Re: relearning C: why does an in-place change to a char* segfault?

On 02/08/2024 19:19, James Kuyper wrote:

On 8/2/24 5:43 AM, Bart wrote:

On 02/08/2024 02:06, Kaz Kylheku wrote:

On 2024-08-01, Bart <bc@freeuk.com> wrote:

It segfaults when the string is stored in a read-only part of the
binary.

>
A string literal creates an array object with static storage duration.
Any attempt to modify that array object has undefined behavior.

>
What's the difference between such an object, and an array like one of
these:
    static char A[100];
    static char B[100]={1};
>
Do these not also have static storage duration? Yet presumably these can
be legally modified.

>
That 1 which initializes B[0] cannot be modified.
>

>
Why not? I haven't requested that those are 'const'. ...

 You don't get a choice in the matter. The C language doesn't permit
numeric literals of any kind to be modified by your code.

My post wasn't about numerical literals. I assumed it was about that '1' value which is stored B's first cell.
However, just in case KK was talking about that unlikely possibly, I covered that as well.
ey can't be,

and don't need to be, declared 'const'. I've heard that in some other
languages, if you call foo(3), and foo() changes the value of it's
argument to 2, then subsequent calls to bar(3) will pass a value of 2 to
bar(). That sounds like such a ridiculous mis-feature that I hesitate to
identify which languages I had heard accused of having that feature, but
it is important to note that C is not one of them.
 Just as 1 is an integer literal whose value cannot be modified,

It can't modified, in a value that would also affect other instances of '1' within that module or produce, because it is very unlikely to be shared.
I don't know of any implementations of this kind of language which do that. (The nearest might FORTRAN IV when '1' was passed by reference to a subroutine, and the subroutine then assigns to that parameter.)
Where it would be more plausible is here:
    const char* B[] = {"A", "A", "A"};
where if you can somehow change that first "A", then the other two could also change if the compiler decides to share those 3 identical strings.