On 2023-06-30 11:11, Michael Van Canneyt via fpc-pascal wrote:
On Fri, 30 Jun 2023, Hairy Pixels via fpc-pascal wrote:
On Jun 30, 2023, at 9:03 AM, Hairy Pixels <generic...@gmail.com>
wrote:
That's why that worked. Hmm with short strings the null terminator
could be truncated when copying right? Something to look out for.
this is what I meant about truncation. S is no longer null terminated
and now the pchar will fail when passed to C libraries right?
var
s: String;
p: Pchar;
begin
s := 'hello';
p := @s;
S will still be null-terminated for ansistrings and shortstrings.
change to
s:=s+s;
p:=@s[1]; // and not @s
writeln(p[Length(s)+1]=#0);
and it will print TRUE.
However, there are operations valid for Pascal shortstrings which would
break this - as an example, older Pascal code used to change the S[0]
(i.e. the length byte) for a shortstring which wouldn't ensure that #0
follows such a moved end of the Pascal shortstring.
Moreover, note that #0 is a valid character in Pascal strings, i.e. in
general it is _not_ guaranteed that the trailing #0 after the end of the
string is the first one encountered when passing the string to a C code.
Obviously, you can ensure this in your own code, but it's important to
stress that neither the compiler nor the FPC RTL check or enforce full
equivalence of Pascal strings and C null-terminated strings.
Tomas
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