GDB Debugger Tips
Lazarus comes with GDB as default debugger.
- This page is for Lazarus 1.0. For older versions see previous version of this page
- Note on GDB 7.5: GDB 7.5 is not supported by the released 1.0. Fixes to support it were made in 1.1.
To get the best possible results, you must ensure that your IDE and Project are both correctly configured.
See Debugger Setup how to set up the IDE and your project in order to use the debugger.
Debug Info Type
When debugging 64 bit applications, it may be necessary to use dwarf.
-g or -gs
You should only use if your gdb version does not support dwarf. There are very few other cases where you need it.
You may need it with "var param" (param by ref) procedure foo(var a: integer); However the IDE deals with this in 99% of all cases.
This is the recommended setting.
This sets the format to Dwarf2 (-gw2 is equivalent to this setting)
use with -godwarfsets if you have a recent gdb. Otherwise sets are not displayed correctly.
This list is in no way complete:
- dwarf allows some properties (those directly mapped to a field)
- stabs (and modern gdb) can do -gp (preserve the case of symbols, instead of getting the all caps stuff).
- stabs has problems with some class type casts. The IDE fixes that in some cases. (Only affects gdb 7.0 and up) 
Inspecting data types (Watch/Hint)
GDB does not know the Pascal string data type. The type information GDB returns does not currently allow for the IDE to differentiate between a PChar (index is 0 based) or a String (index is 1 based).
As a Result "mystring" could be the 10th char in a string, or 11th in a PChar.
Because the IDE can not be sure which one applies, it will show both. The 2 results will be prefixed String/PChar.
Currently the debugger does not support any method execution. Therefore only properties that refer directly to a variable can be inspected. (This only works if using dwarf)
TFoo = Class private FBar: Integer; function GetValue: Integer; public property Bar: Integer read FBar; // Can be inspected (if Dwarf is used) property Value: Integer read GetValue; // Can *not* be inspected end;
Nested Procedures / Function
procedure SomeObject.Outer(NumText: string); var OuterVarInScope: Integer; procedure Nested; var I: Integer; begin WriteLn(OuterVarInScope); end; var OuterVarOutsideScope: Integer; begin Nested; end;
If you step into "Nested", then the IDE allows you to inspect variables from both stack frames.
This is you can inspect: I, OuterVarInScope, NumText (without having to change the current stack frame, in the stack window)
However there are some caveats:
You can also inspect: OuterVarOutsideScope. That is against Pascal scoping rules. This only matters if you have several nested levels, and they all contain a variable of the same name, but the Pascal scoping would hide the variable from the middle frame. Then you get to see the wrong value.
You can not evaluate statements across the 2 frames: "OuterVarnScope-I" does not work.
array [x..y] of Integer;
shows as chars, instead of int. Only gdb 7.2 or up seems to handle it correctly.
Additionally there may be issues when using arrays of (unnamed) records.
Dynamic Arrays will only show a limited amount of their data. As of Lazarus 1.1 the limit can be specified.
SIGFPE is a floating point unit exception.
Source: [forum thread]
SIGFPE exceptions are raised on the next FPU instruction; therefore the error line you get from Lazarus may be off/incorrect.
Delphi, and thus Freepascal, have a non standard FPEMask. A lot of C libraries don't use exceptions to check FPU problems but inspect the FPU status.
Workaround: try changing the mask with SetExceptionMask as early as possible in your program:
uses math; ... SetExceptionMask([exInvalidOp, exDenormalized, exZeroDivide, exOverflow, exUnderflow, exPrecision])
If you think there is something wrong in the FPC code, you can use the $SAFEFPUEXCEPTIONS directive. This will add a FWAIT after every store of a FPU value to memory. Most FPC float and double operations end with storing the result somewhere in memory so the outcome is that the debugger stops on the correct FPC line. FWAIT is a nop for the FPU but will cause an exception to be raised immediately instead of somewhere down your code. It is not done per default because it slows down floating point operations a lot.
Console output for GUI application
On windows the IDE does not have the "Debugger - Console output" window. This is because console applications open there own console window. GUI applications by default have no console. In order to have a console for a GUI application, the compiler settings must be changed (Project options - linking: -WC / -WG)
Error 193 (Error creating process / Exe not found)
For details see here. This issue has been observed under Win XP and appears to be caused by Windows itself. The issue occurs if the path to your app has a space, and a 2nd path/file exists, with a name identical to the part of the path before the space, e.g.:
- Your app: C:\test folder\project1.exe
- Some file: C:\test
SigSegV - even with new, empty application
- In most cases SigSegV is simply an error in your code.
The following applies only, if you get a SigSegv, even with a new empty application (empty form, and no modifications made to unit1).
A SigSegV sometimes can be caused by an incompatibility between GDB and some other products. It is not known if this is a GDB issue or caused by the other product. There is no indication that any of the products listed is in any way at fault. The information provided may only apply to some versions of the named products:
- Comodo firewall
- enable game mode
SigSegV - and continue debugging
gdb.exe has stopped working
GDB itself has crashed. This will be due to a bug in gdb, and may not be fixable in Lazarus. Still it may be worth submitting info (see section "Reporting Bugs" below). Maybe Lazarus can avoid calling the failing function.
There is one already known situation. GDB (6.6 to 7.4 (latest at the time of testing)) may crash while your app is being started, or right after your app stopped. This happen while the libraries (DLL) for your app are loaded (watch the "Debug output" window). In this case go to the debugger options and in the field "debugger_startup_options" enter:
- --eval-command="set auto-solib-add off"
"PC register is not available" ("SuspendThread failed. (winerr 5)")
GDB may fail with this message. This is due to http://sourceware.org/bugzilla/show_bug.cgi?id=14018 If you get this issue you may want to downgrade to GDB 7.2
Can not insert breakpoint
For breakpoints with negative numbers, please see: http://forum.lazarus.freepascal.org/index.php/topic,10317.msg121818.html#msg121818
Win 64 bit
- Requires Lazarus 1.0 with fpc 2.6
- Advised to use dwarf
Using 32 bit Lazarus on 64 bit Windows
Alternatively it is possible to debug apps as 32 bit apps (using the 32bit version of gdb). Once successfully debugged:
- the app can then be cross-compiled to 64 bit, or
- a 2nd Lazarus installation can be used (using a different configuration --primary-config-path option from the 32 bit Lazarus)
When installing the 32 bit Lazarus, ensure you change configuration so the correct 32 bit FPC and GDB are used.
More Win 64 bit problems and solutions
There might be issues on at least some CPUs when using an old version of gdb (e.g. SPARC, with gdb 6.4 as supplied by Debian "Etch"). In particular, background threads might lock up even if the program appears to run correctly standalone.
Mac OS X
Under OS X you usually install GDB with the Apple developer tools. The version of GDB available at the time of writing is 6.3.50
This section is a first approach to collect information about known issues and workarounds. The information provided here depends a lot on feedback from OS X users.
Debugging 32 bit app on 64 bit architecture
It is possible and maybe sometime necessary to debug a 32 bit exe on a 64 bit system.
This is only possible with Lazarus 0.9.29 rev 28640 and up.
TimeOuts (64 Bit only)
It appears that certain commands are not correctly (or incompletely) processed by gdb 6.3.50. Normally GDB finishes each command with a "<gdb>" prompt for the next command. But the version provided by apple sometimes fails to do this. In this case the IDE will have to use a timeout to avoid waiting forever.
This has been observed with:
- Certain Watch expressions (at least if the watched variable was not available in the current selected function)
- Handling an Exception
- Inserting breakpoints (past unit end / none code location)
It can not be guaranteed:
- that GDB will not later return some results from such a command
- that GDB's internal state is still valid
The prompt displayed for timeouts can be switched off in the debugger config.
- Warn On Timeout
- True/False. Auto continue, after a timeout, do not show a warning.
- Set in milliseconds, how long until a timeout detection is triggered (the detection itself may take some more time.)
You may need to "RESET" the debugger after those changes are made
More info see her:
An alternative solution seems to be to use a newer version of GDB (e.g. MacPorts. But this currently fails on other issues)
Hardware exceptions under Mac OS X
When a hardware exception occurs under Mac OS X, such as an invalid pointer access (SIGSEGV) or an integer division-by-zero on Intel processors, the debugger will catch this exception at the Mach level. Normally, the system translates these Mach exceptions into Unix-style signals where the FPC run time library handles them. The debugger is however unable to propagate Mach exceptions in this way.
The practical upshot is that it is impossible under Mac OS X to continue a program in the debugger once a hardware exception has been triggered. This is not FPC-specific, and cannot be fixed by us.
Using Alternative Debuggers
You can download GDB 7.1 (or later) from MacPorts
The following observations have been made:
- MacPorts GDB 7.1 seems to have issues with the stabs debug info from fpc.
- Make sure you select "generate dwarf debug information (-gw)" on the linking tab of the project options
- Watch out for the linker warnings about "unknown stabs"
- If you still have problems ensure that no code at all, is compiled with stabs. Your LCL may contain stabs, and this will end up in your app, even if you compile the app with -gw. So you may have to recompile your LCL with -gw (or without any debug info). Same for any other unit, package, rtl, .... that may have stabs
Even with those rules followed, MacPorts GDB does not always work with fpc compiled apps.
Those observations may be incomplete or wrong, please update them, if you have more information.
- Lazarus Version 1.0 to 1.0.12
In the debugger options configure
EncodeCurrentDirPath = gdfeNone EncodeExeFilename = gdfeNone
And you must use "run param" to specify the actual executable inside the app-bundle ( project.app/Content/MacOS/project or similar)
In Lazarus Version 1.0.14 to 1.2 and up those steps should no longer be required.
Since OS X Mavericks 10.9, Xcode 5 no longer installs gdb by default and not globally.
- For 10.9 you can install an old Xcode 4 in parallel. This is not supported by Apple, so it might break with one of the next versions.
- You can compile and install gdb. See GDB on OS X Maverick.
Please see issue 25157 on mantis
- http://forum.lazarus.freepascal.org/index.php/topic,22529.0.html - about installing Xcode 4.3
- http://forum.lazarus.freepascal.org/index.php/topic,22328.0.html - install gdb
Thread "[Lazarus] Help: OS X Problems" on
- Mac OS X comes with a lot of useful tools for debugging and profiling. Just start /Developer/Applications/Instruments.app and try them out.
The system-provided gdb is ancient (version 6.1.1 in FreeBSD 9) and does not work well with Lazarus.
You can install a newer gdb from the ports tree, e.g.
cd /usr/ports/devel/gdb make -DBATCH install clean
The new gdb is located in /usr/local/bin/gdb
Using a translated GDB (non-English responses from GDB)
The IDE expects responses in English. If GDB is translated this may affect how well the IDE can use it.
In many cases it will still work, but with limits, such as:
- No Message or Class for exceptions
- No live update of threads, while app is running
- Debugger error reported, after the app is stopped (at end of debugging)
- other ...
Please read the entire thread: [http://forum.lazarus.freepascal.org/index.php/topic,20756.msg120720]
Try to run lazarus with
export LANG=en_US.UTF-8 lazarus
- Did you verify your Debugger Setup?
- Did you try Dwarf and Stabs
Check existing Reports
Please check each of the following links
- Search Mantis for category "debugger"
- Search Mantis for text "debugger"
- Search Mantis for text "gdb"
- Search Mantis for text "dwarf"
- Search Mantis for text "stabs"
Bugs in GDB
- ONLY GDB 7.4 (maybe higher / not verified):
- win64 crash with bad dwarf2 symbols
- Solved in GDB 7.4:
- Debuggee doesn't see environment variables set with set environment
- GDP 7.3 and up / *apparently* not present in 7.0 to 7.2
- Win32 fails to continue with "winerr 5" (pc register not available)
- GDB 7.3 to 7.5 (maybe / not tested 7.6.0) / fixed in 7.6.1:
- Wrong array index with dwarf
- GDB crashes, if trying to inspect or watch resourcestrings
- GDB 7.0 to 7.2.xx
- only tested on win32, not known for other platforms
- only, if using dwarf
- GDB 7.6.1 (not tested for 7.5.9 or 7.6.0) and up
- Records (especially: pointer to record) may be mistaken for classes/objects
- The data/values appear to be displayed correct / further tests pending
Issues with GDB 7.5.9 or 7.6
There are various reports (not confirmed) for different platforms about crashes in gdb 7.5.9 or 7.6.
It is highly recommended not to use those versions.
Create a new Report
If you have at any time in the past updated, changed, reinstalled your Lazarus then please check the "Options" dialog ("Environment" or "Tools" Menu) for the version of GDB and FPC used. They may still point to old settings.
- Your Operating system and Version
- Your CPU (Intel, Power, ...) 32 or 64 bit
- Version of
- Lazarus (Latest Release or SVN revision) (Include setting used to recompile LCL, packages or IDE (if a custom compile Lazarus is used))
- FPC, if different from default. Please check the "Options" dialog ("Environment" or "Tools" Menu)
- GDB, please check the "Options" dialog ("Environment" or "Tools" Menu)
- Compiler Settings: (Menu: "Project", "Project Options", indicate all settings you used )
- Page "Code generation"
- Optimization settings (-O???): Level (-O1 / ...) Other (-OR -Ou -Os) (Please always test with all optimizations disabled, and -O1)
- Page "Linking"
- Debug Info: (-g -gl -gw) (Please ensure that at least either -g or -gw is used)
- Smart Link (-XX); This must be OFF
- Strip Symbols (-Xs); This must be OFF
Log info for debug session
- Start Lazarus with the following options:
- On Windows
- you need to create a shortcut to the Lazarus exe (like the one on Desktop) and edit it's properties. Add " --debug-log=C:\laz.log --debug-enable=..." to the command line.
- On Linux
- start Lazarus from a shell and append " --debug-log=/home/yourname/laz.log --debug-enable=..." to the command line
- On Mac/OSX
- start Lazarus from a shell /path/to/lazarus/lazarus.app/Contents/MacOS/lazarus --debug-log=/path/to/yourfiles/laz.log --debug-enable=...
Attach the log file after reproducing the error.
If you can not generate a log file, you may try to open the "Debug output" window (from menu "View", "Debug windows").You must open this before you run your app (F9). And then run your app. Once the error occurs, copy, zip, and attach the content of the "debug output" window.
Experimental Debuggers in pascal
Those are work in progress:
- fpdebug in Lazarus/components/fpdebug and debugger/fpgdbmidebugger