Delphi PChar string is empty when passed to function

I am using Delpi XE6 if that matters.

I am writing a DLL that will receive a response from the Google Directions API, parse the resulting JSON and determine the total travel distance.

GetJSONString_OrDie

works as intended. It gets the url and tries to get a response from Google. If so, it returns the JSON string returned by Google.

ExtractDistancesFromTrip_OrDie

should take a JSON string and parse it to get the total distance of each branch of the path, and keep those distances in the doubling array.

SumTripDistances

takes twins into an array and sums the array and returns the total.

The curious thing is that if I pick these functions from the DLL and put them in the project and call them that way, it works as intended. It is only when they get stuck in the DLL that it happens when something goes wrong. As said, GetJSONString_OrDie

works as intensive and returns a JSON string, but when stepping through the DLL, the ExtractDistancesForTrip

passed PChar string ''

. Why is this?

unit Unit1;



interface
uses
  IdHTTP,
  IdSSLOpenSSL,
  System.SysUtils,
  System.JSON;

type
  arrayOfDouble = array of double;

function GetJSONString_OrDie(url : Pchar) : Pchar;
function ExtractDistancesForTrip_OrDie(JSONstring: Pchar) : arrayOfDouble;
function SumTripDistances(tripDistancesArray: arrayOfDouble) : double;

implementation



{ Attempts to get JSON back from Google Directions API }
function GetJSONString_OrDie(url : Pchar) : PChar;
var
  lHTTP: TIdHTTP;
  SSL: TIdSSLIOHandlerSocketOpenSSL;
begin
  {Sets up SSL}
  SSL := TIdSSLIOHandlerSocketOpenSSL.Create(nil);
  {Creates an HTTP request}
  lHTTP := TIdHTTP.Create(nil);
  {Sets the HTTP request to use SSL}
  lHTTP.IOHandler := SSL;
  try
    {Attempts to get JSON back from Google Directions API}
    Result := PWideChar(lHTTP.Get(url));
  finally
    {Frees up the HTTP object}
    lHTTP.Free;
    {Frees up the SSL object}
    SSL.Free;
  end;
end;

{ Extracts the distances from the JSON string }
function ExtractDistancesForTrip_OrDie(JSONstring: Pchar) : arrayOfDouble;
var
  jsonObject: TJSONObject;
  jsonArray: TJSONArray;
  numberOfLegs: integer;
  I: integer;
begin
  //raise Exception.Create('ExtractDistancesForTrip_OrDie:array of double has not yet been '+
  //                        'implemented.');

  jsonObject:= nil;
  jsonArray:= nil;
  try
    { Extract the number of legs in the trip }
    jsonObject:= TJSONObject.ParseJSONValue(TEncoding.ASCII.GetBytes(JSONstring), 0) as TJSONObject;
    jsonObject:= (jsonObject.Pairs[0].JsonValue as TJSONArray).Items[0] as TJSONObject;
    jsonArray:= jsonObject.Pairs[2].JSONValue as TJSONArray;
    numberOfLegs:= jsonArray.Count;

    {Resize the Resuls arrayOfDouble}
    SetLength(Result, numberOfLegs);

    {Loop through the json and set the result of the distance of the leg}
    {Distance is in km}
    for I := 0 to numberOfLegs-1 do
    begin
      Result[I]:= StrToFloat((((jsonArray.Items[I] as TJSONObject).Pairs[0].JsonValue as TJSONObject).Pairs[1]).JsonValue.Value);
    end;


  finally
    jsonObject.Free;
    jsonArray.Free;
  end;

end;


function SumTripDistances(tripDistancesArray: arrayOfDouble) : double;
var
  I: integer;
begin
  //raise Exception.Create('GetDistanceBetweenPoints_OrDie:double has not yet been ' +
 //                        'implemented.');

 Result:= 0;
 {Loop through the tripDistancesArray, and add up all the values.}
 for I := Low(tripDistancesArray) to High(tripDistancesArray) do
    Result := Result + tripDistancesArray[I];


end;

end.

      

        
        
        
      

    

This is how the functions are called:

program Project3;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  ShareMem,
  IdHTTP,
  IdSSLOpenSSL,
  System.SysUtils,
  System.JSON,
  System.Classes;

type
  arrayOfDouble = array of double;

  testRecord = record
    testString : string;
    testInt : integer;
  end;

function GetJSONString_OrDie(url : Pchar) : PWideChar; stdcall; external 'NMBSGoogleMaps.dll';

function ExtractDistancesForTrip_OrDie(JSONstring: pchar) : arrayOfDouble;  stdcall; external 'NMBSGoogleMaps.dll';

function SumTripDistances(tripDistancesArray: arrayOfDouble) : double; stdcall; external 'NMBSGoogleMaps.dll';


var
  jsonReturnString: string;
  jsonReturnString2: Pchar;
  doubles: arrayOfDouble;
  totalJourney: double;
  uri:Pchar;

  a : testRecord;
  begin
  try
    uri:= 'https://maps.googleapis.com/maps/api/directions/json?origin=Boston,MA&destination=Concord,MA&waypoints=Charlestown,MA|Lexington,MA&key=GETYOUROWNKEY';


    { TODO -oUser -cConsole Main : Insert code here }
    jsonReturnString:= GetJSONString_OrDie(uri); //On step-through, uri is fine.

    jsonReturnString2:= stralloc(length(jsonreturnstring)+1);
    strpcopy(jsonreturnstring2, jsonreturnstring);
    jsonreturnstring2 := 'RANDOMJUNK';

    doubles:= ExtractDistancesForTrip_OrDie(jsonReturnString2); //On step-through, jsonReturnString2 is seen as '', rather than 'RANDOMJUNK'
    totalJourney:= SumTripDistances(doubles);
    WriteLn('The total journey was: ');
    WriteLn(totalJourney);

  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
  readln;
end.

      

        
        
        
      

    

When the first function is called GetJSONString_OrDie

, the uri is passed and validated when viewed or printed. However, after creating a random string and passing it to the ExtractDistancesForTrip_OrDie

function, only sees ''

. If I change ExtractDistancesForTrip_OrDie

to accept an integer, or a record, or anything else - it sees either garbage or random data. It doesn't matter if I am traversing a reference or a value.