Ever wondered why C strings are just character arrays that magically know where they end? It’s all because of the null terminator – that sneaky \0 at the end.
In this video we break it down super simply:
- What a C string actually is (spoiler: just bytes)
- Why old-school assembly needed separate length labels
- How adding a single zero byte fixed everything
- Quick look at ASCII codes for “Hello”
- Why \0 is the perfect terminator (and how to write it in code)
- How loops know when to stop reading the string
- Real example in assembly with and without null termination
Perfect if you’re learning C, diving into low-level programming, or just curious about what’s really happening when you write “hello” in C.
Introduction to Null Terminators 00:00:00
What is a C String? 00:00:11
Problems with Explicit Length in Assembly 00:01:06
Idea of a Special Terminator Symbol 00:01:24
Introduction to ASCII 00:02:20
Why ASCII is Outdated – Move to UTF 00:03:59
Example – “Hello” as ASCII Bytes 00:06:32
Mapping “Hello” to ASCII Codes 00:07:00
How Strings are Stored in Memory 00:08:46
The Null Terminator (\0) Explained 00:09:39
Null is ASCII Code 0 (NUL) 00:10:02
Escape Sequence \0 in Code 00:10:49
How Loops Use Null Terminator to Stop 00:11:55
Null-Terminated Strings in Assembly 00:12:41
Summary of C Strings 00:13:23
Outro and Call to Action 00:13:32
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hey there let’s quickly talk about null terminators in c strings
so first off what’s a c string it’s basically just a character array it’s just an array of numbers
and the numbers will be interpreted as characters and what’s a null terminator it’s pretty easy it’s
just a a way to terminate the string so that a program can automatically determine where the
string ends in assembly a lot of times we’ll have you know a string like we’ll
say oh gosh I don’t want to write this out right now we’ll say you know my
string and we’ll define it as an array of bytes and then we’ll say you know
hello or something right and then later in order to tell the assembly program
how long the string is we’ll use an extra symbol this is not really an
My string length is going to be equal to, this is the ASM assembly by the way in case
you’re wondering, some kind of special symbol, my string length.
And then now we have defined the actual string and then the length of the string.
But this is kind of annoying, right?
We have to define an extra thing to say how long the string is.
And then we have to pass two different things that are kind of the same name later on when
we want to print a string.
But that’s not really the best way to do it.
that’s not really the best way to do it what if instead we just put a special symbol I don’t know
what should I put right now put a special zero I’ll just put like a question mark and we’ll say
that the question mark is a special symbol and then when we want to have part of our program
print the string we can just only give it the string and it can scan the string and as soon
as it sees that special symbol let’s say the question mark for now it’ll know when to stop
processing it or whatever we could allow the program to scan in advance to compute the the
length and maybe store it somewhere so we can keep using it we can allow the program to just
continuously print out characters maybe they’re buffered or something or processing characters
and then just stop when we get to the question mark but the point is it’s a little bit less work
for us to have a special symbol at the very end okay so a spoiler the symbol is going to be a zero
Now let’s look at a special table that is called ASCII.
Well, it’s not, the table’s not called ASCII,
but this is sort of like a scheme that was invented a long time ago.
I forget what this stands for.
It’s like American standard, maybe character something, something.
It’s basically for American purposes.
A long time ago, we decided, you know what?
I’d like to represent characters using one byte per character.
bits or you know a value that can go between 0 and 255 and we’ll just start assigning these numbers
to different characters so you can see here on this ascii table it’s kind of like an old thing
let me turn on my little annotator you can see on this table well the letter h is mapped
to the number 72. so this is this was arbitrary at the time now it’s a standard because everybody
accepts uh you know ascii uh in certain parts of your programs it’s kind of a little out of date
of a little out of date now because you know in the modern era it’s a lot smarter for your programs
to support a world of different characters you know for all different languages than only to
support characters that are common in english and even here there’s probably some stuff missing that
english users would probably want to add like certain accents and symbols and things like that
so you know the the letter o lowercase o it’s 111 uh the letter o uppercase is 79 so we’ve got you
so we’ve got you know a basic idea of mapping so for the purposes of this example i’m just
gonna assume we’re using ascii to make things easier okay so that’s an ascii table um
oh i guess i should say that the modern scheme that we usually use is called utf let me just uh
write that down on a notepad right now so we’ve got utf right now it’s just a different standard
but the same idea you know with ASCII we only had one byte with UTF we have a wide variety of
choices we can do UTF-8 which basically also means one byte it closely map maps to ASCII I
think all or almost all the characters will map to ASCII and then we have UTF-16 which means two
bytes or 16 bits and then we have UTF-32 which is basically four bytes or 32 bits as far as I
while. UTF-32 should be able to handle all the characters that all the languages of the world
commonly use. Just to double check, if you take two to the something power where the power, let’s
say y, two to the y, where y is the number of bits in question, then you can get an idea for the
number of combinations that you can represent. So two to the 32nd power, that’s about four billion.
I should have known that off the top of my head. That’s about four billion possible combinations.
So I don’t know.
I think in my opinion, probably 32 is enough for all symbols in the world, for all languages
in the world.
I’m not sure.
If not, then we would probably want to use UTF-64 if that exists, which definitely would
cover all symbols in the world.
But that’s really not the point of this video.
I just wanted to mention it because if you’re writing an application on your own, you want
to start your own business or you’re, you know, you’re working for someone, you probably
almost certainly do not want to use ASCII.
do not want to use ascii you probably want your database and your program to be able to use utf
16 or 32 because you want to be able to support international users and that’s not going to work
i had a website a long time ago that i wrote the database was in uh it was like latin 8 or
some scheme in the database where it was basically ascii just you know eight bits per character and i
could type characters in their native language and they were kind of disappointed and i lost a
lot of business until i realized oh i should upgrade my database and my program to support
utf but anyway the point of this video is not international character support the point of this
video is what is a null terminator and how do you use it in a c string so i’m going to get back on
Hello. Okay, so we got hello. This would basically be an array of characters or an array of in ASCII
one byte characters. So I’ll put like a number here and a number here and there’ll be five numbers.
What numbers are they? Well, if we’re using ASCII, then we should map these
using the ASCII table. So again, in the ASCII table here, what is capital H?
the number 72 and then the lowercase e that’s going to be the number 101 so i’ll just maybe do
like an arrow here the number 101 and then the capital h is going to be the number 72 and then
we got two l’s lowercase so that’s going to be this right here and that’s going to be the number
108 and then lowercase o that’s going to be the number 111 so basically you know you just go
through the characters you want to represent and map them using the ascii table or hopefully
table or hopefully an actual lookup table inside your program and then you can do the same thing
backwards you know what is what does 72 mean it means h so i’m going to erase this for now
and i’m going to go back to my little notepad here and i’m just going to fill out the numbers
so the first is going to be 72 oh actually before i fill these out hang on let me just
show you that this is basically what we’re trying to translate here so we got like an uppercase h
In many programming languages, definitely C++, which is the one I’m thinking of, we would use double quotes for an actual long string, like a proper string, a literal string, or like an STD string or something.
And we would use an array of single quoted characters to say this is one character that maps to one or more bytes, in this case, one byte.
So I’m going to say H-E-L-L-O and put them all in single quotes.
And then we’ll just sort of like translate the numbers here.
translate the numbers here. So we said the H was 72 because it was capital. It was like a lower
number. And then the E is going to be 101. And then the L is 108. We’ll do two of those 108.
And then the O was a 111. So you can imagine somewhere in an array of data in the computer,
there’s the number 72, 101, 108, you know, whatever. And even further thinking about it,
about it a little more deeply the number 72 is not actually in the machine it’s really a sequence of
ones and zeros right so just imagine that inside the machine there’s just a ton of ones and zeros
for this particular sequence you know five times eight which would be the number of uh you know
eight is like the number of bits per byte since we’re doing one byte characters um what is that
40 let me just double check here i want to make sure i’m saying things that are somewhat correct
that are somewhat correct we’ll do eight times five whoops i think i screwed something up
eight times five 40 yeah okay so you can imagine 40 ones and zeros just inside the machine
anyway so we talked about before not wanting to specify the length of a string so we’ll just
place a special marker instead of a question mark we’ll use the number zero so i’m going to put a
I forgot if I actually pointed this out or not. I’ll just do this to be sure.
Somewhere on this, you can see that the number zero in decimal, or zero in hexa, or zero in octal,
you’re welcome, you know how to do those other systems, is just considered null. So it’s sort
of a control code, or it’s not necessarily a symbol, it’s more of a control code. We’ll say
the number 72 that means h capital h but if we see the number zero that means null and in the
scheme that we’re going to use we will say that null terminates the string that’s why we call it
a null terminator so null terminators are basically just zeros that terminate the string
i could put null here inside of the the character representation just to kind of show you
other languages and we want to do single quoted characters we can usually use an escape sequence
which is just basically slash zero this is not a video about escape sequences but imagine that
every time there is a slash a forward slash and then you know one character after that then the
character will be interpreted zero for example if we just put zero by itself then that actually would
be the ascii code for zero or just the character code for zero in this particular case zero is uh
where the heck is the zero you can’t even find it oh there it is right there that’s like so the
the character zero is represented by the number 48 but that’s not what we actually want we want
the actual number zero because we’re trying to get a null terminator so anyway we couldn’t just put
a regular zero in there because it would be interpreted as 48 instead we put a slash which
tells the computer that the thing that follows is a control character so that will be interpreted as
interpreted as actually a zero or null terminator.
So now that we know how to put a null terminator in an array of characters and we know what
it’s going to look like if we’re just talking about numbers, we now know that if you have
a program that has let’s say a for loop and it’s kind of like going through, you know,
do all the characters, if it’s going through every single character one by one and doing
something to them, printing them, modifying, adding them to an algorithm, whatever.
this for loop or actually this should probably be a while loop since for loops
are kind of limited and the while loop will know when to break because as soon as it sees a
character that is actually just the number zero it’ll say oh i found the null terminator i will
now ignore this and break the loop i won’t do anything with this character because the null
is a terminator it’s not part of the string um what else you know actually i think that’s pretty
much it if you’re in assembly uh i’m just going to go back to yasm assembly for a second because i
like i like to talk about that lately and imagine we have like a string called my string and we say
it’s a sequence of bytes and i say hello you know this is how you would do this in x84 60 x 84 64
yasm assembly on ubuntu or just linux then uh you know this string right here is just a string with
null terminate with no null terminator you want to add a null terminator to the end of it so that
null terminator to the end of it so that you can write a better function that would scan your string
and print it without knowing in advance how long it was then you just do a comma zero at the end
and now you can easily write a function to scan or read or process your strings because this is
really what’s what’s happening under the hood it’s just like a sequence of characters and then it
ends with the zero and there you go that’s the basics of c strings with null terminators
this video i hope you had fun and learned a little bit of stuff i’ll see you in the next video
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