Generating Ubuntu Core File

Recently I've been trying to enable core dumps on Ubuntu. I made a simple program, that would crash (it writes some data out of array).

At first, core dumps weren't generated. This was because ulimit was set to zero, so I run command: ulimit -c unlimited. This didn't help, because most of the core dumps land to apport. I also disabled this by an another simple command: echo "core" > /proc/sys/kernel/core_pattern.

As a result, the core is generated, but it stills cannot be read by gdb. Here is the output of gdb:

root@melon:/home/melon/test# gdb ./a.out ./core.3506 GNU gdb (GDB) 7.5-ubuntu Copyright (C) 2012 Free Software Foundation, Inc.License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>This is free software: you are free to change and redistribute it.There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details.This GDB was configured as "x86_64-linux-gnu".For bug reporting instructions, please see:<http://www.gnu.org/software/gdb/bugs/>...Reading symbols from /home/melon/test/a.out...done.[New LWP 3506] warning: Can't read pathname for load map: Input/output error.Core was generated by `./a.out'.Program terminated with signal 11, Segmentation fault.#0 0x00007fa2dc0f976d in ?? ()(gdb) bt #0 0x00007fa2dc0f976d in ?? ()#1 0x0000000000000000 in ?? ()

And here is the output of file command:

core.3506: ELF 64-bit LSB core file x86-64, version 1 (SYSV), SVR4-style, from './a.out'

And the beginning of ulimit -a:

core file size (blocks, -c) unlimited

And echo of more important /proc/sys/kernel output:

cat /proc/sys/kernel/core_pattern core cat /proc/sys/kernel/core_uses_pid 1 cat /proc/sys/kernel/core_pipe_limit 0

My question is what I am doing wrong? How can I generated a core file, that will be read by gdb?