Difference between revisions of "Logging exceptions"

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==Introduction==
 
==Introduction==
  
'''Stacktrace''' is sometimes called '''Backtrace''' or '''Call stack''' and have meaning of list of stack frames placed on stack containing return address and local variables. Stacktrace is useful to trace back path of execution of nesting procedures.
+
'''Stacktrace''' is sometimes called '''Backtrace''' or '''Call stack'''. It means a list of stack frames placed on 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).
  
  
Compiler have to be directed to get debug information by switches.
+
The compiler has to be directed to get debug information using switches:
  
*-g - generate debug information
+
*-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
 
*-gl - generate line numbers for debug information
*-gw - generate older dwarf debug information
+
*-gs - generate older stabs debug information
 +
*-gw - generate dwarf2 debug information
 
*-Xg - use external debug symbol file
 
*-Xg - use external debug symbol file
 
  
  
 
===Unit SysUtils===
 
===Unit SysUtils===
Contains some routines useful for debugging exceptions.
+
This unit contains some routines useful for debugging exceptions.
  
 
<syntaxhighlight>{ Exception handling routines }
 
<syntaxhighlight>{ Exception handling routines }
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===Unit System===
 
===Unit System===
Contains some stack related routines:
+
This unit contains some stack related routines:
  
 
<syntaxhighlight>function SysBackTraceStr(Addr:Pointer): ShortString; // Default address to string converter assigned to BackTraceStrFunc
 
<syntaxhighlight>function SysBackTraceStr(Addr:Pointer): ShortString; // Default address to string converter assigned to BackTraceStrFunc
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Procedural variable '''BackTraceStrFunc''' responsible to translate memory address to string debug information. Default behavior is implemented by '''SysBackTraceStr'''.
 
Procedural variable '''BackTraceStrFunc''' responsible to translate memory address to string debug information. Default behavior is implemented by '''SysBackTraceStr'''.
  
====STAB====
+
====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.
  
If STAB debugging information is selected (compiler switch -gl), unit '''lineinfo''' is automatically included to program and '''BackTraceStrFunc''' function is remapped to '''StabBackTraceStr'''.
+
====Stabs====
 +
If the stabs format (-gs) is used, the '''BackTraceStrFunc''' function is remapped to '''StabBackTraceStr'''.
  
 
====DWARF====
 
====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'''.
If DWARF debugging information is selected (compiler switch -gw), unit '''lnfodwrf''' is automatically included to program and '''BackTraceStrFunc''' function is remapped to '''DwarfBacktraceStr'''.
 
 
 
  
 
===Unit LCLProc===
 
===Unit LCLProc===
 
+
This Lazarus unit has some debug-related functions:
 
<syntaxhighlight>// Debugging
 
<syntaxhighlight>// Debugging
 
procedure RaiseGDBException(const Msg: string);
 
procedure RaiseGDBException(const Msg: string);
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'''FPC help:'''
+
==Dump current call stack==
 +
See also FPC help on dumping stack/exception details:
 
* [http://www.freepascal.org/docs-html/rtl/system/dumpexceptionbacktrace.html DumpExceptionBackTrace]
 
* [http://www.freepascal.org/docs-html/rtl/system/dumpexceptionbacktrace.html DumpExceptionBackTrace]
 
* [http://www.freepascal.org/docs-html/rtl/system/dump_stack.html dump_stack]
 
* [http://www.freepascal.org/docs-html/rtl/system/dump_stack.html dump_stack]
 
 
==Dump current call stack==
 
  
 
<syntaxhighlight>procedure DumpCallStack;
 
<syntaxhighlight>procedure DumpCallStack;
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===System ExceptProc===
 
===System ExceptProc===
  
If unhandled exception occurs than ExceptProc procedure variable is executed. Default behavior initialized by procedure '''InitExceptions''' in unit '''System''' can be redefine to custom handler. As example implementation default procedure '''CatchUnhandledException''' can be taken.
+
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:
 
<syntaxhighlight>procedure CatchUnhandledException(Obj: TObject; Addr: Pointer; FrameCount: Longint; Frames: PPointer);
 
<syntaxhighlight>procedure CatchUnhandledException(Obj: TObject; Addr: Pointer; FrameCount: Longint; Frames: PPointer);
 
var
 
var
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===TApplication.OnException===
 
===TApplication.OnException===
  
This event can be used to override default application wide exceptions handling. Custom logging mechanism could provide show custom dialog, log to file, console, sending report to mail, logging to HTTP server, e.g.
+
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.
  
 
<syntaxhighlight>procedure TMainForm.CustomExceptionHandler(Sender: TObject; E: Exception);
 
<syntaxhighlight>procedure TMainForm.CustomExceptionHandler(Sender: TObject; E: Exception);
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===Handling thread exceptions===
 
===Handling thread exceptions===
  
Handling exceptions which are raised in thread have to be done manually. Thread main TThread method '''Execute''' is called form function '''ThreadProc''' located in unit '''Classes'''.  
+
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'''.  
 
<syntaxhighlight>function ThreadProc(ThreadObjPtr: Pointer): PtrInt;
 
<syntaxhighlight>function ThreadProc(ThreadObjPtr: Pointer): PtrInt;
 
begin
 
begin
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end;</syntaxhighlight>
 
end;</syntaxhighlight>
  
In this function Execute method is enclosed in try-except block and all exceptions are handled in way of assigning exception object to '''FatalException''' property of TThread objec as last occurred exception object. So exceptions are not displayed to user at all.
+
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 application own try-except block should be inserted to catch all unhandled exceptions by custom exception handler.
+
In every thread in the application, a separate try-except block should be inserted to catch all unhandled exceptions by a custom exception handler.
  
 
<syntaxhighlight>procedure TMyThread.Execute;
 
<syntaxhighlight>procedure TMyThread.Execute;
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end;</syntaxhighlight>
 
end;</syntaxhighlight>
  
Then '''CustomExceptionThreadHandler''' can get exception call stack and manage showing error message or logging log report to file. As handler is executed from thread showing message dialog have to be done thread safe using '''Synchronize''' method.
+
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.
  
 
<syntaxhighlight>procedure TMainForm.CustomExceptionHandler(Thread: TThread; E: Exception);
 
<syntaxhighlight>procedure TMainForm.CustomExceptionHandler(Thread: TThread; E: Exception);
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===Using map file===
 
===Using map file===
  
Use compiler switch -Xm to generate map file.
+
Use compiler switch -Xm to generate map file. '''what is a map file? why can this be useful? BigChimp, March 2013'''
  
 
===Exceptions in DLL===
 
===Exceptions in DLL===
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[[Category:Debugging]]
 
[[Category:Debugging]]
 +
[[Category:FPC]]
 +
[[Category:Lazarus]]

Revision as of 12:55, 14 March 2013

Introduction

Stacktrace is sometimes called Backtrace or Call stack. It means a list of stack frames placed on 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).


The compiler has to be directed to get 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
  • -gw - generate dwarf2 debug information
  • -Xg - use external debug symbol file


Unit SysUtils

This unit contains some routines useful for debugging exceptions.

{ Exception handling routines }
function ExceptObject: TObject;
function ExceptAddr: Pointer;
function ExceptFrameCount: Longint;
function ExceptFrames: PPointer;

Unit System

This 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 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

Call stack of 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 to many places in code.

try
  // Some operation which raise 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 $', HexStr(PtrUInt(Addr), SizeOf(PtrUInt) * 2), ' :');
  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. what is a map file? why can this be useful? BigChimp, March 2013

Exceptions in DLL

See also

External links

  • esprinter - tool to get stacktrace from running FreePascal program specified by process id and thread id (for Win32)