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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: does char *str="abcd"; alloc addressable memory?
Date: Thu, 09 Jul 2020 10:07:54 -0700
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Keith Thompson <Keith.S.Thompson+u@gmail.com> writes:

> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>
>> John Forkosh <forkosh@panix.com> writes:
>>
>>> Malcolm McLean <malcolm.arthur.mclean@gmail.com> wrote:
>>>
>>>> On Monday, 6 July 2020 12:57:06 UTC+1, Bonita Montero  wrote:
>>>>
>>>>> The compiler should give you an error for casting a string-literal to
>>>>> a pointer to non-const characters.  If cou cast it explicitly you may
>>>>> modify the caracters and that usually works, but it's not guaranteeed.
>>>>> Most platforms will make a writable copy of a read-only mapped page.
>>>>
>>>> It doesn't warn because const was a late addition to C, and string
>>>> literals were originally char *s.
>>>> You can declare a writeable string with
>>>>
>>>> char writeable[] = "Overwrite me";
>>>>
>>>> this is also seldom a good idea.
>>>
>>> As mentioned upthread
>>>    char *str = ( argc>1? argv[1] :  (char *)"01234&56789" );
>>> compiles without the -Wwrite-strings warning, but still
>>> segfaults.  And
>>>    char str[] = ( argc>1? argv[1] : "01234&56789" );
>>> gives an error
>>>    tester.c:7:16: error: invalid initializer
>>>        7 |   char str[] = ( argc>1? argv[1] : "01234&56789" );
>>>          |                ^
>>>
>>> As suggested downthread, I can easily
>>>    char default[99] = "whatever",
>>>         *str = ( argc>1? argv[1] : default );
>>> but I'd prefer the more compact notation if possible.
>>> Any way to get that to work?
>>
>> It's compound literals to the rescue!
>>
>>     char *str =   argc > 1  ? argv[1]  : (char[]){ "01234&56789" };
>>
>> No errors, no undefined behavior, no problem.
>
> One thing to keep in mind is that the object created by a compound
> literal has automatic storage duration.  In this case I don't think
> that matters, but it is a difference between string literals and
> compound literals.  (A compound literal outside a function body
> creates an object with static storage duration.)

Yes, good point, and that is worth both knowing and remembering.
I didn't mention it only because the declaration in question
occurs right at the start of main() and so it will not, in this
case, be a problem.  But it is important to know for other
potential uses.