G ++不会使用-O0编译我的源代码，但是会使用-O2 -DNDEBUG编译我的源代码，我该如何解决呢？

Question

I am writing a firmware for an ARM microcontroller. I have built the cross-compiler without syscalls, actually my RTOS (ChibiOS) provides a simple syscalls implementation.

All my code is written in C excepting some parts where I only use C++ for linking with Eigen library (a C++ template library for linear algebra, it is only headers).

If I compile my source example code with -O2 -DNDEBUG (as far as I know with NDEBUG the code won't need assert()) everything compiles fine and the firmware works.

If I compile my source example code with -O0 I have the following:

Linking build/ch.elf
/home/noether/workspace/tool-chains/arm-none-eabi-4.6.2/lib/gcc/arm-none-eabi/4.6.2    /../../../../arm-none-eabi/lib/thumb/cortex-m4/libc.a(lib_a-abort.o): In function `abort':
/home/noether/workspace/tool-chains/summon-arm-toolchain/build/arm-none-eabi/thumb /cortex-m4/newlib/libc/stdlib/../../../../../../../gcc-4.6.2/newlib/libc/stdlib  /abort.c:63: undefined reference to `_exit'
/home/noether/workspace/tool-chains/arm-none-eabi-4.6.2/lib/gcc/arm-none-eabi/4.6.2/../../../../arm-none-eabi/lib/thumb/cortex-m4/libc.a(lib_a-signalr.o): In function `_kill_r':
/home/noether/workspace/tool-chains/summon-arm-toolchain/build/arm-none-eabi/thumb/cortex-m4/newlib/libc/reent/../../../../../../../gcc-4.6.2/newlib/libc/reent/signalr.c:61: undefined reference to `_kill'
/home/noether/workspace/tool-chains/arm-none-eabi-4.6.2/lib/gcc/arm-none-eabi/4.6.2/../../../../arm-none-eabi/lib/thumb/cortex-m4/libc.a(lib_a-signalr.o): In function `_getpid_r':
/home/noether/workspace/tool-chains/summon-arm-toolchain/build/arm-none-eabi/thumb/cortex-m4/newlib/libc/reent/../../../../../../../gcc-4.6.2/newlib/libc/reent/signalr.c:96: undefined reference to `_getpid'
collect2: ld returned 1 exit status
make: *** [build/ch.elf] Error 1

It does not matter if I put -DNDEBUG, I have the same output. I am using those flags as well, -fno-exceptions and fno-rtti. If I do not use/link the Eigen library (the only C++ stuff), g++ compiles the source fine even with -O0.

Actually, I implemented a simple _kill _getpid and _exit functions, and the code compiles, but the code goes from 13KB to 130KB, and It crashes (maybe I haven't written well those functions).

What I want is to remove this stuff from my code (abort, etc) if I use -O0, as it is done (I guess) with -O2.

Thank you very much. Let me know if you need more information.

Answer 1

These references are almost certainly due to using assert() ; that will call abort() on failure, which in turn will try to raise a signal (using the kill() syscall in this implementation) to abort the process. Obviously, this isn't possible if you can't make syscalls.

Building with -DNDEBUG (like you do in the optimised version) will fix this; it causes the assert() macro to generate no code, so there will be no references to abort() . The optimisation level itself shouldn't make any difference.

Alternatively, if you want to preserve your assertions, you could implement your own assert() macro that doesn't require any syscalls.

Answer 2

You could simply implement stubs for those missing syscalls and link them to your code. That will keep the linker quiet, but you might make the stubs do something sensible or useful. exit() might disable interrupts and loop indefinitely or force a reset. _kill() might hook into your RTOS to terminate a thread, or simply call _exit(), and _getpid() might return some dummy value or an RTOS thread ID.