C-gdb-step-by-step-runtime-debug

Step 1 : Create a file test.c with programming mistakes

#include <stdio.h>
#include <stdlib.h>

void fun_2()
{
	int a[5] = {1,2,3,4,5};
	int *p = a;

	a[5] = 400;
	a[6] = 400;
	*p = 100;
}

void fun_1()
{
	int a[5] = {1,2,3,4,5};
	int *p;

	p += 3;

	*p = 100;

	p += 3;

	*p = 200;
}

int main()
{
  char *p;

  // Allocation #1 of 30 bytes
  p = (char *) malloc(30);
  free(p);

  // Allocation #2 of 12 bytes
  p = (char *) malloc(8);
  free(p);

  // Allocation #3 of 16 bytes
  p = (char *) malloc(25);
  free(p);

  fun_1();
  fun_2();

  return 0;
}

Step 2 : Compile program and create executable
$ gcc -g test.c -o test $ ./test Segmentation fault (core dumped)

Step 3 : Segmentation fault is seen in above test. Let us run gdb

$ gdb test 
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
Copyright (C) 2020 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:
<http://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...
(gdb) r
Starting program: /home/test/Desktop/training/valgrind/test 

Program received signal SIGSEGV, Segmentation fault.
0x0000555555555245 in fun_1 () at test.c:21
21		*p = 100;
(gdb) q
A debugging session is active.

	Inferior 1 [process 112408] will be killed.

Quit anyway? (y or n) y

Step 4 : SIGSEGV is seen at 0x0000555555555245 in fun_1( ) at test.c Line number 21

Step 5 : Add break points to do step by step debugging using gdb

$ gdb test 
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04) 9.2
Copyright (C) 2020 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:
<http://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...
(gdb) b main
Breakpoint 1 at 0x1271: file test.c, line 29.
(gdb) b fun_1
Breakpoint 2 at 0x11fe: file test.c, line 15.
(gdb) list
15	{
16		int a[5] = {1,2,3,4,5};
17		int *p;
18	
19		p += 3;
20	
21		*p = 100;
22	
23		p += 3;
24	
(gdb) list main
24	
25		*p = 200;
26	}
27	
28	int main()
29	{
30	  char *p;
31	
32	  // Allocation #1 of 30 bytes
33	  p = (char *) malloc(30);
(gdb) b 30
Breakpoint 3 at 0x127d: file test.c, line 33.
(gdb) 
Note: breakpoint 3 also set at pc 0x127d.
Breakpoint 4 at 0x127d: file test.c, line 33.
(gdb) r
Starting program: /home/test/Desktop/training/valgrind/test 

Breakpoint 1, main () at test.c:29
29	{
(gdb) s

Breakpoint 3, main () at test.c:33
33	  p = (char *) malloc(30);
(gdb) s

Breakpoint 2, fun_1 () at test.c:15
15	{
(gdb) s
16		int a[5] = {1,2,3,4,5};
(gdb) s
19		p += 3;
(gdb) s
21		*p = 100;
(gdb) s

Program received signal SIGSEGV, Segmentation fault.
0x0000555555555245 in fun_1 () at test.c:21
21		*p = 100;
(gdb) s

Program terminated with signal SIGSEGV, Segmentation fault.
The program no longer exists.
(gdb) q



(gdb) r
Starting program: /home/test/gdb

Program received signal SIGSEGV, Segmentation fault.
0x0000555555555245 in fun_1 () at prg1.c:21
21		*p = 100;
(gdb) 
(gdb) run
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/test/gdb

Program received signal SIGSEGV, Segmentation fault.
0x0000555555555245 in fun_1 () at prg1.c:21
21		*p = 100;
(gdb)
(gdb) help
List of classes of commands:

aliases -- Aliases of other commands.
breakpoints -- Making program stop at certain points.
data -- Examining data.
files -- Specifying and examining files.
internals -- Maintenance commands.
obscure -- Obscure features.
running -- Running the program.
stack -- Examining the stack.
status -- Status inquiries.
support -- Support facilities.
tracepoints -- Tracing of program execution without stopping the program.
user-defined -- User-defined commands.

Type "help" followed by a class name for a list of commands in that class.
Type "help all" for the list of all commands.
Reading symbols from test...
(gdb) 
(gdb) q
test@test:~/gdb$
(gdb) list
15	{
16		int a[5] = {1,2,3,4,5};
17		int *p;
18	
19		p += 3;
20	
21		*p = 100;
22	
23		p += 3;
24	
(gdb)
(gdb) set listsize unlimited
(gdb) list
1	#include <stdio.h>
2	#include <stdlib.h>
3	
4	void fun_2()
5	{
6		int a[5] = {1,2,3,4,5};
7		int *p = a;
8	
9		a[5] = 400;
10		a[6] = 400;
11		*p = 100;
12	}
13	
14	void fun_1()
15	{
16		int a[5] = {1,2,3,4,5};
17		int *p;
18	
19		p += 3;
20	
21		*p = 100;
22	
23		p += 3;
24	
25		*p = 200;
26	}
27	
28	int main()
29	{
30	  char *p;
31	
32	  // Allocation #1 of 30 bytes
33	  p = (char *) malloc(30);
34	  free(p);
(gdb) b main
Note: breakpoint 5 also set at pc 0x555555555271.
Breakpoint 6 at 0x555555555271: file prg1.c, line 29.
(gdb) list
24	
25		*p = 200;
26	}
27	
28	int main()
29	{
30	  char *p;
31	
32	  // Allocation #1 of 30 bytes
33	  p = (char *) malloc(30);
(gdb) b 29
Note: breakpoints 5 and 6 also set at pc 0x555555555271.
Breakpoint 7 at 0x555555555271: file prg1.c, line 29.
(gdb) b 31
Breakpoint 8 at 0x55555555527d: file prg1.c, line 33.
(gdb) b 32
Note: breakpoint 8 also set at pc 0x55555555527d.
Breakpoint 9 at 0x55555555527d: file prg1.c, line 33.
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/test/gdb 

Breakpoint 5, main () at prg1.c:29
29	{
(gdb) c
Continuing.

Breakpoint 8, main () at prg1.c:33
33	  p = (char *) malloc(30);
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x0000555555555245 in fun_1 () at prg1.c:21
21		*p = 100;
(gdb) 
Breakpoint 7 at 0x555555555271: file prg1.c, line 29.
(gdb) b 31
Breakpoint 8 at 0x55555555527d: file prg1.c, line 33.
(gdb) b 32
Note: breakpoint 8 also set at pc 0x55555555527d.
Breakpoint 9 at 0x55555555527d: file prg1.c, line 33.
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/test/gdb 

Breakpoint 5, main () at prg1.c:29
29	{
(gdb) c
Continuing.

Breakpoint 8, main () at prg1.c:33
33	  p = (char *) malloc(30);
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x0000555555555245 in fun_1 () at prg1.c:21
21		*p = 100;
(gdb) 
(gdb) n
33	  p = (char *) malloc(30);
(gdb) n
34	  free(p);
(gdb) n
37	  p = (char *) malloc(8);
(gdb) n 3
42	  free(p);
(gdb) 
(gdb) b 43
Breakpoint 1 at 0x12cb: file prg1.c, line 44.
(gdb) r
Starting program: /home/test/gdb 
Breakpoint 1, main () at prg1.c:44
44	  fun_1();
(gdb) s
fun_1 () at prg1.c:15
15	{
(gdb) r
The program being debugged has been started already.
Start it from the beginning? (y or n) y
Starting program: /home/test/gdb

Breakpoint 1, main () at prg1.c:44
44	  fun_1();
(gdb) n

Program received signal SIGSEGV, Segmentation fault.
0x0000555555555245 in fun_1 () at prg1.c:21
21		*p = 100;
(gdb) 
(gdb) b main
Breakpoint 1 at 0x1271: file prg1.c, line 29.
(gdb) until 43
The program is not being run.
(gdb) r
Starting program: /home/test/gdb

Breakpoint 1, main () at prg1.c:29
29	{
(gdb) until 43
main () at prg1.c:44
44	  fun_1();
(gdb) 
(gdb) print p
$3 = (int *) 0x5555555552fc <__libc_csu_init+12>
(gdb) print &p
$4 = (int **) 0x7fffffffddf8
(gdb) print *p
$5 = 1230389504
(gdb) p/d p
$6 = 93824992236284
(gdb) p/u p
$7 = 93824992236284
(gdb) p/t p
$8 = 10101010101010101010101010101010101001011111100
(gdb) p/c p
$9 = 252 '\374'
(gdb) 
(gdb) info args
No arguments.
(gdb) info b
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x0000555555555271 in main 
                                                   at prg1.c:29
	breakpoint already hit 1 time
(gdb) w p
Cannot use this setting with the "with" command
(gdb) watch p
Hardware watchpoint 2: p
(gdb) info w
Ambiguous info command "w": warranty, watchpoints, win.
(gdb) info watch
Num     Type           Disp Enb Address            What
2       hw watchpoint  keep y                      p
(gdb) info frame
Stack level 0, frame at 0x7fffffffde50:
 rip = 0x5555555552cb in main (prg1.c:44); saved rip = 0x7ffff7de50b3
 source language c.
 Arglist at 0x7fffffffde28, args: 
 Locals at 0x7fffffffde28, Previous frame's sp is 0x7fffffffde50
 Saved registers:
  rbp at 0x7fffffffde40, rip at 0x7fffffffde48
(gdb) info local
p = 0x5555555592a0 ""
(gdb) 
(gdb) b main
Breakpoint 1 at 0x1271: file prg1.c, line 29.
(gdb) b 10
Breakpoint 2 at 0x11d6: file prg1.c, line 10.
(gdb) 
(gdb) watch p
Hardware watchpoint 2: p
(gdb)
(gdb) clear
Deleted breakpoints 1 3 No breakpoint at this line.
(gdb) 
(gdb) delete
Delete all breakpoints? (y or n) y
(gdb) 
(gdb) info b
No breakpoints or watchpoints.
(gdb) b main
Breakpoint 5 at 0x555555555271: file prg1.c, line 29.
(gdb) disable 5
(gdb) info b
Num     Type           Disp Enb Address            What
5       breakpoint     keep n   0x0000555555555271 in main at prg1.c:29
(gdb) enable 5
(gdb) info b
Num     Type           Disp Enb Address            What
5       breakpoint     keep y   0x0000555555555271 in main at prg1.c:29
(gdb) 
(gdb) b main
Breakpoint 1 at 0x1271: file prg1.c, line 29.
(gdb) r
Starting program: /home/test/gdb

Breakpoint 1, main () at prg1.c:29
(gdb) kill
Kill the program being debugged? (y or n) y
[Inferior 1 (process 59946) killed]
(gdb) 


(gdb) bt
#0  fun_1 () at prg1.c:15
#1  0x00005555555552d5 in main () at prg1.c:44
(gdb) 
(gdb) up
#1  0x00005555555552d5 in main () at prg1.c:44
44	  fun_1();
(gdb) down
#0  fun_1 () at prg1.c:16
16		int a[5] = {1,2,3,4,5};
(gdb) up
#1  0x00005555555552d5 in main () at prg1.c:44
44	  fun_1();
(gdb) down
#0  fun_1 () at prg1.c:16
16		int a[5] = {1,2,3,4,5};
(gdb) 
(gdb) finish
Run till exit from #0  fun_1 () at prg1.c:15

Program received signal SIGSEGV, Segmentation fault.
0x0000555555555245 in fun_1 () at prg1.c:21
21		*p = 100;
(gdb) 
(gdb) s 
fun_1 () at prg1.c:15
15	{
(gdb) return
Make fun_1 return now? (y or n) y
#0  main () at prg1.c:45
45	  fun_2();
(gdb)

GDB Cheat sheet
Sl.No	Command	Description	Example
1	run r	This command used to run or restart the program in gdb	(gdb) r Starting program: /home/test/gdb Program received signal SIGSEGV, Segmentation fault. 0x0000555555555245 in fun_1 () at prg1.c:21 21 p = 100; (gdb) (gdb) run The program being debugged has been started already. Start it from the beginning? (y or n) y Starting program: /home/test/gdb Program received signal SIGSEGV, Segmentation fault. 0x0000555555555245 in fun_1 () at prg1.c:21 21 p = 100; (gdb)
2	help help <class name> help <command>	Provide the details regarding all the classes of commands	(gdb) help List of classes of commands: aliases -- Aliases of other commands. breakpoints -- Making program stop at certain points. data -- Examining data. files -- Specifying and examining files. internals -- Maintenance commands. obscure -- Obscure features. running -- Running the program. stack -- Examining the stack. status -- Status inquiries. support -- Support facilities. tracepoints -- Tracing of program execution without stopping the program. user-defined -- User-defined commands. Type "help" followed by a class name for a list of commands in that class. Type "help all" for the list of all commands.
3	quit q	To quit from GDB	Reading symbols from test... (gdb) (gdb) q test@test:~/gdb$
4	list line_num list function list location list first,last	To print the program, by default it will print 10 lines	(gdb) list 15 { 16 int a[5] = {1,2,3,4,5}; 17 int p; 18 19 p += 3; 20 21 p = 100; 22 23 p += 3; 24 (gdb)
5	set listsize count set listsize unlimited	Set attribute of a command. See set listsize for example.	(gdb) set listsize unlimited (gdb) list 1 #include <stdio.h> 2 #include <stdlib.h> 3 4 void fun_2() 5 { 6 int a[5] = {1,2,3,4,5}; 7 int p = a; 8 9 a[5] = 400; 10 a[6] = 400; 11 p = 100; 12 } 13 14 void fun_1() 15 { 16 int a[5] = {1,2,3,4,5}; 17 int p; 18 19 p += 3; 20 21 p = 100; 22 23 p += 3; 24 25 p = 200; 26 } 27 28 int main() 29 { 30 char p; 31 32 // Allocation #1 of 30 bytes 33 p = (char *) malloc(30); 34 free(p);
6	continue c continue number	Continue till next breakpoint hits	(gdb) b main Note: breakpoint 5 also set at pc 0x555555555271. Breakpoint 6 at 0x555555555271: file prg1.c, line 29. (gdb) list 24 25 p = 200; 26 } 27 28 int main() 29 { 30 char p; 31 32 // Allocation #1 of 30 bytes 33 p = (char ) malloc(30); (gdb) b 29 Note: breakpoints 5 and 6 also set at pc 0x555555555271. Breakpoint 7 at 0x555555555271: file prg1.c, line 29. (gdb) b 31 Breakpoint 8 at 0x55555555527d: file prg1.c, line 33. (gdb) b 32 Note: breakpoint 8 also set at pc 0x55555555527d. Breakpoint 9 at 0x55555555527d: file prg1.c, line 33. (gdb) r The program being debugged has been started already. Start it from the beginning? (y or n) y Starting program: /home/test/gdb Breakpoint 5, main () at prg1.c:29 29 { (gdb) c Continuing. Breakpoint 8, main () at prg1.c:33 33 p = (char ) malloc(30); (gdb) c Continuing. Program received signal SIGSEGV, Segmentation fault. 0x0000555555555245 in fun_1 () at prg1.c:21 21 p = 100; (gdb) Breakpoint 7 at 0x555555555271: file prg1.c, line 29. (gdb) b 31 Breakpoint 8 at 0x55555555527d: file prg1.c, line 33. (gdb) b 32 Note: breakpoint 8 also set at pc 0x55555555527d. Breakpoint 9 at 0x55555555527d: file prg1.c, line 33. (gdb) r The program being debugged has been started already. Start it from the beginning? (y or n) y Starting program: /home/test/gdb Breakpoint 5, main () at prg1.c:29 29 { (gdb) c Continuing. Breakpoint 8, main () at prg1.c:33 33 p = (char ) malloc(30); (gdb) c Continuing. Program received signal SIGSEGV, Segmentation fault. 0x0000555555555245 in fun_1 () at prg1.c:21 21 *p = 100; (gdb)
7	next n next number	Execute next line of code	(gdb) n 33 p = (char ) malloc(30); (gdb) n 34 free(p); (gdb) n 37 p = (char ) malloc(8); (gdb) n 3 42 free(p); (gdb)
8	step s step number	Execute next line of code also it steps into the function	(gdb) b 43 Breakpoint 1 at 0x12cb: file prg1.c, line 44. (gdb) r Starting program: /home/test/gdb Breakpoint 1, main () at prg1.c:44 44 fun_1(); (gdb) s fun_1 () at prg1.c:15 15 { (gdb) r The program being debugged has been started already. Start it from the beginning? (y or n) y Starting program: /home/test/gdb Breakpoint 1, main () at prg1.c:44 44 fun_1(); (gdb) n Program received signal SIGSEGV, Segmentation fault. 0x0000555555555245 in fun_1 () at prg1.c:21 21 *p = 100; (gdb)
9	until untli line_number	until always stops your program if it attempts to exit the current stack frame	(gdb) b main Breakpoint 1 at 0x1271: file prg1.c, line 29. (gdb) until 43 The program is not being run. (gdb) r Starting program: /home/test/gdb Breakpoint 1, main () at prg1.c:29 29 { (gdb) until 43 main () at prg1.c:44 44 fun_1(); (gdb)
10	print variable-name p variable-name p file-name::var-name p ‘file-name’::var-name p *array-var@length p/x variable p/d variable p/u variable p/t variable p/c variable	This command is used to print the value in the object	(gdb) print p $3 = (int ) 0x5555555552fc <__libc_csu_init+12> (gdb) print &p $4 = (int ) 0x7fffffffddf8 (gdb) print p $5 = 1230389504 (gdb) p/d p $6 = 93824992236284 (gdb) p/u p $7 = 93824992236284 (gdb) p/t p $8 = 10101010101010101010101010101010101001011111100 (gdb) p/c p $9 = 252 '\374' (gdb)
11	info args info locals info catch info frame info line info line number info signals info handle info args info breakpoints info break info break number info watchpoints info registers info threads	Provide the info about cmd args, break point, watch point, local variables.,	(gdb) info args No arguments. (gdb) info b Num Type Disp Enb Address What 1 breakpoint keep y 0x0000555555555271 in main at prg1.c:29 breakpoint already hit 1 time (gdb) w p Cannot use this setting with the "with" command (gdb) watch p Hardware watchpoint 2: p (gdb) info w Ambiguous info command "w": warranty, watchpoints, win. (gdb) info watch Num Type Disp Enb Address What 2 hw watchpoint keep y p (gdb) info frame Stack level 0, frame at 0x7fffffffde50: rip = 0x5555555552cb in main (prg1.c:44); saved rip = 0x7ffff7de50b3 source language c. Arglist at 0x7fffffffde28, args: Locals at 0x7fffffffde28, Previous frame's sp is 0x7fffffffde50 Saved registers: rbp at 0x7fffffffde40, rip at 0x7fffffffde48 (gdb) info local p = 0x5555555592a0 "" (gdb)
12	break main break 10 break func if a == 10 break filename:function break filename:line-num	Setting breakpoint	(gdb) b main Breakpoint 1 at 0x1271: file prg1.c, line 29. (gdb) b 10 Breakpoint 2 at 0x11d6: file prg1.c, line 10. (gdb)
13	watch p	Setting watchpoints	(gdb) watch p Hardware watchpoint 2: p (gdb)
14	clear clear function clear line-number	Clearing breakpoints	(gdb) clear Deleted breakpoints 1 3 No breakpoint at this line. (gdb)
15	delete delete breakpoint-num delete range	Deleting breakpoint watchpoints	(gdb) delete Delete all breakpoints? (y or n) y (gdb)
16	disable breakpoint enable breakpoint	Disable enable break points	(gdb) info b No breakpoints or watchpoints. (gdb) b main Breakpoint 5 at 0x555555555271: file prg1.c, line 29. (gdb) disable 5 (gdb) info b Num Type Disp Enb Address What 5 breakpoint keep n 0x0000555555555271 in main at prg1.c:29 (gdb) enable 5 (gdb) info b Num Type Disp Enb Address What 5 breakpoint keep y 0x0000555555555271 in main at prg1.c:29 (gdb)
17	kill	Used to kill the current running program	(gdb) b main Breakpoint 1 at 0x1271: file prg1.c, line 29. (gdb) r Starting program: /home/test/gdb Breakpoint 1, main () at prg1.c:29 (gdb) kill Kill the program being debugged? (y or n) y [Inferior 1 (process 59946) killed] (gdb)
18	tui enable tui disable	To enable or disable text user interface
19	layout next layout prev layout src layout asm layout split layout regs	Switch b/w different layout of TUI
20	backtrace bt	Print the trace of which function the control in.	(gdb) bt #0 fun_1 () at prg1.c:15 #1 0x00005555555552d5 in main () at prg1.c:44 (gdb)
21	up down up number down number	Move up and down b/w the stacks frames	(gdb) up #1 0x00005555555552d5 in main () at prg1.c:44 44 fun_1(); (gdb) down #0 fun_1 () at prg1.c:16 16 int a[5] = {1,2,3,4,5}; (gdb) up #1 0x00005555555552d5 in main () at prg1.c:44 44 fun_1(); (gdb) down #0 fun_1 () at prg1.c:16 16 int a[5] = {1,2,3,4,5}; (gdb)
22	finish f	Runs until the current function is finished	(gdb) finish Run till exit from #0 fun_1 () at prg1.c:15 Program received signal SIGSEGV, Segmentation fault. 0x0000555555555245 in fun_1 () at prg1.c:21 21 *p = 100; (gdb)
23	return	Return to the previous stack frame	(gdb) s fun_1 () at prg1.c:15 15 { (gdb) return Make fun_1 return now? (y or n) y #0 main () at prg1.c:45 45 fun_2(); (gdb)