Logging exceptions
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Introduction
A stacktrace is sometimes called backtrace or call stack. This is a list of stack frames placed on the stack containing return address and local variables. A stacktrace is therefore useful to trace back the path of execution of your program (following the calls to procedures).
In order to obtain a stacktrace, the compiler first has to be directed to generate debug information using switches:
- -g - generate debug information (in the default output format for the platform concerned, which is dwarf2 on a lot of platforms)
- -gl - generate line numbers for debug information
- -gs - generate older stabs debug information; don't use both -gs and -gw
- -gw - generate dwarf2 debug information; don't use both -gs and -gw
- -Xg - use external debug symbol file
SysUtils unit
SysUtils contains some routines useful for debugging exceptions.
{ Exception handling routines }
function ExceptObject: TObject;
function ExceptAddr: Pointer;
function ExceptFrameCount: Longint;
function ExceptFrames: PPointer;
System unit
The system unit contains some stack related routines:
function SysBackTraceStr(Addr:Pointer): ShortString; // Default address to string converter assigned to BackTraceStrFunc
procedure Dump_Stack(var f : text;bp:pointer); // Dump stack to text file
procedure DumpExceptionBackTrace(var f:text); // Dump backtrace to text file
Procedural variable BackTraceStrFunc responsible to translate memory address to string debug information. Default behavior is implemented by SysBackTraceStr.
Line information
If the line info debug output is selected (compiler switch -gl), the unit lineinfo is automatically included in the program. This unit makes sure that debuggers/exception handlers can find the line numbers of running code. It can be useful if you do not want to deploy a production executable with full debug info, but you do want useful information when errors occur.
Stabs
If the old stabs format (-gs) is used, the BackTraceStrFunc function is remapped to StabBackTraceStr.
DWARF
If the dwarf debugging format is selected (compiler switch -gw), the unit lnfodwrf is automatically included in the program and BackTraceStrFunc function is remapped to DwarfBacktraceStr.
Unit LCLProc
This Lazarus unit has some debug-related functions:
// Debugging
procedure RaiseGDBException(const Msg: string);
procedure RaiseAndCatchException;
procedure DumpExceptionBackTrace;
procedure DumpStack;
function GetStackTrace(UseCache: boolean): string;
procedure GetStackTracePointers(var AStack: TStackTracePointers);
function StackTraceAsString(const AStack: TStackTracePointers; UseCache: boolean): string;
function GetLineInfo(Addr: Pointer; UseCache: boolean): string;
Dump current call stack
See also FPC help on dumping stack/exception details:
procedure DumpCallStack;
var
I: Longint;
prevbp: Pointer;
CallerFrame,
CallerAddress,
bp: Pointer;
Report: string;
const
MaxDepth = 20;
begin
Report := '';
bp := get_frame;
// This trick skip SendCallstack item
// bp:= get_caller_frame(get_frame);
try
prevbp := bp - 1;
I := 0;
while bp > prevbp do begin
CallerAddress := get_caller_addr(bp);
CallerFrame := get_caller_frame(bp);
if (CallerAddress = nil) then
Break;
Report := Report + BackTraceStrFunc(CallerAddress) + LineEnding;
Inc(I);
if (I >= MaxDepth) or (CallerFrame = nil) then
Break;
prevbp := bp;
bp := CallerFrame;
end;
except
{ prevent endless dump if an exception occured }
end;
ShowMessage(Report);
end;
Dump exception call stack
The call stack of an exception can be obtained through SysUtils functions ExceptAddr, ExceptFrames and ExceptFrameCount.
uses SysUtils;
procedure DumpExceptionCallStack(E: Exception);
var
I: Integer;
Frames: PPointer;
Report: string;
begin
Report := 'Program exception! ' + LineEnding +
'Stacktrace:' + LineEnding + LineEnding;
if E <> nil then begin
Report := Report + 'Exception class: ' + E.ClassName + LineEnding +
'Message: ' + E.Message + LineEnding;
end;
Report := Report + BackTraceStrFunc(ExceptAddr);
Frames := ExceptFrames;
for I := 0 to ExceptFrameCount - 1 do
Report := Report + LineEnding + BackTraceStrFunc(Frames[I]);
ShowMessage(Report);
Halt; // End of program execution
end;
Handling exceptions
Manual exception handling
Manual executions of exception handler can be inserted in many places in code.
try
// ... some operation which should raise an exception...
raise Exception.Create('Test error');
except
on E: Exception do
DumpExceptionCallStack(E);
end;
System ExceptProc
If an unhandled exception occurs, the ExceptProc procedure variable is executed. Default behaviour is initialized by procedure InitExceptions in unit System. If you want to, this procedure can be reassigned to a custom handler.
An example:
procedure CatchUnhandledException(Obj: TObject; Addr: Pointer; FrameCount: Longint; Frames: PPointer);
var
Message: string;
i: LongInt;
hstdout: ^Text;
begin
hstdout := @stdout;
Writeln(hstdout^, 'An unhandled exception occurred at $', sysBackTraceStr(addr), ' :');
if Obj is exception then
begin
Message := Exception(Obj).ClassName + ' : ' + Exception(Obj).Message;
Writeln(hstdout^, Message);
end
else
Writeln(hstdout^, 'Exception object ', Obj.ClassName, ' is not of class Exception.');
Writeln(hstdout^, BackTraceStrFunc(Addr));
if (FrameCount > 0) then
begin
for i := 0 to FrameCount - 1 do
Writeln(hstdout^, BackTraceStrFunc(Frames[i]));
end;
Writeln(hstdout^,'');
end;
TApplication.OnException
This event can be used to override default application wide exceptions handling. A custom logging mechanism could provide show custom dialog, log to file, console, sending report to mail, logging to HTTP server, e.g.
procedure TMainForm.CustomExceptionHandler(Sender: TObject; E: Exception);
begin
DumpExceptionCallStack;
Halt; // End of program execution
end;
procedure TMainForm.FormCreate(Sender: TObject);
begin
Application.OnException := @CustomExceptionHandler;
end;
procedure TMainForm.ButtonClick(Sender: TObject);
begin
raise Exception.Create('Test');
end;
Handling thread exceptions
Handling exceptions which are raised in threads has to be done manually. The main thread TThread method Execute is called from the function ThreadProc located in unit Classes.
function ThreadProc(ThreadObjPtr: Pointer): PtrInt;
begin
...
try
Thread.Execute;
except
Thread.FFatalException := TObject(AcquireExceptionObject);
end;
...
end;
In this function, the Execute method is enclosed in a try-except block and all exceptions are handled by assigning the exception object to the FatalException property of TThread object as last occurred exception object. So exceptions are not displayed to the user at all.
In every thread in the application, a separate try-except block should be inserted to catch all unhandled exceptions by a custom exception handler.
procedure TMyThread.Execute;
begin
try
// some erroneous code
except
on E: Exception do
CustomExceptionThreadHandler(Self, E);
end;
end;
Then CustomExceptionThreadHandler can get the exception call stack and manage showing error messages or logging log reports to file. As the handler is executed from a thread, showing message dialog has to be done thread safe using the Synchronize method.
procedure TMainForm.CustomExceptionHandler(Thread: TThread; E: Exception);
begin
Thread.Synchronize(DumpExceptionCallStack);
end;
Using map file
Use compiler switch -Xm to generate map file.
Exceptions in DLL
todo
See also
- Profiling
- Creating a Backtrace with GDB
- MultiLog - A logging package
- log4delphi - A logging package
External links
- esprinter - tool to get stacktrace from running FreePascal program specified by process id and thread id (for Win32)
- PascalBugReports - project intended to mimic EurekaLog/MadExcept but has not been worked on for a while