On 2020-01-01 01:58, JJ Merelo wrote:
El mar., 31 dic. 2019 a las 21:56, ToddAndMargo via perl6-users (<perl6-us...@perl.org>) escribió:
On 2019-12-31 09:17, JJ Merelo wrote:
> Hi,
>
> El mar., 31 dic. 2019 a las 5:54, Todd Chester via perl6-users
> (<perl6-us...@perl.org <mailto:perl6-us...@perl.org>>) escribió:
>
> Hi All,
>
> https://docs.raku.org/language/nativecall.html#Passing_and_returning_values
>
> Did anyone else discover the mistake, hopefully not the hard way like I
> did. Anyone get weird gibberish printed out like I did?
>
> my$string="FOO";
> # The lifetime of this variable must be equal to the required lifetime of
> # the data passed to the C function.
> my$array=CArray[uint8].new($string.encode.list);v
>
> The mistake is that "C" strings are terminated by a chr(0) -- in
> "C" terms chr(0) is called a nul or 0x00.
>
> If you don't terminate your C string, the reading functions keep
> careening
> until it finds a chr(0). You have to tack a chr(0) onto the end.
>
> my$array=CArray[uint8].new($string.encode.list);
> $array [$array.elems] = 0;
>
> $array should be 70 79 70 0 not 70 79 79
> You can test this with `say "$array"` (include the quotes).
>
> Maybe JJ will pick up on this and get it fixed.
>
>
>
> I don't always read this list, but I do read issues. If you put it
> there, it will be definitely get addressed when possible.
>
> --
> JJ
Hi JJ,
I have a rule about not posting to that list, so maybe
I could answer that with I will not address issues that are not raised in the GitHub repo (which is not a list, BTW), but anyway, here you go. The thing is, first, that the example has little to do with the structure of the string itself; it focuses on the fact that the string has to be encoded. But anyway, let's check if that is true, with this little example:
------ >8 ---------- const-char.c ----------------------#include <stdio.h>
void set_foo ( const char *foo) {
printf("%s", foo);
}----------------- 8< --------------------------------------
And this Raku script that uses it
-------------------------- >8 --------------- null-terminated-or-not.p6 --------------use NativeCall;
my $string = "FOO";
my $array = CArray[uint8].new($string.encode.list);
sub set_foo(CArray[uint8]) is native('const-char') { * }
set_foo( $array );----------------- 8< --------------------------------------
This works without a glitch, printing FOO to the terminal. As a matter of fact, $array.elems returns 3, so its "encode" is not actually adding any null-termination at the end. I'm no expert in C myself, and have not been for at least 25 years. But I know stackoverflow, which says (https://stackoverflow.com/a/1253336/891440)
Why do C strings work this way? It's because the PDP-7 microprocessor, on which UNIX and the C programming language were invented, had an ASCIZ string type. ASCIZ meant "ASCII with a Z (zero) at the end."
Is this the only way to store strings? No, in fact, it's one of the worst ways to store strings. For non-trivial programs, APIs, operating systems, class libraries, you should avoid ASCIZ strings like the plague.
Plagues are bad. Let's avoid them for the upcoming year. Other than that, happy new year to every one.
--JJ
Hi JJ,
Wow, you spent a lot of time on that. Thank you!
You are of course correct.
The example works and that is the end of it. So no
mistake to be addressed.
You are a teacher. Would you have wrote that example
with out addressing the nature of a "C" string and why
it requires a nul at the end?
You are a teacher. Would you have given an example that
only worked in the example?
You are a teacher. What would you have made the
purpose of that document? To teach how to use the
module? Or so just put down things with no practical
application? "It is intuitively obvious and left up to
the student to figure out on his own?"
This is what happens in reality when you do not add the
nul to the end of a "C" string. C just keep careening
until it finds a nul. Here you go:
So you are 100% correct in your assertions. And why
this error will not be corrected. And a good example
of why I don't post there anymore.
-T
-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Computers are like air conditioners. They malfunction when you open windows ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
