Wrong line numbers from addr2line

I try to find the exact line of a call in the backtrace in C++ program. Right now I am using these lines (from the man page of backtrace) to get the trace:

  void *bt_buffer[1000];
  char **bt_strings;
  int bt_nptrs = backtrace(bt_buffer, 1000);
  bt_strings = backtrace_symbols(bt_buffer, bt_nptrs);

In bt_strings I find lines of the form

./prog() [0x402e42]

Now I take the address (the hex string) and feed it to addr2line. That sometimes results in apparently wrong line numbers. Internet search led me to this post, in which it is shown that

readelf -wl ./prog

indicates where the line really is, or rather how many lines the symbol has moved to the current line.

edit: This happens when I compile with -g -O0, i.e. explicetly without optimisations. The compiler is gcc 4.6.3 Is there another compiler flag that I miss?

My problem is the following: I need to automate this. I need my program to create a backtrace (done), extract the file (done) and the line number (fail).

I could of course call readelf and parse the output, but that's not really suitable, as the output differs from symbol to symbol depending on what exactly happened. Sometimes The address of a symbol is in one line and the information about the line offset in the next line...

To sum up:

Is there an elegant way to get the exact line number of a function call in the backtrace from within the program during runtime?

edit: example code:

#define UNW_LOCAL_ONLY
#include <libunwind.h>
#include <execinfo.h>
#include <iostream>
#include <stdlib.h>

void show_backtrace()
{
  // get current address
  void* p = __builtin_return_addr