out parameter and "ShowMessage" function - delphi

Question

I have a function declare like this :
function execProc(ProcName,InValues:PChar;out OutValues:PChar):integer; //The "OutValues" is a out parameter.
And I call this function like this:
procedure TForm1.Button6Click(Sender: TObject);
var
v:integer;
s:pchar;
begin
Memo1.Clear;
v := execProc(pchar('PROC_TEST'),pchar('aaa'),s);
showmessage(inttostr(v)); //mark line
Memo1.Lines.Add(strpas(s));
end;
when i delete the mark line(showmessage(inttostr(v))),i will have a correct result display in the Memo1,but if i keep use the showmessage(), the memo1 will dispaly an error string : "Messag" ,Why?
Thanks for any help!
function execProc(ProcName,InValues:PChar;out OutValues:PChar):integer;
var
str: TStrings;
InValue,OutValue: string;
i,j,scount: integer;
begin
Result := -100;
i := 0;
j := 0;
str := TStringList.Create;
try
sCount := ExtractStrings(['|'], [], InValues, str);
with kbmMWClientStoredProc1 do
begin
Close;
Params.Clear;
StoredProcName := StrPas(ProcName);
FieldDefs.Updated := False;
FieldDefs.Update;
for i := 0 to Params.Count - 1 do
begin
if (Params[i].ParamType = ptUnknown) or
(Params[i].ParamType = ptInput) or
(Params[i].ParamType = ptInputOutput) then
begin
inc(j);
InValue := str[j-1];
Params[i].Value := InValue;
end;
end;
try
ExecProc;
for i := 0 to Params.Count - 1 do
begin
if (Params[i].ParamType = ptOutput) or
(Params[i].ParamType = ptInputOutput) then
OutValue := OutValue + '|' + Params[i].AsString;
end;
OutValues := PChar(Copy(OutValue,2,Length(OutValue)-1));
Result := 0;
except
on E:Exception do
begin
if E.Message = 'Connection lost.' then Result := -101;//服务器连接失败
if E.Message = 'Authorization failed.' then Result := -102;//身份验证失败
Writelog(E.Message);
end;
end;
end;
finally
str.Free;
end;
end;

Answer 1

The problem is in the design of your interface and the use of PChar.
OutValues := PChar(Copy(OutValue,2,Length(OutValue)-1));
This is implemented by making an implicit, hidden, local string variable which holds the value
Copy(OutValue,2,Length(OutValue)-1)
When the function returns, that string variable is destroyed and so OutValues points at deallocated memory. Sometimes your program appears to work but that's really just down to chance. Any small change can disturb that, as you have observed.
The problem is easy enough to fix. Simply use string parameters rather than PChar. This will make the code easier to read as well as making it work correctly.
function execProc(ProcName, InValues: string; out OutValues: string): integer;