Local string array initialization - arrays

Question

I have sporadic problems (access violation in unit System).
Application is running 24 x 7 and it happens one - two times in a week.
I have a procedure with local string array, and I found there are cases I assign non initialised array member to string variable like in code below.
Could it be a reason for access violation?
procedure tform1.DoSomething;
var
sarr: array [1..4] of String;
s: String;
begin
sarr[1] := 'aaaa';
sarr[2] := 'bbbb';
s := sarr[3]; // Can I get here an access violation???
end;
Real function code below
exception happens when obj.opcode = cmdp_done
function is called from thread message queue. obj is created in another thread, and
sent in PostThreadMessage as msg.lparam
procedure ORTR_ProcessFiscalResponse(obj: TDataForOrpak);
const
maxdim = 4;
var
s: array [1..maxdim] of String;
i, n, fiscalNr, statusToSend: Integer;
sql, pdf, ortrName, docType, rut: String;
ortr: TCustomizedOrtr;
oFiscal: TFiscalDevice;
fpds: TFPDeviceState;
begin
try
case obj.opcode of
cmdp_status: N := 3;
cmdp_done: N := 2;
plcl_docissued: N := 4;
plcl_docfailed: N := 1;
else
Exit;
end;
for i:=1 to n do
s[i] := GetTerm(obj.ident, i, ';');
if s[1] = '' then
Exit;
statusToSend := 0;
ortrName := GetTerm(s[1], 1, '+');
fiscalNr := StrToIntDef(GetTerm(s[1]+'+', 2, '+'), 999999);
docType := s[3];
rut := s[4];
ortr := TCustomizedOrtr.GetTerminalByName(ortrName) as TCustomizedOrtr;
if ortr = nil then
Exit;
if (ortr.FPState = fps_idle) or (ortr.fiscalNr <> fiscalNr) then begin
if (StrToIntDef(s[2], 0) <> 0) and (obj.opcode = cmdp_done) then
fiscal_Confirm(obj.otherdevname, obj.ident);
if obj.opcode = plcl_docissued then begin
try
PLCL_SetDocState(s[1], rut, false, StrToInt(s[2]), StrToInt(docType));
except
AddToLogFile('*** PLCL_SetDocState', log_exceptions);
end;
end;
Exit;
end;
oFiscal := fiscal_Assigned(ortr.ctlPump.PumpID) as TFiscalDevice;
case obj.opcode of
plcl_docissued:
begin
ortr.authData.ECRReceiptNr := s[2];
pdf := StringFromHexPresentation(obj.rawdata);
sql := format(sql_PaperlessAdd, [
ToLocalSQL_DateTime(ortr.ctlPump.FinalTime),
ToLocalSQL_Integer(ortr.ctlPump.PumpID),
ToLocalSQL_String(pdf)]);
try
UpdateLocalDB(sql);
except
AddToLogFile('*** PL save pdf', log_exceptions);
end;
try
PLCL_SetDocState(s[1], rut, true, StrToInt(s[2]), StrToInt(docType));
except
AddToLogFile('*** PLCL_SetDocState', log_exceptions);
end;
ortr.FPState := fps_idle;
ortr.currStage := TTextIndexType(0);
ortr.currStage := tivirt_towelcome; // VADIM
ExternalProcessPumpState(ortr.authData.gsPump.PumpID);
end;
plcl_docfailed:
ortr.FPState := fps_plerror;
cmdp_status:
begin
ortr.FPError := StrToIntDef(s[3], 0);
fpds := TFPDeviceState(StrToIntDef(s[2], 0));
ortr.FPState := fpds;
if (fpds in [fps_nocomm, fps_error]) and (ortr.fiscalMode = 1) and
(ortr.authData = nil) and (ortr.fiscalNr < 0) then
SpecialInterface.SendFiscalNrToPromax(-ortr.fiscalNr, '0');
case fpds of
fps_nopaper: statusToSend := wph_prnpaperout;
fps_nocomm: statusToSend := wph_prncommfailure;
fps_error: statusToSend := wph_prngenericfailure;
end;
end;
cmdp_done:
begin
if ortr.fiscalMode = 1 then begin
if ortr.authData = nil then begin // DRY GOOD
SpecialInterface.SendFiscalNrToPromax(-fiscalNr, s[2]);
end
else begin
ortr.authData.ECRReceiptNr := s[2];
ExternalProcessPumpState(ortr.authData.gsPump.PumpID);
end
end;
if StrToIntDef(s[2], 0) <> 0 then
fiscal_Confirm(obj.otherdevname, obj.ident);
statusToSend := wph_prnidle;
ortr.FPState := fps_idle;
ortr.currStage := ti_takereceipt;
end;
end;
if (statusToSend <> 0) and (oFiscal <> nil) then
PostNonVisualCommand(nv_devicestate, statusToSend, Integer(oFiscal));
finally
obj.free;
end;
end;

Answer 1

Your initial piece of code, the tform1.DoSomething routine, is unable of producing an access violation:
For the static array variable sarr, memory is allocated for all its elements.
The string variable s, as well as the elements in sarr, are automatically initialized to empty. 1
Thus you are simply assigning an empty string, and s remains empty.
Concerning your actual code, assuming that it does produce the access violation, my guess would be that:
the obj parameter still refers to an already destroyed object,
that obj.opcode reads an invalid piece of memory, but since it is compared to an numerical value, will pass,
that Exit is called in de case else clause, and
that obj.Free fails in the finally clause.
1 All string variables are initilized to empty, except string function results:
If the function exits without assigning a value to Result or the function name, then the function's return value is undefined.
The missing compiler warning is still a bug.