Any reason why compiling this with ldc would cause the exe to crash?

Compiling with DMD (using either declaration of palindrome works just fine though)


// ----

module test;

import std;

void main()
{
    char[] palindrome = cast(char[])"able was I ere I saw elba";

   //char[] palindrome = ['a','b','l','e','w','a','s','I','e','r','e','I','s','a','w','e','l','b','a'];

    writeln(palindrome);

    // note: The line below causes the exe to crash when compiled with ldc
    // but only if using the first version of palindrome.

    writeln(palindrome.reverse);
}

// ---

On Wed, Feb 02, 2022 at 11:21:52PM +0000, forkit via Digitalmars-d-learn wrote: [...]
>     char[] palindrome = cast(char[])"able was I ere I saw elba";

String literals are immutable by default. Casting immutable to mutable
is UB (Undefined Behaviour).


[...]
>     writeln(palindrome.reverse);

Especially because .reverse mutates its argument.  So you're attempting to overwrite immutable data here. That's probably what caused the crash: the literal is put in the read-only segment and the OS killed the program when it tried to write to data in that read-only segment.


T

-- 
People who are more than casually interested in computers should have at least some idea of what the underlying hardware is like. Otherwise the programs they write will be pretty weird. -- D. Knuth

On Wednesday, 2 February 2022 at 23:30:50 UTC, H. S. Teoh wrote:
> On Wed, Feb 02, 2022 at 11:21:52PM +0000, forkit via Digitalmars-d-learn wrote: [...]
>>     char[] palindrome = cast(char[])"able was I ere I saw elba";
>
> String literals are immutable by default. Casting immutable to mutable
> is UB (Undefined Behaviour).
>
>
> [...]
>>     writeln(palindrome.reverse);
>
> Especially because .reverse mutates its argument.  So you're attempting to overwrite immutable data here. That's probably what caused the crash: the literal is put in the read-only segment and the OS killed the program when it tried to write to data in that read-only segment.
>
>
> T

that explains ldc perhaps (although i don't really get it. It's cast to mutable and being assigned to mutable.

in any case... ldc doesn't like it, but dmd is fine with this ??

On Thursday, 3 February 2022 at 01:39:33 UTC, forkit wrote:
>

oops!  forgot the .dup

char[] palindrome = cast(char[])"able was I ere I saw elba".dup;

;-)

On Thu, Feb 03, 2022 at 01:39:33AM +0000, forkit via Digitalmars-d-learn wrote:
> On Wednesday, 2 February 2022 at 23:30:50 UTC, H. S. Teoh wrote:
> > On Wed, Feb 02, 2022 at 11:21:52PM +0000, forkit via Digitalmars-d-learn wrote: [...]
> > >     char[] palindrome = cast(char[])"able was I ere I saw elba";
> > 
> > String literals are immutable by default. Casting immutable to
> > mutable is UB (Undefined Behaviour).
> > 
> > 
> > [...]
> > >     writeln(palindrome.reverse);
> > 
> > Especially because .reverse mutates its argument.  So you're attempting to overwrite immutable data here. That's probably what caused the crash: the literal is put in the read-only segment and the OS killed the program when it tried to write to data in that read-only segment.
[...]
> that explains ldc perhaps (although i don't really get it. It's cast to mutable and being assigned to mutable.

Assigning the literal to char[] simply creates a mutable slice of the immutable data. That's technically UB. Then .reverse modifies the array in-place, which means you violated immutable.


> in any case... ldc doesn't like it, but dmd is fine with this ??

UB doesn't mean a guaranteed crash, it means the result will be implementation-dependent (and likely not what was intended). It's possible that dmd didn't put the literal in a read-only segment, or there may be some other reason. In any case, just because it worked by chance does not mean it's OK to simply cast immutable to mutable and then proceed to mutate it.


T

-- 
Let X be the set not defined by this sentence...

On Thursday, 3 February 2022 at 01:57:12 UTC, H. S. Teoh wrote:
>

would be nice if the compiler told me something though :-(

i.e. "hey, dude, you really wanna to that?"

On Thu, Feb 03, 2022 at 02:01:34AM +0000, forkit via Digitalmars-d-learn wrote: [...]
> would be nice if the compiler told me something though :-(
> 
> i.e. "hey, dude, you really wanna to that?"

Mark your function @safe, and the compiler will stop you from unsafe casts of this nature. That's part of the reason we have @safe. ;-)


T

-- 
Everybody talks about it, but nobody does anything about it!  -- Mark Twain