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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: relearning C: why does an in-place change to a char* segfault?
Date: Tue, 13 Aug 2024 17:43:09 -0700
Organization: A noiseless patient Spider
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References: <IoGcndcJ1Zm83zb7nZ2dnZfqnPWdnZ2d@brightview.co.uk> <20240801114615.906@kylheku.com>
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Kaz Kylheku <643-408-1753@kylheku.com> writes:

> On 2024-08-01, Mark Summerfield <mark@qtrac.eu> wrote:
>
>> This program segfaults at the commented line:
>>
>> #include <ctype.h>
>> #include <stdio.h>
>>
>> void uppercase_ascii(char *s) {
>>     while (*s) {
>>         *s = toupper(*s); // SEGFAULT
>>         s++;
>>     }
>> }
>>
>> int main() {
>>     char* text = "this is a test";
>
> The "this is a test" object is a literal.  It is part of the
> program's image.  When you try to change it, you're making your
> program self-modifying.
>
> The ISO C language standard doesn't require implementations to
> support self-modifying programs;  the behavior is left undefined.
>
> It could work in some documented, reliable way, in a given
> implementation.
>
> It's the same with any other constant in the program.  [...]

That is wrong both technically and practically.  And obviously
so.