Why do strings in C need to be null terminated?

Question

Just wondering why this is the case. I\'m eager to know more about low level languages, and I\'m only into the basics of C and this is already confusing me.

Do langu

Answer 1

C has no notion of strings by itself. Strings are simply arrays of chars (or wchars for unicode and such).

Due to those facts C has no way to check i.e. the length of the string as there is no "mystring->length", there is no length value set somewhere. The only way to find the end of the string is to iterate over it and check for the \0.

There are string-libraries for C which use structs like

struct string {
    int length;
    char *data;
};

to remove the need for the \0-termination but this is not standard C.

Languages like C++, PHP, Perl, etc have their own internal string libraries which often have a seperate length field that speeds up some string functions and remove the need for the \0.

Some other languages (like Pascal) use a string type that is called (suprisingly) Pascal String, it stores the length in the first byte of the string which is the reason why those strings are limited to a length of 255 characters.

Answer 2

They need to be null terminated so you know how long they are. And yes, they are simply arrays of char.

Higher level languages like PHP may choose to hide the null termination from you or not use it at all - they may maintain a length, for example. C doesn't do it that way because of the overhead involved. High level languages may also not implement strings as an array of char - they could (and some do) implement them as lists of arrays of char, for example.

Answer 3

Because in C strings are just a sequence of characters accessed viua a pointer to the first character.

There is no space in a pointer to store the length so you need some indication of where the end of the string is.

In C it was decided that this would be indicated by a null character.

In pascal, for example, the length of a string is recorded in the byte immediately preceding the pointer, hence why pascal strings have a maximum length of 255 characters.

Answer 4

Think about what memory is: a contiguous block of byte-sized units that can be filled with any bit patterns.

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A character is simply one of those bit patterns. Its meaning as a string is determined by how you treat it. If you looked at the same part of memory, but using an integer view (or some other type), you'd get a different value.

If you have a variable which is a pointer to the start of a bunch of characters in memory, you must know when that string ends and the next piece of data (or garbage) begins.

Example

Let's look at this string in memory...

H e l l o , w o r l d ! \0 
^
|
+------ Pointer to string

...we can see that the string logically ends after the ! character. If there were no \0 (or any other method to determine its end), how would we know when seeking through memory that we had finished with that string? Other languages carry the string length around with the string type to solve this.

I asked this question when my underlying knowledge of computers was limited, and this is the answer that would have helped many years ago. I hope it helps someone else too. :)

Answer 5

They are null-terminated because whole plenty of Standard Library functions expects them to be.

Answer 6

It is a convention - one could have implemented it with another algorithm (e.g. length at the beginning of the buffer).

In a "low level" language such as assembler, it is easy to test for "NULL" efficiently: that might have ease the decision to go with NULL terminated strings as opposed of keeping track of a length counter.