2025  05:33 PM            15,960 hi.out
               1 File(s)         15,960 bytes
               0 Dir(s)  99,378,515,968 bytes free

C:\Users\cd-desk\Documents\dev\sysinc>

Run

C:\Users\cd-desk\Documents\dev\sysinc>podman exec -it 9a ./mnt/hi.out
i wrote this on windows lol meow

C:\Users\cd-desk\Documents\dev\sysinc>

• Windows Development: It’s easy.

It’s not

• Windows opened a window asking me what to do when I tried to run this.

C:\Users\cd-desk\Documents\dev\sysinc>.\hi.out

C:\Users\cd-desk\Documents\dev\sysinc>

Exercise

• Windows users:
  • Get this to run in PowerShell
• Mac users:
  • Get this to run on Mac, the .sh should mostly work. Right?
• Linux users:
  • Develop a “portable” binary that works across distributions.

Fin