Quick question about null terminator string!

Question

Hello,
I always feel that a char array should end with a "\0".
But just to clear up something here, if I do:
char *a[10];
and I fill out the array with 10 characters, then, should we *always* append the "\0" at the end of this array?
And if so, should it be appended at a[9] location or a[10] ??? It confuses me a little because since the array is supposed to take 10 characters, then does the compiler save an extra space at the 11th character location for the null terminator string?
OR
Is it up to us to make sure we fill the array with up to 9 characters making sure to save the 10th location in the array for the null terminator string?
So in a nutshell, once we finish up with a char array do we do this:
a[9] = '\0';
or this:
a[10] = '\0';
I excuse the simple question but this has been haunting me for a while, but thanks for all replies!!!
r

Answer 1

Ok, a few misconceptions here.
I always feel that a char array should end with a "\0".
This is not strictly true.  This is only true of the data is to be treated as a string.  Often you will use a char array to hold binary data.  Normally such binary data will NOT end in a NUL character.  In fact, most binary data may contain
NUL characters sprinkled throughout the data.
char *a[10];
This is NOT an array of ten characters.  This is an array of ten pointers to characters (or character arrays).  You probably intended
char a[10];
You then state
I fill out the array with 10 characters, then, should we *always* append the "\0" at the end of this array?
Again, this depends on whether you intend the data to be a string or binary data.  I will assume you mean string from here on out.  For a string, yes you should probably always append a '\0' Of course you don't actually have room for that NUL --
see the following answer.  (Actually there are some string APIs that let you pass in a number of characters to process and those will work with no terminating NUL, but frankly you are playing with fire when you do that so I would recommend always putting
the trailing NUL in myself.)
And if so, should it be appended at a[9] location or a[10]
The declaration "char a[10]" causes the compiler to reserve *exactly* ten bytes of data for that array.  If you write to location a[10], that is the
eleventh byte and will cause undefined behavior.  At best you might not notice anything bad.  At worst it could cause any sort of crazy data corruption.  It could easily crash your program.
You would need to write the NUL character at a[9].   Note that writing NUL at a[9] will overwrite the last byte that you put into that array.
As a side note here:  This is why you try to always use a proper string class like std::string when using C++ -- it takes care of all of that for you, including growing the string larger if you fill it up.  Then you don't need to worry
about these pesky details that are easy to get wrong.