Re: Why does gdb stop at a different line than “i b” shows while returning from function?

[wempwer@gmail.com]

On Fri, Aug 03, 2012 at 11:15:42AM +0200, Joachim Protze wrote:
> The behaviour is similar with setting a breakpoint at empty line 7 ("b
> 7") or function header line 13 ("b 13"): "i b" shows try5.c:7 resp.
> try5.c:13, but the program stops at line 8 resp. 15.
> 

I would understand this because there are no instructions at line 7 and 13 so breakpoints have no effect on these lines and are automatically moved to the nearest next instruction. Is my understanding correct? 

However, my example is a bit different. I moved from 'display' to 'main' frame and set a breakpoint. I don't understand why gdb put the breakpoint on line 8 instead of 9, because this is the actual next instruction that's going to be executed in 'frame 1' and in fact when you resume the execution it stops at line 9 in 'main'. I think the breakpoint should be set at line 9 or if it must be set at line 8 it should not be triggered at line 9. There are no empty instructions such as '{' anywhere along the way in this situation.


> The thing gets even fancier, when you rerun the program after setting
> the breakpoints: Now "i b" lists your breakpoint at line 9, while my
> breakpoints remain at line 7 resp. 13.
> 
> - Joachim
> 

That's right, I didn't notice that before. Maybe it's a bug? I include the full log:

Reading symbols from /home/ja/gdb/learning/try5...done.
(gdb) b 7
Breakpoint 1 at 0x4004fb: file try5.c, line 7.
(gdb) b 13
Breakpoint 2 at 0x40051e: file try5.c, line 13.
(gdb) b display 
Note: breakpoint 2 also set at pc 0x40051e.
Breakpoint 3 at 0x40051e: file try5.c, line 15.
(gdb) r
Starting program: /home/ja/gdb/learning/try5 

Breakpoint 1, main () at try5.c:8
(gdb) c
Continuing.

Breakpoint 2, display (x=3) at try5.c:15
(gdb) f 1
#1  0x000000000040050c in main () at try5.c:8
(gdb) b
Breakpoint 4 at 0x40050c: file try5.c, line 8.
(gdb) c
Continuing.
i is 0.
i is 1.
i is 2.

Breakpoint 4, main () at try5.c:9
(gdb) c
Continuing.
[Inferior 1 (process 13038) exited normally]
(gdb) i b
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x00000000004004fb in main at try5.c:7
	breakpoint already hit 1 time
2       breakpoint     keep y   0x000000000040051e in display at try5.c:13
	breakpoint already hit 1 time
3       breakpoint     keep y   0x000000000040051e in display at try5.c:15
	breakpoint already hit 1 time
4       breakpoint     keep y   0x000000000040050c in main at try5.c:8
	breakpoint already hit 1 time
(gdb) r
Starting program: /home/ja/gdb/learning/try5 

Breakpoint 1, main () at try5.c:8
(gdb) i b
Num     Type           Disp Enb Address            What
1       breakpoint     keep y   0x00000000004004fb in main at try5.c:7
	breakpoint already hit 1 time
2       breakpoint     keep y   0x000000000040051e in display at try5.c:13
3       breakpoint     keep y   0x000000000040051e in display at try5.c:15
4       breakpoint     keep y   0x000000000040050c in main at try5.c:9
(gdb)