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From: Bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: relearning C: why does an in-place change to a char* segfault?
Date: Fri, 2 Aug 2024 19:33:20 +0100
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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.