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ICS-PA0: GDB for Beginners

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Recording experiments about use GDB to debug a core file.

code of file test.c

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int actual_calc(int a, int b){
  int c;
  c=a/b;
  return 0;
}

int calc(){
  int a;
  int b;
  a=13;
  b=0;
  actual_calc(a, b);
  return 0;
}

int main(){
  calc();
  return 0;
}

Executing our script and configuring core dumps

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$ gcc -ggdb test.c -o test.out
$ ./test.out
Floating point exception (core dumped)

-ggdb option to gcc will ensure that the debugging session using GDB; it adds GDB specific debugging information to the test.out binary.

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$ man core
CORE(5)              Linux Programmer's Manual             CORE(5)

NAME
       core - core dump file
DESCRIPTION
		`core dumped file` contains an image of the process's memory at the time of termination. This image  can  be  used  in  a  debugger  (e.g., gdb(1))  to  inspect  the  state of the program at the time that it terminated.

Now we know what is “core dumped”. Thus, let’s see core file:

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$ ls
core.1813.1000.8.test.out.1729779410  intOverflow.c  test.out
intOverflow                           test.c
$
$ file core.1813.1000.8.test.out.1729779410
core.1813.1000.8.test.out.1729779410: ELF 64-bit LSB core file, x86-64, version 1 (SYSV), SVR4-style, from './test.out', real uid: 1000, effective uid: 1000, real gid: 1000, effective gid: 1000, execfn: './test.out', platform: 'x86_64'

We can see that this is 64-bit core file, user id is 1000, platform is “x86_64”. Notice: 8 in core filename that it was a signal 8 which terminate the program. Sigal 8 is SIGFPE, means Floating Point Exception.

Using GDB

So try to use GDB to analyze this core dump file

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$ gdb ./test.out ./core.1813.1000.8.test.out.1729779410

Then it displayed as following:
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GNU gdb (Ubuntu 12.1-0ubuntu1~22.04.2) 12.1
Copyright (C) 2022 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<https://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from ./test.out...
[New LWP 1813]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Core was generated by `./test.out'.
Program terminated with signal SIGFPE, Arithmetic exception.
#0  0x000056423afe413b in actual_calc (a=13, b=0) at test.c:3
3               c = a / b;
(gdb)

This is the first time that I try to use gdb to analyze a file. Following the output, it shows SIGFPE is an arithmetic exception. Besides, there is the frame called #0 which is the procedure terminated. This frame displays memory address, function (use procedure in tutorial) name, parameter value, and the line number in “test.c”.

Then we can see what is the line of code (line 3), c=a/b, but “b=0”, it is the reason of exception.

Backtracing

And try backtracing, (command bt), it shows as:

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(gdb) 
#0  0x000056423afe413b in actual_calc (a=13, b=0) at test.c:3
#1  0x000056423afe4171 in calc () at test.c:12
#2  0x000056423afe418a in main () at test.c:17

It is the reverse order of things that happen: frame #0 (the first frame) is the last function which was being executed by the program when it crashed, and frame #2 was the very first frame called when the program was started.

Power user tip: if you use multiple threads, you can use the command thread apply all bt to obtain a backtrace for all threads which were running as the program crashed!

Frame inspection

Inspect each frame, the matching source code (if it is available), and each variable step by step:

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(gdb) f 2
#2  0x000056423afe418a in main () at test.c:17
17              calc();
  • f 2 means ‘jump into’ frame 2
  • list to list the source code (Only useful when you have listing source code permission)
  • p a means print the value of variable “a”

alt

Conclusion

Finish this tutorial, I learned some usage of GDB. It is not too difficult to learn, but my English writing skill is still poor.

Have a nice day!
By Le4f,
2024.10.25