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GDB Debugger Tips

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Introduction

Lazarus comes with GDB as default debugger. From Lazarus 2.0 onwards, this default has changed to LLDB on MacOs.

See also

Setup (GDB and LLDB)

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.

Setup Video Tutorial

Other

General

Debug Info Type (GDB and LLDB)

Note: Those setting apply only to the project or package to which they are set. If changing settings in either project or package options, it should be ensured that this is done for all packages and the project.
Otherwise your project will have mixed debug info, which can lead to a degraded debug experience.
The setting can be applied to project and packages using Additions and Overrides

Note: When compiling 32Bit apps (native or cross) FPC defaults to "Stabs" (At least on some OS). It is recommended to change your configuration to "Dwarf".

For 64 bit applications, FPC supports only "Dwarf". (Older FPC also support "Stabs", but that is not recommended)

Stabs (only GDB)

-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.

The LLDB based debugger does not support Stabs

Dwarf (GDB and LLDB)

Dwarf 2 (-gw)

This sets the format to Dwarf2. This is the most basic dwarf setting.

Dwarf 2 with sets (-gw -godwarfsets)

This setting adds the ability to inspect sets: "type TFoo=set of (a,b,c);". This is borrowed from the Dwarf 3 specs, but supported by most versions of GDB (any GDB from version 7 upwards should do).

text
This is the recommended setting.
Dwarf 3 (-gw3)

Dwarf 3 can encode additional info for some types (such as strings and arrays). It also preserves the case of identifiers in the debug info.

However there are still issues with the produced debug info. Some info may be incorrectly encoded, and other is not understood by GDB. In some cases this can lead to gdb crashing. This setting can be used, when using the FpDebug based debugger (add on package for the IDE)

Differences

This list is in no way complete:

Inspecting data types (Watch/Hint)

Strings

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[10]" could be the 10th char in a string, or 11th in a PChar.

Because the IDE cannot be sure which one applies, it will show both. The 2 results will be prefixed String/PChar.

Properties

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)

pascal
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

pascal
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 cannot evaluate statements across the 2 frames: "OuterVarnScope-I" does not work.

Warning: You may see a wrong value. If there is in any other unit a global (or otherwise visible to GDB scoping rules) variable, with the same name as the local var from the outer frame, then that global var will be shown. There is no warning for that. The safe way is to explicitly select the outer stack frame in which the local var is defined, and check the value.

Arrays

pascal
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.

If Watches/Hint values are not displaying correctly disable REGVARS. Set -OoNOREGVAR in "Custom Options".

Specifying GDB disassembly flavor

GDB allows one to specify the assembly flavor (intel or AT&T style) to be used when displaying assembly code. If one wants to change to a specific flavor this can be changed in the AssemblerStyle property in Lazarus's debugger options for GDB (feature available from Lazarus 2.0). Alternatively (and the only way to achieve this from inside Lazarus in older versions before Lazarus 2.0) one can specify this option by passing the set disassembly-flavor instruction to Debugger_Startup_options. Several syntax variations can be used (depending on GDB version), a few variations are shown below:

text
 -ex "set disassembly-flavor intel"
 --eval-command="set disassembly-flavor intel"
 -eval-command="set disassembly-flavor intel"
 --eval-command "set disassembly-flavor intel"

Note that it may be necessary to reset the debugger (Run | Reset Debugger) before GDB will load with the new setting.

Windows

Environment and Unicode / None-Latin based locale

GDB in combination with Lazarus can have issues, if your environment (such as Username, or PATH, or ...) contain none Latin chars.

This is not an issue with any particular GDB version. Rather it depends on how GDB was build. To work correctly under Lazarus GDB needs to have been build using Cygwin.

There are several builds available provided by the Lazarus team. Some of them are build with Cygwin. For 64bit: https://sourceforge.net/projects/lazarus/files/Lazarus%20Windows%2064%20bits/Alternative%20GDB/

For 32bit the builds have not yet been tested. https://sourceforge.net/projects/lazarus/files/Lazarus%20Windows%2032%20bits/Alternative%20GDB/

Console output for GUI application

On Windows, the IDE does not have the "Debugger - Console output" window. This is because console applications open their 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 / Compiler Options / Config and Target / Win32 Gui Application: -WC / -WG)

Debugging applications with Administrative privileges

On Windows Vista+, in Project Options setting the manifest file permissions for the program to "as Administrator" will run the program with Administrator privileges. If your IDE is not running as Administrator, debugging will appear to start ok, the program will show in the tasklist, but its GUI will not show.

So please be aware that you have to match privilege level between the IDE (and gdb) and the applicationt to be debugged.

Win 64 bit

Using 32 bit Lazarus on 64 bit Windows

Alternatively it is possible to debug applications as 32 bit applications (using the 32bit version of gdb). Once successfully debugged:

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

See also http://forum.lazarus.freepascal.org/index.php/topic,13188.0/topicseen.html

Win CE

Debugger does not find any source files

Menu: "Tools" Page: "Debugger"/"Generic" Field: "Additional search path"

Put your the drive letter of your project directory in this field.

Linux

Known Problems

macOS

With the release of macOS 10.14 (Mojave) in 2018, gdb was replaced by lldb', and is no longer supported.

This section is a first approach to collect information about known issues and workarounds. The information provided here depends a lot on feedback from macOS users.

From Lazarus 2.0 onwards the IDE has an LLDB based debugger. The GDB based debugger can also be used, but requires a lot of work building and code-signing the gdb binary and is not supported.

Known Problems (GDB)

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:

It cannot be guaranteed:

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.

TimeOutForEval
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 here:

An alternative solution seems to be to use a newer version of GDB (see below).

Hardware exceptions under macOS

When a hardware exception occurs under macOS, 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 macOS 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 (GDB)

You can install GDB 7.1 (or later) using MacPorts, fink or homebrew.

The following observations have been made:

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, 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

text
 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.

Xcode 5

Since Mavericks 10.9, Xcode 5 no longer installs gdb by default and not globally.

Please see issue 25157 on mantis

Thread "[Lazarus] Help: macOS Problems" on

Frozen UI with GDB

To start a program from the shell you need to run "open <program1>.app", direct access to ./program1 will not give you access to the UI. To get gdb working from inside your IDE you have to specify this path as run parameter. Choose from menu Run > Run parameter and enter the full path at host application. For instance, /User/<yourid>/sources/myproject/project1.app/Content/MacOS/project1

Links

FreeBSD

FreeBSD 11

The system-provided gdb is ancient (version 6.1.1 in FreeBSD 11.4) and does not work well with Lazarus.

You can install a newer gdb from the ports tree, e.g.

bash
cd /usr/ports/devel/gdb
make -DBATCH install clean

The new gdb is located in "/usr/local/bin/gdb".

FreeBSD 12

The system-provided gdb is ancient (version 6.1.1 in FreeBSD 12.3) and it does not work well with Lazarus.

You can install a newer gdb from the ports tree, e.g.

bash
cd /usr/ports/devel/gdb
make -DBATCH install clean

The new gdb is located in "/usr/local/bin/gdb".

It may need the following project option ("Project Options" dialog):

FreeBSD 13

FreeBSD 13.0 has removed the ancient gdb from the base system and replaced it with lldb.

You can still install a recent gdb from the ports tree.

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:

Please read the entire thread: [http://forum.lazarus.freepascal.org/index.php/topic,20756.msg120720 http://forum.lazarus.freepascal.org/index.php/topic,20756.msg120720](/wiki/Http://forum.lazarus.freepascal.org/index.php/topic%2C20756.msg120720_http://forum.lazarus.freepascal.org/index.php/topic%2C20756.msg120720)

Try to run lazarus with

text
 export LANG=en_US.UTF-8
 lazarus

Known Problems / Errors reported by the IDE

Message    

OS

Description

 

During startup program exited normally

 

 

All

In rare cases the IDE displays the message "During startup program exited normally". This message appears when the debugged application is closed. If this happens no breakpoints in the app will have been triggered, and the app will have run as if no debugger was present.

This error can happen for various reasons. There are at least 2 known:

There may be more. The 2nd only happens on repeated runs of the debugger.

The IDE tries to work around those, but it does not always succeed. It is likely that in cases where it currently fails, this must be worked around by user set config (see below).

Even if you can work around, please consider to send a logfile: #Log_info_for_debug_session

Ways to work around: (all on the options / debugger page)

Lazarus 1.2.2 and before
go to the debugger options and in the field "debugger_startup_options" enter:
--eval-command="set auto-solib-add off"

Lazarus 1.2.4 and higher
go to the debugger options and set the field "DisableLoadSymbolsForLibraries" to "True"

This can only be used, if you do not debug within libraries (if you have not written your own lib)

It will add a very slight increase to the time it takes to start the debugger, as gdb must be reloaded each time.

Try any of the values available for "InternalStartBreak" (in the property grid of the options)

SIGFPE

 

 

All

SIGFPE is a floating point unit exception.

Source: http://forum.lazarus.freepascal.org/index.php/topic,21586.0.html 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:

pascal
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.

Error 193 (Error creating process / Exe not found)

 

 

Windows

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

 

 

Windows

Note
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
http://forum.lazarus.freepascal.org/index.php/topic,7065.msg60621.html#msg60621

BitDefender
enable game mode

SigSegV - and continue debugging

 

 

Windows, Mac

http://sourceware.org/bugzilla/show_bug.cgi?id=10004

http://forum.lazarus.freepascal.org/index.php/topic,18121.msg101798.html#msg101798

On Windows Open/Save/File or System Dialog cause gdb to crash

See next entry "gdb.exe has stopped working"

Or download the "alternative GDB" 7.7.1 (Win32) from the Lazarus SourceForge site.

"Step over" steps into function (Win 64)

 

 

Windows 64

Sometimes pressing F8 does not step over a function, but steps into it.

It is possible to continue with F8, and sometimes GDB will step back into the calling function, after 1 or 2 further steps (stepping over the remainder of the function).

In Lazarus 2.0 a workaround was added, but needs to be enabled. Go to Tools > Options > Debugger and enable (in the property grid) "FixIncorrectStepOver".

gdb.exe has stopped working / SigSegv with Open/Close Dlg

 

 

Windows

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).

Lazarus 1.2.2 and before
go to the debugger options and in the field "debugger_startup_options" enter:
--eval-command="set auto-solib-add off"

Lazarus 1.2.4 and higher
go to the debugger options and set the field "DisableLoadSymbolsForLibraries" to "True"

This can only be used, if you do not debug within libraries (if you have not written your own lib)

internal-error: clear_dangling_display_expressions

 

 

Windows, maybe other

At the end of the debug session, the debugger reports:

text
internal-error: clear_dangling_display_expressions: 
Assertion `objfile->pspace == solib->pspace' failed.

This is an error in gdb, and sometimes solved by

Lazarus 1.2.2 and before
go to the debugger options and in the field "debugger_startup_options" enter:
--eval-command="set auto-solib-add off"

Lazarus 1.2.4 and higher
go to the debugger options and set the field "DisableLoadSymbolsForLibraries" to "True"

This can only be used, if you do not debug within libraries (if you have not written your own lib)

"PC register is not available" ("SuspendThread failed. (winerr 5)")

 

 

Windows

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

Cannot insert breakpoint

 

 

All

For breakpoints with negative numbers Please see: http://forum.lazarus.freepascal.org/index.php/topic,10317.msg121818.html#msg121818

You may also want to try:

Lazarus 1.2.2 and before
go to the debugger options and in the field "debugger_startup_options" enter:
--eval-command="set auto-solib-add off"

Lazarus 1.2.4 and higher
go to the debugger options and set the field "DisableLoadSymbolsForLibraries" to "True"

This can only be used, if you do not debug within libraries (if you have not written your own lib)

For breakpoints with positive numbers This may happen due to incorrect Debugger Setup. Ensure smart linking is disabled. It usually means that the breakpoint is in a procedure that is not called, and not included in your exe.

On Windows, if it happens despite correct setup, then add -Xe (external linker) to the custom options.

Reporting Bugs

Check existing Reports

Please check each of the following links

Bugs in GDB

Description

OS

GDB Version(s)

Reference

win64 crash with bad dwarf2 symbols

win64

GDB 7.4 (maybe higher)

http://sourceware.org/bugzilla/show_bug.cgi?id=14014

Debuggee doesn't see environment variables set with set environment

win

Solved in GDB 7.4

http://sourceware.org/bugzilla/show_bug.cgi?id=10989

Win32 fails to continue with "winerr 5" (pc register not available)
apparently not present in 7.0 to 7.2

win

GDB 7.3 and up (maybe 6.x too)
Appears to be fixed in 7.7.1

http://sourceware.org/bugzilla/show_bug.cgi?id=14018

Wrong array index with dwarf

*

GDB 7.3 to 7.5 (maybe 7.6.0)
fixed in 7.6.1

http://sourceware.org/bugzilla/show_bug.cgi?id=15102

GDB crashes, if trying to inspect or watch resourcestrings
only tested on win32, not known for other platforms only, if using dwarf

*

GDB 7.0 to 7.2.xx

Records (especially: pointer to record) may be mistaken for classes/objects
The data/values appear to be displayed correct / further tests pending
This also affect the ability to inspect shortstring
You can workaround this by manually typecasting.

*

GDB 7.6.1 and up (tested and failing up to GDB 8.1)

https://sourceware.org/bugzilla/show_bug.cgi?id=16016

Object variables (Members) of the current methods class (self.xxx) cannot be watched/inspected
To work around use upper case or prefix with self.

*

GDB 7.7 up to 7.9.0 (incl)
Workaround present in Lazarus 1.4 and up

https://sourceware.org/bugzilla/show_bug.cgi?id=17835

gdb cannot continue after SIGFPE or SIGSEGV happen on windows

win

*

https://sourceware.org/bugzilla/show_bug.cgi?id=10004

GDB truncates values for "const x = qword(...)"

Win 64 bit
maybe others

Fixed in GDB 7.7 and up, maybe fibed between 7.3 and 7.7

GDB error for inspecting interfaces (IUnknown etc)

DWARF 3

https://sourceware.org/bugzilla/show_bug.cgi?id=25101

GDB can not display structures (e.g. class) with dyn array inside

DWARF 3

https://sourceware.org/bugzilla/show_bug.cgi?id=25102

GDB crash on ^AnsiString

DWARF 3

https://sourceware.org/bugzilla/show_bug.cgi?id=25103

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 (GDB and LLDB)

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.

Basic Information

Optimization settings (-O???): Level (-O1 / ...) Other (-OR -Ou -Os) (Please always test with all optimizations disabled, and -O1)

Log info for debug session

Start Lazarus with the following options:

text
 --debug-log=LOG_FILE  --debug-enable=DBG_CMD_ECHO,DBG_STATE,DBG_DATA_MONITORS,DBGMI_QUEUE_DEBUG,DBGMI_TYPE_INFO,DBG_ERRORS

:;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 cannot generate a log file, you may try to open the "Debug output" window (from menu "View" -> "Ide Internals").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.

Running the test-case

If you run a different version of GDB you may want to run the test case. Please note, that not all tests have yet the correct expectations for each possible setup. Therefore some tests may fail on some systems.

The debugger test are in the directory:

In the below the 1.3 path is used. If using 1.2.x substitute as appropriate.

One time setup

Create directory components\lazdebuggergdbmi\test\Logs: Optional, but helps keeping test result in one place

Create config files: in components\lazdebuggergdbmi\test\

copy fpclist.txt.sample to fpclist.txt

copy gdblist.txt.sample to gdblist.txt

open the 2 txt files and edit the path to fpc/gdb, also edit the version (the test may run different test, for different versions). It is possible to specify more than one gdb/fpc

Running the test

See also Lazarus Debugger Implementation#Testcases (Backend)

Links

External Links

Experimental Debuggers in Pascal

Those are work in progress:

Related forum topics

From the Free Pascal wiki, page "GDB Debugger Tips", as it stood on 2022-05-28 after a change by Trev, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's Licensing page. If you have concerns about this material, message the admin.

Revision 3, last changed by WikiArchive .