Given the following code sippet:
1: int main(int argc, char **argv) {
2: int i;
3:
4: i = i + 1;
5:
6: ...
7:
8: }
When you set a breakpoint on line 2: in the above code and start debugging in GDB the execution actually stops on line 4:. This is the correct behavior of course BUT is it possible to query GDB for the correct stop point number before debugging... right after the creation of the breakpoint.
(gdb) b 2
Breakpoint 1 at 0x400547: file main.cpp, line 2.
This is what info b shows:
(gdb) info b $bpnum
Num Type Disp Enb Address What
1 breakpoint keep y 0x0000000000400547 in main(int, char**) at main.cpp:2
and you need to do this for address in the column "Address":
(gdb) info line *0x0000000000400547
Line 4 of "main.cpp" starts at address 0x400547 <main(int, char**)+11> and ends at 0x400550 <main(int, char**)+20>.
As you can see the real breakpoint on "Line 4 of main.cpp"
Probably you are looking for
info b
(which is short for info breakpoints) and gives the number in the first "column".
If you want to break when the variable gets modified, use a watch point instead. GDB will always break at the nearest executable code line after the breakpoint (if set by line). A variable declaration isn't executable code and therefore the assignment on the next (non-empty) line will be used.
If what you are looking for is to know at exactly which line it stops, I think there is no such thing on the GDB prompt. However, you can still use a watch point and customize what happens on your break and watch points using commands.
From the comment skwllsp wrote, here's how it would look:
(gdb) info b
Num Type Disp Enb Address What
1 breakpoint keep y 0x0000000000400547 in main(int, char**) at main.cpp:2
Which says there is one breakpoint (1), which is enabled and it shows you the line, too. Disposition (Disp) and enabled (Enb) are useful, but please read the manual for a proper description. I can also warmly recommend the book "Art of Debugging" from Nostarch.
