How can I know how the exit() function work? - c

Question

I wrote a small program to find how the exit() function works in Linux.
#include <unistd.h>
int main()
{
exit(0);
}
And then I compiled the program with gcc.
gcc -o example -g -static example.c
In gdb, when I set a breakpoint, I got these lines.
Dump of assembler code for function exit:
0x080495a0 <+0>: sub $0x1c,%esp
0x080495a3 <+3>: mov 0x20(%esp),%eax
0x080495a7 <+7>: movl $0x1,0x8(%esp)
0x080495af <+15>: movl $0x80d602c,0x4(%esp)
0x080495b7 <+23>: mov %eax,(%esp)
0x080495ba <+26>: call 0x80494b0 <__run_exit_handlers>
End of assembler dump.
(gdb) b 0x080495a3
Function "0x080495a3" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (0x080495a3) pending.
(gdb) run
Starting program: /home/jack/Documents/overflow/example
[Inferior 1 (process 2299) exited normally]
The program does not stop at the breakpoint. Why? I use -static to compile the program, why does the breakpoint pend until the library loads into the memory?

Answer 1

You're asking gdb to break on a function called 0x080495a3. You'll need to use b *0x080495a3 instead.
(gdb) help break
Set breakpoint at specified line or function.
break [LOCATION] [thread THREADNUM] [if CONDITION]
LOCATION may be a line number, function name, or "*" and an address.
As the help says, The * tells gdb it's an address you want to break on.
From your example:
Function "0x080495a3" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (0x080495a3) pending.
The "pending" means that the breakpoint is waiting until a function called 0x080495a3 is loaded from a shared library.
You might also be interested in break-range:
(gdb) help break-range
Set a breakpoint for an address range.
break-range START-LOCATION, END-LOCATION
where START-LOCATION and END-LOCATION can be one of the following:
LINENUM, for that line in the current file,
FILE:LINENUM, for that line in that file,
+OFFSET, for that number of lines after the current line
or the start of the range
FUNCTION, for the first line in that function,
FILE:FUNCTION, to distinguish among like-named static functions.
*ADDRESS, for the instruction at that address.
The breakpoint will stop execution of the inferior whenever it executes
an instruction at any address within the [START-LOCATION, END-LOCATION]
range (including START-LOCATION and END-LOCATION).

Answer 2

It looks like that you're trying to set a breakpoint at a function named 0x080495a3. Instead try b *0x080495a3 to indicate to GDB that you want to break at a specific address.

Answer 3

0x080495a3 is an address of the line on which you are willing to apply break point. But the format for gdb is b (function name or line number). So You have 2 ways to do this.
1) do an l after your gdb session has started. It will list you the code in C. And then apply a break point using the line number else
2) if you want to use the address, use b *0x080495a3 way to set a break point.
This way you will be able to halt at line
0x080495a3 <+3>: mov 0x20(%esp),%eax