[avr-libc-dev] .bss section unitialized

[Marian Buschsieweke]

Dear avr-libc developers,

tl;dr: Does anyone know what could be the problem when the .bss section is
uninitialized?

I'm trying to build RIOT [1] using avr-libc 2.0.0 and GCC 6.4.0 for the
Arduino UNO (ATmega328p) and the Arduino Mega2560 (ATmega2560), but I ran into a
bunch of strange bugs. Upon investigation with AVaRICE and an AVR Dragon it
seems that the .bss section (or at least part of it) remains uninitialized, as
at least the variables I investigated on contained random values before they
were first used.

I have to admit that I have trouble to get AVaRICE working reliable. (E.g.
setting breakpoints through gdb doesn't seem to work for me, but inline
assembly using `asm("break");` statements seem to work reliable for me.) So
maybe the problem is completely different and reading random values is caused
by my fragile AVaRICE setup... At least when reading the same variable multiple
times without rebooting the Arduino I consistently get the same value, which
gives me some confidence in the results.

How to reproduce:
- Clone git://github.com/RIOT-OS/RIOT
- cd into examples/default
- Add the following lines to the Makefile after assignment of the
  BOARD_PROVIDES_NETIF variable:

CFLAGS += -std=gnu99 # <-- Only required if asm("break"); is used
BOARD_PROVIDES_NETIF += arduino-mega2560
USEMODULE += w5100
CFLAGS += -DW5100_PARAM_CS=ARDUINO_PIN_10
CFLAGS += -DW5100_PARAM_EVT=ARDUINO_PIN_2

 - Run "make BOARD=arduino-mega2560"
 - Add a breakpoint in file sys/net/gnrc/pktbuf_static/gnrc_pktbuf_static.c in
   line 84 before `mutex_lock(&_mutex);` in function
   `void gnrc_pktbuf_init(void)`.
    - You could insert `asm("break");` into the code (and run
      make BOARD=arduino-mega2560 again)
 - Investigate the contents of _mutex. It contains a random value, even though
   it should contain `NULL`
 - Btw: "make BOARD=arduino-mega2560 flash" runs avrdude for you to flash the
   board

Compiling with other toolchains (other versions of GCC or/and avr-libc) result
in binaries that have similar issues.

Does someone know what the problem could be?

Kind regards,
Marian

[1]: http://riot-os.org/

-------------------------------------------------------------
M.Sc. Marian Buschsieweke
Dept. Communication and Networked Systems (ComSys)
Institute for Intelligent Cooperating Systems (IKS)
Otto-von-Guericke-University of Magdeburg
Universitätsplatz 2, Building 29, Room 314
39106 Magdeburg
Germany

http://www.comsys.ovgu.de/Team/Marian+Buschsieweke.html
Tel.: +49 - 391 - 67 - 52673
Fax:  +49 - 391 - 67 - 41161

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