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GNU Debugger

Resources

Debug Information Generation

Generate DWARF debugging information in an executable?

To debug with gdb, compile with -g. This generates DWARF information, which is used by the debugger to understand what the program is doing.

gcc -g -o program source.c

Use -O to compile with optimizations, but this can adversely affect the debug experience i.e. -O and -g are orthogonal and independent. To get a reasonable level of performance and a good debug experience, use -Og. And to get an even better debug experience, use -g3 rather than just -g.

gcc -g3 -Og -o program source.c

You can use the readelf utility to look at the debug info.

Resources

  • GDB Tips by Greg Law - Tip #1

References

Text User Interface

Start the text user interface in gdb?

TUI (Text User Interface) mode allows you to see the context of what you’re debugging!

gdb -tui ./executable

You can also type layout src to switch to the TUI mode.

layout src

The shortcut ctrl–x a will also start the TUI.

ctrl+l to redraw the screen reset in TUI mode.

Resources

  • GDB Tips by Greg Law - Tip #2

References

Enter SingleKey mode?

Press ctrl-x s in TUI mode to enable SingleKey mode. In this mode you can use r to run, c to continue, s for step, and q to leave SingleKey mode, without pressing enter.

Resources

  • GDB Tips by Greg Law - Tip #65

References

Cycle through assembly and source code windows in GDB TUI?

ctrl-x 2

Resources

  • GDB Tips by Greg Law - Tip #3

References

Switch focus between GDB TUI windows?

Description

(gdb) focus src
(gdb) focus cmd

Resources

  • GDB Tips by Greg Law - Tip #3

References

Command History

Navigate through command history?

Description

  • ctrl-p for previous commands
  • ctrl-n for next commands

Resources

  • GDB Tips by Greg Law - Tip #3

References

Starting Execution

Begin debugging session of an executable?
  • start: starts debugging session by running program line-by-line.
  • run: starts debugging session running program as usual.

Resources

  • Boost.Asio C++ Network Programming - Chapter 7

References

Attaching Process

Start debugging an already running process?

Attaching to a process requires root privileges.

After debugger gets attached to the process, the program will freeze as if it is on a breakpoint.

sudo gdb
(gdb) attach 12345

Resources

References

Detaching Process

Detach debugger from a debugging process?

Description

(gdb) detach

After detaching, the program will continue its normal flow as there was no debugger attached to it at first place.

Resources

References

Source Listing

Show source code in debugging session?

list displays 10 lines of source code. To see how many lines of source code will be displayed enter show listsize. To adjust the lines of source code displayed enter set listsize 20.

Resources

  • Boost.Asio C++ Network Programming - Chapter 7

References

Breakpoints

How many breakpoints does GDB offer?
  1. Hardware watchpoints: watch foo
  2. Software watchpoints: watch $rax
  3. Location watchpoints: watch -l foo
  4. Read-only watchpoint: rwatch foo
  5. Access watchpoints: awatch foo

Resources

  • GDB Tips by Greg Law - Tip #7

References

Get information about breakpoints, watchpoints and catchpoints?

Description

(gdb) info breakpoints

Resources

  • GDB Tips by Greg Law - Tip #13

References

Breakpoint Creation

Set breakpoints on a program?

Description

(gdb) break 50
(gdb) break *main
(gdb) break *main+50
(gdb) break source.cpp:main+50

Resources

  • Boost.Asio C++ Network Programming - Chapter 7

References

Run program with an immediate stop on main function?

Description

Use start to put temporary breakpoint on main and then run the program or starti to stop at the very first instruction of the program.

Resources

  • GDB Tips by Greg Law - Tip #4

References

Use regex to select multiple breakpoints for an operation? 
(gdb) rbreak std::sto[ifd]

Resources

  • GDB Tips by Greg Law - Tip #14

References

Breakpoint Removal

Delete a breakpoint from a program?

Description

(gdb) delete 1

Resources

  • Boost.Asio C++ Network Programming - Chapter 7

References

Breakpoint Conditional

Set condition for a breakpoint? 
(gdb) condition 3 e == 42
(gdb) break func(long) if e == 42

Resources

  • GDB Tips by Greg Law - Tip #15

References

Conditionally break execution only when iteration index has increased to 3?

Description

(gdb) break source.cpp:10 if index == 3

Resources

References

Breakpoint Skipping

Skip over a number of breakpoint hits?

To ignore the next 10000000 times that breakpoint 1 is hit.

(gdb) ignore 1 10000000
(gdb) info break 1

breakpoint already hit 1000 times

(gdb) ignore 1 999
(gdb) run

Resources

  • GDB Tips by Greg Law - Tip #5
  • GDB Tips by Greg Law - Tip #6

References

Breakpoint Availability

Disable an already existing breakpoint? 
(gdb) disable <breakpoint index>

Resources

References

Enable an already disabled breakpoint? 
(gdb) enable <breakpoint index>

Resources

References

Breakpoint Saving

Save breakpoints of a debugging session? 
(gdb) save breakpoints program.gdb

Resources

References

Restore saved breakpoints on a debugging session?

Description

(gdb) source program.gdb

Resources

References

Manually write the content of a file in which breakpoints were saved? 
(gdb) break source.cpp:8
(gdb) break source.cpp:22

Note that lines might have been changed during sessions.

Resources

References

Watchpoint Creation

Watch changes of a variable only when a criteria is met?

Description

(gdb) watch foo[90].number if bar[90].number == 255

Resources

  • GDB Tips by Greg Law - Tip #7

References

Track a particular location in memory rather than the value of an expression? 
(gdb) watch -l expression

Convinience variables also work nicely with location watchpoints:

(gdb) print * __errno_location()
$2 = 0
(gdb) watch -l $2

Resources

  • GDB Tips by Greg Law - Tip #9
  • GDB Tips by Greg Law - Tip #10

References

Execution Flow

Step through program execution in debugging session?

Description

  • continue: Will resume the execution of the program until it completes.
  • step: Executes program one more step. Step might be one line of source code or one machine instruction.
  • next: Executes program similar to step, but it only continues to the next line in the current stack frame and will not step into functions.

Resources

  • Boost.Asio C++ Network Programming - Chapter 7

References

Memory Examination

Examine a memory address or register?

Display the memory contents at a given address using x [address] short for examine. You can customise the output with an optional format and length argument.

(gdb) x/[length][format] [address]
(gdb) x/2x 0x0ffffffff7c544f0
(gdb) x/2x $rsp

Resources

  • GDB Tips by Greg Law - Tip #63

References

Thread Tracking

Get information of threads?

Description

To get information about the spawned threads:

(gdb) info threads

This will show thread ID, how the OS sees them, and the name the thread was given by the program.

Stepping through a thread that releases other threads, gets all threads out of sync. This can be changed with set scheduler-locking on, meaning only the current thread will run. Beware of deadlock through, the thread you're running might block on a lock held by another not running thread.

A middle ground can be found with set scheduler-locking step, which has scheduler-locking on when using step, but disables it when you use continue, allowing you to travel between breakpoints without having to constantly switch between the threads.

Resources

  • GDB Tips by Greg Law - Tip #64

References

Backtrace

List the innermost or outermost frames of backtrace?

Description

If you want to examine the backtrace of multiple frames but you're very deep in the stack, you can use backtrace n to show the n innermost frames. Similarly, you can use stacktrace -n to show the n outermost frames.

(gdb) bt full
(gdb) bt 1
(gdb) bt -1

Resources

  • GDB Tips by Greg Law - Tip #59

References

Core Dump Control

Where the coredumps are stored? 
coredumpctl list

Resources

References

Store the core dump of your program into a file?

Description

coredumpctl dump $PWD/program --output program.core

Resources

References

Load the stored coredump into debugging session?

Description

(gdb) target core program.core

Resources

References

Select a core section to inspect the cause of termination?

When frame is selected from a core dump, program will be set to the corresponding state.

(gdb) frame 0
(gdb) p variable_causing_trouble

Resources

References

Core Dump Generation

Create a core dump from the state of an executable in debugging session? 
(gdb) gcore program.core

Resources

References

Memory Inspection

Print the value of an object? 
(gdb) print bag_size

Resources

  • Boost.Asio C++ Network Programming - Chapter 7

References

Inspect the type of a variable? 
(gdb) whatis random_number

Resources

  • Boost.Asio C++ Network Programming - Chapter 7

References

Memory Modification

Modify the value of a variable? 
(gdb) set var random_number = 5

Resources

  • Boost.Asio C++ Network Programming - Chapter 7

References

Shell Command Execution

Run shell commands in debugging session?

Description

(gdb) shell pwd

Resources

  • Boost.Asio C++ Network Programming - Chapter 7

References

Debugger Configuration

Automatically run commands on GDB startup?

If you have commands you want to run every time you debug, or if you want to definne a function to pretty print a specific data structure, you can put them in .gdbinit file on the top project directory, or in your home directory.

.gdbinit

set logging enabled on

You may need to enable auto-loading for the directory by adding the following line in ~/.config/gdb/gdbinit file.

set auto-load-safe-path /path/to/dir

You can also enable it for all directories with set auto-load-safe-path /.

Resources

  • GDB Tips by Greg Law - Tip #62

References