g++4.8 hides variables from gdb - c++

Question

After recently upgrading to g++ 4.8.1 I've found that debugging is completely impossible in gdb. g++ seems to hide all variables from gdb, regardless of optimization options. In the following session, runner.cpp is as follows:
#include <vector>
using namespace std;
int main(void) {
vector<int> arr;
int a = 3;
int b = 2;
b = a + 3;
arr.push_back(1);
arr.push_back(2);
arr.push_back(3);
arr.push_back(4);
return 0;
}
This is the result:
Script started on Tue 14 Jul 2015 01:11:14 PM PDT
me#ministation:~/Development/clib$ g++ -g -O0 runner.cpp
me#ministation:~/Development/clib$ gdb -q ./a.out
Reading symbols from /home/me/Development/clib/a.out...done.
(gdb) break 11
Breakpoint 1 at 0x40095c: file runner.cpp, line 11.
(gdb) run
Starting program: /home/me/Development/clib/a.out
warning: no loadable sections found in added symbol-file system-supplied DSO at 0x7ffff7ffa000
Breakpoint 1, main () at runner.cpp:11
11 arr.push_back(1);
(gdb) print a
$1 = {i = {0, 1045149306}, d = 1.2904777690891933e-08} ## I have no idea what this means
(gdb) print b
$2 = {i = {0, 1068498944}, d = 0.0625}
(gdb) print arr
No symbol "arr" in current context.
(gdb) info locals
No locals.
(gdb) next
12 arr.push_back(2);
(gdb)
13 arr.push_back(3);
(gdb) print arr
No symbol "arr" in current context.
(gdb) next
14 arr.push_back(4);
(gdb)
16 return 0;
(gdb) print arr
No symbol "arr" in current context.
(gdb) q
A debugging session is active.
Inferior 1 [process 6392] will be killed.
Quit anyway? (y or n) y
me#ministation:~/Development/clib$
Script done on Tue 14 Jul 2015 01:12:05 PM PDT
I've seen somewhat similar posts where the -O0 flag was recommended, but it doesn't seem to work here. The exact same session after compiling with g++4.6 produces the expected results. Any ideas on what's causing this with g++4.8?

Answer 1

The particular problem here is that the meaning of the DW_AT_high_pc tag in the debug info has been extended to also mean an offset.
Originally, a function ranging from 0x804dd8e to 0x804ddae was encoded as
DW_AT_low_pc : 0x804dd8e
DW_AT_high_pc : 0x804ddae
Now it also can be encoded as
DW_AT_low_pc : 0x804dd8e
DW_AT_high_pc : 0x20
which saves a bit of space in the debug info.
Older versions of GDB only recognize the first version, and interpret the second in a way that there can't be any variables in this range.
Possible solutions are to compile with -gdwarf-2 or to upgrade GDB.

Answer 2

gdb and gcc need to be compatible versions when it comes to debug format (and they are generally quite well compatible over many versions, but SOMETIMES compatibility is broken from one version of a compiler to another).
I have had exactly the same problem. The solution is either to throw some flag at gcc to compile with "older style debug format", or to upgrade the version of gdb to one that "works" with the debug format given by the newer version of gcc.
It would be NICE if gdb actually said something like "Your debug format is X, you need debug format Y or Z for gdb to work correctly", but I'm not 100% sure that can be reliably determined (for example when a bug is found in the generation that is fixed, and need the corresponding fix in the debugger, because the same bug appears in both the "produce" and "consume" parts of the debug-symbol handler - so an older, buggy version can't read the newer unbuggy format and vice versa)