_sbrk exists, linker can't find it

Hello,

I'm trying to compile a program for a stm32f4-discovery board using code 
sourcery g++ lite and I'm having a peculiar problem. My program uses the 
"new operator" and I had to implement the _sbrk function.

Here is the linker command:
arm-none-eabi-g++ -mthumb -mcpu=cortex-m4 -march=armv7e-m 
-mfix-cortex-m3-ldrd -o main.elf -O2  -g -Wall -Werror 
-Wl,-T/home/li/dev/arm_rc/ld/cs3.ld  main.o start_cs3.o newlib_stubs.o

_sbrk is in newlib_stubs.c (.o). Knowing that order of the files on the 
command line counts, I tried moving newlib_stubs.o as first/last 
argument but with no success.

What could be wrong?

Note that the C version of the program that uses malloc is compiling 
just fine (arm-none-eabi-gcc instead of arm-none-eabi-g++)

Thanks,
Liviu