Back to TILs

C++ debugging with GDB

Date: 2023-02-07Last modified: 2024-11-02

C++debug gdb gdb-break-condition gdb-info gdb-scripting gdb-time-travel gdb-tui gui-ddd gui-nemiver rr

Table of contents

Compilation flags
Breakpoint
Running
Breakpoint condition
Watchpoint
Time travel (rr record and replay)
Multiprocess debugging
Disassembly
TUI enable
Displaying informations
Finishing
Example code
Scripting
Possible output
References

Compilation flags

Use -ggdb or -g3 to include all information available to gdb.

By using -g only a limited amount of information is added to the executable.

See the vídeo Debugging with Macros (-g3,-ggdb) for good explanation about the debug level flags.

Breakpoint

[b]reak function_name
[b]reak filename:line
[b]reak *memory address

Running

Use the command run to start the program execution.

$ gdb bin/gdb_test_01

$ gdb -tui bin/gdb_test_01

Create breakpoint at function my_func_01:

(gdb) break my_func_01(int, float) 
Breakpoint 1 at 0x3a74

Execute the program with breakpoint

(gdb) run
Starting program: /data/home/geraldo/git/Intmain/language_cpp/bin/gdb_test_01 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7fffd889d700 (LWP 194516)]
[New Thread 0x7fffd809c700 (LWP 194517)]
[New Thread 0x7fffc789b700 (LWP 194518)]
[New Thread 0x7fffc709a700 (LWP 194519)]
[New Thread 0x7fffb6899700 (LWP 194520)]
[New Thread 0x7fffae098700 (LWP 194521)]
[New Thread 0x7fffa5897700 (LWP 194522)]

Thread 1 "gdb_test_01" hit Breakpoint 1, 0x0000555555557a74 in my_func_01(int, float) ()

Continue execution after breakpoint

(gdb) cont
Continuing.
s = 10 + 3.141492 = 13.141492
[Thread 0x7fffa5897700 (LWP 194522) exited]
[Thread 0x7fffae098700 (LWP 194521) exited]
[Thread 0x7fffb6899700 (LWP 194520) exited]
[Thread 0x7fffc709a700 (LWP 194519) exited]
[Thread 0x7fffc789b700 (LWP 194518) exited]
[Thread 0x7fffd809c700 (LWP 194517) exited]
[Thread 0x7fffd889d700 (LWP 194516) exited]
[Inferior 1 (process 194512) exited normally]
(gdb)

Breakpoint condition

[condition] <breakpoint #> <condition>

Updates the breakpoint indicated by the given number so that execution of the program stops at that point only if condition is true. condition is expressed in C syntax, and can only use variables and functions that are available in the scope of the breakpoint location.

Create the breakpoint #1 at line 112

(gdb) list
104	}
105
106	int my_func_02(int x) {
107	  return x + 42;
108	}
109
110	void loop() {
111	  for( int i=0; i<10; i++) {
112	    fmt::print("i = {}\n", i);
113	  }
(gdb) break 112
Breakpoint 1 at 0x3d92: file gdb_test_01.cpp, line 112.

Update the breakpoint #1 condition

(gdb) condition 1 i == 7

Run the program, and the breakpoint only will be actioned for i == 7

(gdb) run
Starting program:
/data/home/geraldo/git/Intmain/language_cpp/bin/gdb_test_01
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7fffd889c700 (LWP 490606)]
[New Thread 0x7fffd809b700 (LWP 490607)]
[New Thread 0x7fffcf89a700 (LWP 490608)]
[New Thread 0x7fffbf099700 (LWP 490609)]
[New Thread 0x7fffb6898700 (LWP 490610)]
[New Thread 0x7fffae097700 (LWP 490611)]
[New Thread 0x7fffa5896700 (LWP 490612)]
s = 10 + 3.141492 = 13.141492
i = 0
i = 1
i = 2
i = 3
i = 4
i = 5
i = 6

Thread 1 "gdb_test_01" hit Breakpoint 1, loop () at gdb_test_01.cpp:112
112	    fmt::print("i = {}\n", i);

Check backtrace

(gdb) backtrace 
#0  loop() () at gdb_test_01.cpp:112
#1  0x0000555555557edc in main(int, char**) (argc=1, argv=0x7fffffffe3b8) at gdb_test_01.cpp:121

Continue the execution as well

(gdb) continue
Continuing.
i = 7
i = 8
i = 9
[Thread 0x7fffa5896700 (LWP 490612) exited]
[Thread 0x7fffae097700 (LWP 490611) exited]
[Thread 0x7fffb6898700 (LWP 490610) exited]
[Thread 0x7fffbf099700 (LWP 490609) exited]
[Thread 0x7fffcf89a700 (LWP 490608) exited]
[Thread 0x7fffd809b700 (LWP 490607) exited]
[Thread 0x7fffd889c700 (LWP 490606) exited]
[Inferior 1 (process 490602) exited normally]
(gdb)

Another option to condition is ignoring an amount of hits on an specific breakpoint. To ignore 1000 hits on breakpont #2 you can use:

ignore 2 1000

Watchpoint

Watchpoints - more than watch and continue

Time travel (rr record and replay)

$ sudo apt install rr

$ sudo rr record -n ./prog args

$ sudo rr replay   # last
$ sudo rr replay /path/to/file

Time travel debugging with rr debugger

Multiprocess debugging

Multiprocess debugging in GDB

Disassembly

If you do not have access to the source code of a function and wish to set a breakpoint on a particular instruction, call disassemble function_name (where function_name is the name of the procedure); this command will allow you to see the memory address of each instruction.

For the following code:

int my_func_02(int x) {
  return x + 42;
}

We can get something like:

(gdb) disassemble my_func_02(int)
Dump of assembler code for function _Z10my_func_02i:
   0x0000000000003d60 <+0>:	push   %rbp
   0x0000000000003d61 <+1>:	mov    %rsp,%rbp
   0x0000000000003d64 <+4>:	mov    %edi,-0x4(%rbp)
   0x0000000000003d67 <+7>:	mov    -0x4(%rbp),%eax
   0x0000000000003d6a <+10>:	add    $0x2a,%eax         ; 0x2a = 42
   0x0000000000003d6d <+13>:	pop    %rbp
   0x0000000000003d6e <+14>:	ret
End of assembler dump.

TUI enable

Type tui enable on gdb prompt
-tui on command line

Displaying informations

(gdb) help p
print, inspect, p
Print value of expression EXP.
Usage: print [[OPTION]... --] [/FMT] [EXP]

FMT:

o(octal)
x(hex)
d(decimal)
u(unsigned decimal)
t(binary)
f(float)
a(address)
i(instruction)
c(char)
s(string)
z(hex, zero padded on the left)

Print in binary format

(gdb) p /t 1024
$1 = 10000000000
(gdb) p /t 7
$2 = 111

Print in hex format

(gdb) p /x 1024
$1 = 0x400
(gdb) p /x 7
$2 = 0x7

Finishing

kill stop debugging session
quit stop gdb

Example code

void my_func_01( int a, float pi )
{
  auto s = fmt::format( "{} + {} = {}", a, pi, a + pi );
  fmt::print( "s = {}\n", s );
}

int my_func_02( int x )
{
  return x + 42;
}

void loop()
{
  for( int i = 0; i < 10; i++ ) {
    fmt::print( "i = {}\n", i );
  }
}

int main( [[maybe_unused]] int argc, [[maybe_unused]] char **argv )
{
  int   a  = 10;
  float pi = 3.141492;
  my_func_01( a, pi );
  loop();
  return 0;
}

Scripting

By default during the startup, GDB executes the file .gdbinit.
Use --command=<filename> to specify a script

set pagination off
set logging file gdb.output
set logging on

set $var = 0 # yes, you can declare variables ...

break function1 if param1 == 32
    command 1
    print param2
    print param3->member1
    continue
end

break file.c:142 if x > 4
    command 2
    print y
    call checker_function
    continue
end

break function2 if $var++ < 3
    command 3
    print $var
    backtrace full
    continue
end

run

set logging off
quit

Possible output

s = 10 + 3.141492 = 13.141492
i = 0
i = 1
i = 2
i = 3
i = 4
i = 5
i = 6
i = 7
i = 8
i = 9