Using gdb
I figured I'll write a mini-tutorial on how to use gdb, because there's not that many places where they teach you how to use gdb effectively.
Let's say you have a program and it crashes, what do you do?
Example code:
```
void func() { char *p = 0; *p = 0x69; }
int main() { func(); }
```
Next:
```
gdb a.out
```
Followed by the `run` command:
```
(gdb) run
Starting program: /home/d/a.out
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Program received signal SIGSEGV, Segmentation fault.
0x000055555555513d in func () at test.c:1
1 void func() { char *p = 0; *p = 0x69; }
(gdb)
```
You can see where it crashes, but you'd like a stacktrace...
```
gdb) bt
#0 0x000055555555513d in func () at test.c:1
#1 0x0000555555555155 in main () at test.c:3
(gdb)
```
(NB: `bt` stands for backtrace)
You can go up a frame and check local variables:
```
(gdb) up
#1 0x0000555555555155 in main () at test.c:3
3 int main() { func(); }
(gdb) info local
No locals.
```
Or down a frame and check local variables:
```
(gdb) down
#0 0x000055555555513d in func () at test.c:1
1 void func() { char *p = 0; *p = 0x69; }
(gdb) info local
p = 0x0
```
You can continue after the segfault:
```
(gdb) cont
Continuing.
Program terminated with signal SIGSEGV, Segmentation fault.
The program no longer exists.
(gdb)
```
Now we can re-run it, but before we do, we can set a breakpoint:
```
(gdb) break func
Breakpoint 1 at 0x555555555131: file test.c, line 1.
(gdb)
```
Now we run it again, it will stop at the breakpoint:
```
(gdb) run
Starting program: /home/d/a.out
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Breakpoint 1, func () at test.c:1
1 void func() { char *p = 0; *p = 0x69; }
(gdb)
```
We can single step through it:
```
(gdb) step
Program received signal SIGSEGV, Segmentation fault.
0x000055555555513d in func () at test.c:1
1 void func() { char *p = 0; *p = 0x69; }
(gdb)
```
We can also try the command `next`, which is similar to `step` but skips over subroutines.
We can get help from gdb at any time using `help`.
Another useful `info` command is `info reg`, which shows CPU registers.
Also useful is `disas` command, which disassembles the code.
