Debugging by Neural Lantern - Dailymotion

Neural Lantern

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00:00hey there i'd like to talk to you about debugging your program's output with xxd

00:06what am i talking about well there's a program called xxd that can sort of inspect

00:16at the byte level the output of some text file or binary file it's really useful for

00:23inspecting things why would you want to debug the output of your program well i don't know

00:27maybe when you print a message from your program the program might be printing some extra characters

00:32somewhere or some garbage and maybe maybe you don't notice because you're sort of looking at

00:36your program's output in the terminal but then maybe if your output is being sent to another

00:41program uh for the purposes of automation or like you're getting a grade from a class or something

00:47like that then um well it can it can sort of it can sort of get you without you realizing what's

00:53wrong so let me just give you an example real fast of what i'm talking about i have a simple

01:00assembly program here this is not a video about how to write assembly code so just assume that

01:04you know if you want to know assembly see my other videos but um i'm just going to print a simple

01:10message i'm just going to print hello i feel awesome and then the length of the message i'm

01:15specifying here you don't know you don't need to know too much about this for this particular video

01:19i'm just printing a string in assembly that's all i'm doing um and then here you can see i'm just

01:24kind of like printing it and then i'm like exiting so if we go back to the terminal here i can type

01:28make run this is not a make file video see my other videos i want to type make run just to compile and

01:35run the program and then here's the message that it prints right hello i feel awesome it looks good in

01:41the terminal i can't be 100 sure though because think about this what if i corrupted the message on

01:47purpose this can happen in assembly all the time if you get the length wrong or if you add extra

01:53characters at the end of it or even if you have you know a corrupted register somewhere that either

01:59reads from the wrong spot to get a string or maybe it writes over the string for some reason there's a

02:05lot of things that can go wrong so for now just suppose that i've corrupted my message somehow by

02:11i don't know let's put a zero in between uh the string so i'm going to say that it's going to be

02:17like h e l zero l o and what that should do uh well it should look like just the word hello

02:25but if i'm automating this program or sending it in to get a grade at a school somewhere then

02:30this should you know not match what is expected if i'm automating with like another system in some

02:38way but it should look okay in the terminal let's see i'm going to do clear and make run

02:44notice how the message looks the same hello i feel awesome so if i was automating you know if i was

02:51sending this string to another program for automation i might be convinced that i'm not doing anything

02:56wrong and i'll i'll think it's the other side that is wrong or or computers are just buggy or something

03:01like that but actually i've outputted a corrupted message for some reason

03:04so how can i really be sure that my message is what i think i'm seeing in the terminal

03:09that's the xxd program pretty easy xxd you can install it with i think sudo apt install xxd let

03:18me see if i have it on this system oh it's already installed okay good so um i'm going to do

03:25clear and make run again so there it is uh i would like to just redirect the standard output pipe

03:31to a file with a special command in the shell let me just show you real fast

03:36i am definitely printing to std out the standard pipe which is just a number one file descriptor

03:44if you don't understand that i have other videos but um i'm going to run the program again but then

03:49i'm going to redirect the output to uh a file and i'm going to call it std out.txt it's probably a good

03:56idea to eliminate spaces i think i've made mistakes in other videos where i didn't put a space

03:59before or after this little redirector symbol and i ended up creating a file just called the

04:04number two or the number one but uh pipe number one is standard output so i'm going to say anything

04:09that comes uh to pipe one which is standard output i'm just going to redirect it to a text file

04:14so when i hit enter notice how nothing prints to the screen but

04:18the standard output text file that i specified is now created

04:23if i spill the contents of std out it still looks normal

04:27so don't let that trick you but watch what happens if i use xxd

04:32xxd std out basically xxd is going to read it as a binary file

04:37and tell me what is the value of every single character in there

04:41file io i forgot to change the title of that that's not this video but notice right here

04:48where it says h-e-l-l-o and then right before that there's like a little dot

04:54if you look at the dot let's see what is the dot it's one two three four five six it's the sixth thing

04:59one two three four five six the dot is this

05:03one two three four five six seven eight nine ten twelve thirteen fourteen fifteen

05:13oh there's fifteen characters but there's not that many i think probably they're packing them by two

05:17here let me see if that makes more sense now one two three four five six one two three four five six

05:24yeah that makes a lot more sense so the sixth uh byte not the sixth uh word uh is basically

05:33oa and that maps to this dot right here and if you recall oa is a decimal 10 which is uh the

05:43beginning of a new line you know like a carriage return line feed um 13 comma 10 so that's what it

05:49is right there and then so now we know that there's xxd is figuring out here or helping us understand

05:56that there's a new line between slash main and then the message which if you recall that's exactly

06:00what happened and then when we look into our hello string there's a dot in the middle here

06:07what is that dot well we just have to figure out what number that corresponds to so

06:11the oa was that dot right there so we just got to go a couple more bytes forward one two three four

06:18bytes forward one two three four bytes forward it's this zero right here

06:26so xxd even though the terminal didn't help us originally xxd has helped us realize that we're

06:32actually printing a zero under the hood and we can't see it in the terminal so we have now debugged

06:39our output you know depending on where you print this you might see nothing like in the terminal it

06:45didn't really show anything but if you're on a web page somewhere or some other type of thing that's

06:50going to print the data for you you might see a weird symbol i mean it really depends this is not

06:54something you'll see everywhere but you might see a weird symbol that kind of looks like a black diamond

06:59with a question mark inside of it or some other weird symbol if you start seeing weird symbols all

07:03over the place or just in a place where you know there's not supposed to be a weird symbol that might

07:09be an indication to you that you have weird uh values sitting inside of your string and then you

07:14should probably try to debug with xxd so that's it that's the basics for debugging with xxd just you

07:20know file output anyway for your information if you wanted to debug the uh standard error pipe you could

07:27also do this you know because the error pipe is just a two um and that's basically it i'm not going to

07:36debug right now because it's that's too easy but i hope you enjoyed this video thank you for watching

07:41i hope you learned a little bit and had a little bit of fun i'll see you in the next video

07:48hey everybody thanks for watching this video again from the bottom of my heart i really appreciate it

07:53i do hope you did learn something and have some fun if you could do me a please a small little favor

07:58could you please subscribe and follow this channel or these videos or whatever it is you do on the current

08:05social media website that you're looking at right now it would really mean the world to me and it'll

08:09help make more videos and grow this community so we'll be able to do more videos longer videos better

08:14videos or just i'll be able to keep making videos in general so please do do me a kindness and uh and

08:22subscribe you know sometimes i'm sleeping in the middle of the night and i just wake up because i know

08:26somebody subscribed or followed it just wakes me up and i get filled with joy that's exactly what happens

08:31every single time so you could do it as a nice favor to me or you could you could troll me if you

08:35want to just wake me up in the middle of the night just subscribe and then i'll i'll just wake up i

08:39promise that's what will happen also uh if you look at the middle of the screen right now you should see

08:45a qr code which you can scan in order to go to the website which i think is also named somewhere at the

08:49bottom of this video and it'll take you to my main website where you can just kind of like see

08:54all the videos i published and the services and tutorials and things that i offer and all that good stuff and

09:00uh if you have a suggestion for uh uh clarifications or errata or just future videos that you want to

09:09see please leave a comment or if you just want to say hey what's up what's going on you know just send

09:14me a comment whatever i also wake up for those in the middle of the night i get i wake up in a cold

09:18sweat and i'm like it would really it really mean the world to me i would really appreciate it so

09:23again thank you so much for watching this video and um enjoy the cool music as as i fade into the

09:32darkness which is coming for us all

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Master Debugging with xxd: Inspect Program Output at a Byte Level for Automation & Clean Code!

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Debugging

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