p1-stats

GDB Tutorial

GDB is a command line debugger. It runs primarily on Linux. On Windows 10, you can use GDB via WSL. On macOS, GDB is unreliable, so we recommend using LLDB instead (LLDB Tutorial).

GDB is harder to learn compared to most visual debuggers. However, you might want to use GDB in situations like these:

GDB uses commands. Here’s a handy reference card.

Quick Start

If you’re a first time user, skip this section.

Compile your program with the -g flag and start GDB in text user interface (TUI) mode. EECS 281 students: remove optimization flags like -O3.

$ pwd
/Users/awdeorio/src/eecs280/p1-stats
$ make stats_tests.exe
g++ -Wall -Werror -pedantic -g --std=c++11 stats_tests.cpp stats.cpp p1_library.cpp -o stats_tests.exe
$ gdb -tui stats_tests.exe
b main breakpoint on main function
b 29 breakpoint on line 29 of current file
b stats.cpp:16 breakpoint on line 16 of stats.cpp
r run or rerun
s step into
n step over (AKA next)
refresh refresh TUI view
p myvar print variable myvar
q quit

Prerequisites

At this point, you should already have a folder for your project (instructions). Your folder location might be different. You should have downloaded and unpacked the starter files already (instructions).

$ pwd
/Users/awdeorio/src/eecs280/p1-stats
$ ls
Makefile     main_test.in           p1_library.cpp  stats.h
launch.json  main_test.out.correct  p1_library.h    stats_public_test.cpp
main.cpp     main_test_data.tsv     stats.cpp       stats_tests.cpp

You should have function stubs in stats.h and main.cpp. If you simply want to download a copy with the stubs already in place, use these commands.

$ wget --no-clobber https://eecs280staff.github.io/p1-stats/stats.cpp -O stats.cpp
$ wget --no-clobber https://eecs280staff.github.io/p1-stats/main.cpp -O main.cpp

Install

GDB works best on Linux, including WSL. If you’re running macOS, use LLDB instead. If you’re on CAEN Linux, GDB and GCC are already installed. Your versions might be different.

$ sudo apt-get install g++ gdb
$ g++ --version
g++ (Ubuntu 9.3.0-17ubuntu1~20.04) 9.3.0
$ gdb --version
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2

Compile

GDB debugs a compiled executable. Compile the executable you plan to debug.

$ pwd
/Users/awdeorio/src/eecs280/p1-stats
$ make clean
rm -rvf *.exe *~ *.out *.dSYM *.stackdump
$ make stats_tests.exe
g++ -Wall -Werror -pedantic -g --std=c++11 stats_tests.cpp stats.cpp p1_library.cpp -o stats_tests.exe

PITFALL: GDB debugging will be very hard to understand if there are no debugging symbols. Double check the output of make and verify that you see -g being used in the commands. The EECS 280 defaults include -g.

Optionally run your executable at the command line before debugging. With EECS 280 project 1 starter files and functions stubs in stats_tests.cpp, we get an assertion failure.

$ ./stats_tests.exe
test_sum_small_data_set
stats_tests.exe: stats.cpp:16: double sum(std::vector<double>): Assertion `false' failed.
Aborted (core dumped)

Run

To run inside GDB, prefix your executable name with gdb.

$ gdb stats_tests.exe
...
Reading symbols from stats_tests.exe...
(gdb)

The r command runs the program. You can ignore any error about “raise.c: No such file or directory.”.

(gdb) r
Starting program: /vagrant/src/eecs280/p1-stats-awdeorio/stats_tests.exe
test_sum_small_data_set
stats_tests.exe: stats.cpp:16: double sum(std::vector<double>): Assertion `false' failed.

Program received signal SIGABRT, Aborted.

Use the back trace command (bt) to see the stack at the time of failure. You can ignore all the standard library functions. Just pay attention to functions that you wrote. This is a great way to get a quick look at the root cause of a segfault.

(gdb) bt
#0  ...
#1  ...
#2  ...
#3  ...
#4  0x0000555555559244 in sum (v=std::vector of length 3, capacity 3 = {...})
    at stats.cpp:16
#5  0x0000555555557a5c in test_sum_small_data_set () at stats_tests.cpp:47
#6  0x0000555555557996 in main () at stats_tests.cpp:30

Quit. Pro-tip: Control-D will quit, and a second Control-D will confirm the y or n prompt.

(gdb) q
A debugging session is active.

  Inferior 1 [process 1090] will be killed.

Quit anyway? (y or n) y

Input redirection

Skip this subsection on your first time through the tutorial. You can use input redirection to avoid typing program input each time you run (for debugging) a program.

Without input redirection, you can type input into GDB after running your program.

$ gdb main.exe
...
(gdb) r
Starting program: /vagrant/src/eecs280/p1-stats-awdeorio/main.exe
enter a filename
main_test_data.tsv
enter a column name
B
...

With input redirection, use a run command like this r < main_test.in.

$ gdb main.exe
...
(gdb) r < main_test.in

Arguments and options

Skip this subsection for EECS 280 project 1. You’ll need it for project 2 and beyond.

Arguments and options are inputs to a program typed at the command line. Arguments are often required. Options (AKA flags or switches) start with a hyphen (-), and are typically optional.

Arguments example from project 2: resize.exe is the name of the program, and the arguments are horses.ppm, horses_400x250.ppm, 400, and 250.

$ ./resize.exe horses.ppm horses_400x250.ppm 400 250
$ gdb resize.exe
(gdb) r horses.ppm horses_400x250.ppm 400 250

Options example from project 5: main.exe is the name of the program. train_small.csv and test_small.csv are arguments. --debug is an option.

$ ./main.exe train_small.csv test_small.csv --debug
$ gdb main.exe
(gdb) r train_small.csv test_small.csv --debug

Debug

Start GDB with your executable.

$ gdb stats_tests.exe
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
...
Reading symbols from stats_tests.exe...
(gdb)

Set a breakpoint on the main function (b main) and run the program (r).

(gdb) b main
Breakpoint 1 at 0x3989: file stats_tests.cpp, line 29.
(gdb) r
Starting program: /home/awdeorio/src/eecs280/p1-stats/stats_tests.exe

Breakpoint 1, main () at stats_tests.cpp:29
29  {

Pro-tip: start is a shortcut for b main followed by r.

Pro-tip: Other ways to set breakpoints:

b main breakpoint on main function
b 29 breakpoint on line 29 of current file
b stats.cpp:16 breakpoint on line 16 of stats.cpp

Step over to the next line of code with n, do this twice. Our test fails because we haven’t implemented sum() yet. You can ignore any error about “raise.c: No such file or directory.”.

(gdb) n
30    test_sum_small_data_set();
(gdb) n
test_sum_small_data_set
stats_tests.exe: stats.cpp:16: double sum(std::vector<double>): Assertion `false' failed.

Program received signal SIGABRT, Aborted.

Rerun the program.

(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/awdeorio/src/eecs280/p1-stats/stats_tests.exe

Breakpoint 1, main () at stats_tests.cpp:29
29  {

Step over to the next line of code (n), then step into the function with s.

(gdb) n
30    test_sum_small_data_set();
(gdb) s
test_sum_small_data_set () at stats_tests.cpp:39
39  {

List a few lines of surrounding code with l (that’s a lowercase L).

(gdb) l
34
35    return 0;
36  }
37
38  void test_sum_small_data_set()
39  {
40    cout << "test_sum_small_data_set\n";
41
42    vector<double> data;
43    data.push_back(1);

Step over to the next line of code (n) a couple times until you’re on this line of code. Pro-tip: just hit Enter to repeat your previous command.

(gdb) n
40    cout << "test_sum_small_data_set\n";
(gdb) n
test_sum_small_data_set
42    vector<double> data;
(gdb) n
43    data.push_back(1);
(gdb) n
44    data.push_back(2);
(gdb) n
45    data.push_back(3);
(gdb) n
47    assert(sum(data) == 6);

Print the value of a variable with p. If you have trouble viewing the contents of the vector in the previous step, see the Pretty Printing STL Containers with gdb section.

(gdb) p data
$3 = std::vector of length 3, capacity 4 = {1, 2, 3}

Quit. Pro-tip: Control-D will quit, and a second Control-D will confirm the y or n prompt.

(gdb) q
A debugging session is active.

  Inferior 1 [process 1090] will be killed.

Quit anyway? (y or n) y

Debug with TUI mode

Start GDB in TUI mode with your executable and hit Enter a few times until you see the (gdb) prompt. TUI mode shows your source code at the top with a GDB prompt at the bottom. It uses the ncurses library for text-based user interfaces.

$ gdb -tui stats_tests.exe
┌──stats_tests.cpp─────────────────────────────────────────────────────────────┐
│   29          {                                                              │
│   30            test_sum_small_data_set();                                   │
│   31            test_sum_small_data_set();                                   │
│   32                                                                         │
│   33            // Call your test functions here                             │
│   34                                                                         │
│   35            return 0;                                                    │
│   36          }                                                              │
│   37                                                                         │
│   38          void test_sum_small_data_set()                                 │
│   39          {                                                              │
│   40            cout << "test_sum_small_data_set\n";                         │
│   41                                                                         │
└──────────────────────────────────────────────────────────────────────────────┘
exec No process In:                                                L??   PC: ??
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
--Type <RET> for more, q to quit, c to continue without paging--
Type "apropos word" to search for commands related to "word"...
Reading symbols from stats_tests.exe...
(gdb)

Set a breakpoint on the main function (b main) and run the program (r).

┌──stats_tests.cpp─────────────────────────────────────────────────────────────┐
│B+>29          {                                                              │
│   30            test_sum_small_data_set();                                   │
│   31            test_sum_small_data_set();                                   │
│   32                                                                         │
│   33            // Call your test functions here                             │
│   34                                                                         │
│   35            return 0;                                                    │
│   36          }                                                              │
│   37                                                                         │
│   38          void test_sum_small_data_set()                                 │
│   39          {                                                              │
│   40            cout << "test_sum_small_data_set\n";                         │
│   41                                                                         │
└──────────────────────────────────────────────────────────────────────────────┘
native process 1154 In: main                           L29   PC: 0x555555557989
Reading symbols from stats_tests.exe...
(gdb) b main
Breakpoint 1 at 0x3989: file stats_tests.cpp, line 29.
(gdb) r
Starting program: /vagrant/src/eecs280/p1-stats-awdeorio/stats_tests.exe

Breakpoint 1, main () at stats_tests.cpp:29
(gdb)

Pro-tip: start is a shortcut for b main followed by r.

Pro-tip: Other ways to set breakpoints:

b main breakpoint on main function
b 29 breakpoint on line 29 of current file
b stats.cpp:16 breakpoint on line 16 of stats.cpp

Step over to the next line of code with n and notice the line cursor > advance.

┌──stats_tests.cpp─────────────────────────────────────────────────────────────┐
│B+ 29          {                                                              │
│  >30            test_sum_small_data_set();                                   │
│   31            test_sum_small_data_set();                                   │
│   32                                                                         │
│   33            // Call your test functions here                             │
│   34                                                                         │
│   35            return 0;                                                    │
│   36          }                                                              │
│   37                                                                         │
│   38          void test_sum_small_data_set()                                 │
│   39          {                                                              │
│   40            cout << "test_sum_small_data_set\n";                         │
│   41                                                                         │
└──────────────────────────────────────────────────────────────────────────────┘
native process 1161 In: main                           L30   PC: 0x555555557991
(gdb) b main
Breakpoint 1 at 0x3989: file stats_tests.cpp, line 29.
(gdb) r
Starting program: /vagrant/src/eecs280/p1-stats-awdeorio/stats_tests.exe

Breakpoint 1, main () at stats_tests.cpp:29
(gdb) n
(gdb)

Again, step over to the next line code with n. Our test fails because we haven’t implemented sum() yet. You can ignore any error about “raise.c: No such file or directory.”.

...
stats_tests.exe: stats.cpp:16: double sum(std::vector<double>): Assertion `false' failed.

Program received signal SIGABRT, Aborted.

Refresh the TUI interface and rerun the program. Sometimes, a crash can cause random characters to appear in TUI mode, which is why we use refresh.

(gdb) refresh
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y

At this point, you should be back at the breakpoint on main that you set earlier.

┌──stats_tests.cpp─────────────────────────────────────────────────────────────┐
│B+>29          {                                                              │
│   30            test_sum_small_data_set();                                   │
│   31            test_sum_small_data_set();                                   │
│   32                                                                         │
│   33            // Call your test functions here                             │
│   34                                                                         │
│   35            return 0;                                                    │
│   36          }                                                              │
│   37                                                                         │
│   38          void test_sum_small_data_set()                                 │
│   39          {                                                              │
│   40            cout << "test_sum_small_data_set\n";                         │
│   41                                                                         │
└──────────────────────────────────────────────────────────────────────────────┘
native process 1209 In: main                           L29   PC: 0x555555557989
(gdb) refresh
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /vagrant/src/eecs280/p1-stats-awdeorio/stats_tests.exe

Breakpoint 1, main () at stats_tests.cpp:29
(gdb)

Step over to the next line of code (n), then step into the function with s.

┌──stats_tests.cpp─────────────────────────────────────────────────────────────┐
│B+ 29          {                                                              │
│   30            test_sum_small_data_set();                                   │
│   31            test_sum_small_data_set();                                   │
│   32                                                                         │
│   33            // Call your test functions here                             │
│   34                                                                         │
│   35            return 0;                                                    │
│   36          }                                                              │
│   37                                                                         │
│   38          void test_sum_small_data_set()                                 │
│  >39          {                                                              │
│   40            cout << "test_sum_small_data_set\n";                         │
│   41                                                                         │
└──────────────────────────────────────────────────────────────────────────────┘
native process 1209 In: test_sum_small_data_set        L39   PC: 0x5555555579a2
Start it from the beginning? (y or n) y
Starting program: /vagrant/src/eecs280/p1-stats-awdeorio/stats_tests.exe

Breakpoint 1, main () at stats_tests.cpp:29
(gdb) n
(gdb) s
test_sum_small_data_set () at stats_tests.cpp:39
(gdb)

Step over to the next line of code (n) a couple times until you’re on this line of code. If the user interface ever starts to look messed up, just refresh it. Pro-tip: just hit Enter to repeat your previous command.

┌──stats_tests.cpp─────────────────────────────────────────────────────────────┐
│   42            vector<double> data;                                         │
│   43            data.push_back(1);                                           │
│   44            data.push_back(2);                                           │
│   45            data.push_back(3);                                           │
│   46                                                                         │
│  >47            assert(sum(data) == 6);                                      │
│   48                                                                         │
│   49            cout << "PASS!\n";                                           │
│   50          }                                                              │
│   51                                                                         │
│   52          // Add the test function implementations here.                 │
│                                                                              │
│                                                                              │
└──────────────────────────────────────────────────────────────────────────────┘
native process 1209 In: test_sum_small_data_set        L47   PC: 0x555555557a3d
(gdb) n
(gdb) n
(gdb) n
(gdb) n
(gdb) n
(gdb) n
(gdb) refresh
(gdb)

Print the value of a variable with p. If you have trouble viewing the contents of the vector in the previous step, see the Pretty Printing STL Containers with gdb section.

(gdb) p data
$3 = std::vector of length 3, capacity 4 = {1, 2, 3}

Quit. Pro-tip: Control-D will quit, and a second Control-D will confirm the y or n prompt.

(gdb) q
A debugging session is active.

  Inferior 1 [process 1090] will be killed.

Quit anyway? (y or n) y

Pro-tips

Use the up arrow key to cycle through previously used commands.

Use TAB completion to automatically complete the name of a command or a variable.

Use Emacs keyboard shortcuts to enter and edit your current command.

Next steps

Return to the main set up tutorial.

Pretty-printing STL Containers with gdb

Some installations of gdb don’t ship with pretty printing support for STL containers like vector.

If you have this problem, you’ll see something like this when you try to view the contents of a vector while debugging in GDB or VS Code (some VS Code configurations use GDB under the hood).

Install Subversion and Python. If you’re using Cygwin, install additional packages with the Cygwin installer.

Get the GDB STL pretty printers. These are written in Python.

$ mkdir ~/.gdb.d/
$ svn co svn://gcc.gnu.org/svn/gcc/trunk/libstdc++-v3/python ~/.gdb.d/python

Use this command to create a .gdbinit file in your home directory.

$ echo -e 'python\nimport os\nimport sys\nsys.path.insert(0, os.path.expanduser("~/.gdb.d/python"))\nfrom libstdcxx.v6.printers import register_libstdcxx_printers\nregister_libstdcxx_printers (None)\nend' > ~/.gdbinit

Double check everything:

$ cat ~/.gdbinit
python
import os
import sys
sys.path.insert(0, os.path.expanduser("~/.gdb.d/python"))
from libstdcxx.v6.printers import register_libstdcxx_printers
register_libstdcxx_printers (None)
end
$ ls -R ~/.gdb.d/
/Users/awdeorio/.gdb.d/:
python

/Users/awdeorio/.gdb.d/python:
Makefile.am  Makefile.in  hook.in  libstdcxx

/Users/awdeorio/.gdb.d/python/libstdcxx:
__init__.py  v6

/Users/awdeorio/.gdb.d/python/libstdcxx/v6:
__init__.py  printers.py  xmethods.py

Finally, restart GDB or VS Code and try to check the contents of vector again.