--- title: Bare Metal ARM - Compiler flag analysis lang: en redirect_from: - /arm-bare-1 - /arm-gcc-1 description: "Trying out different GCC compiler flags for embedded / microcontroller programming, specifically for ARM (arm-none-eabi-gcc)." --- So... I'm doing software development for microcontrollers at work with a commercial toolchain. I also evaluated the arm-none-eabi-gcc toolchain in the past (briefly). One thing that continues to confuse me is the different compiler and linker options used in various online examples, tutorials and library Makefiles. What I especially wanted to know is: What happens before `main()` with each combination of options? Do the variables get initialized (i.e. does the `.data` section get copied from flash to RAM) and is the `.bss` section properly zeroed? *TL;DR: See last paragraph* Some libraries, e.g. the [libopencm3 project][locm], as well as the [ARM CMSIS][cmsis] contain explicit startup code that do these tasks. This post uses the latest version of the [GNU embedded toolchain for ARM][1]. At the time of writing, that's 8‑2019‑q3‑update. This post will take a look at: - using `-nostartfiles` vs. not using it - using nano.specs vs. nosys.specs vs. not using any specs file I start with a simple `main()` function that should suffice to analyze what I want. This example *won't* run on an actual Cortex-M3 microcontroller, since it lacks the vector table. I might (read: probably not ;)) look into this in a later post. I'm gonna use two different variants, one `main()` function that exits/returns and one that doesn't. Note the latter is the usual case when programming microcontrollers. For sake of simplicity, lets do it like this: ~~~ c static int zerobss; static int globalinit = 42; int main() { int localvar = 21; #ifdef DOEXIT return 0; #else while(1); #endif } ~~~ To test the effect of various options, I wrote the following bash script which loops over all combinations: ~~~ bash #!/bin/bash # make.sh export PATH=/opt/gcc-arm-none-eabi-8-2019-q3-update/bin/:$PATH COMMONFLAGS="-mcpu=cortex-m3 -mthumb " FILES=main.c STARTOPT=("" -nostartfiles) SPECS=("" "-specs=nosys.specs" "-specs=nano.specs") EXIT=("" "-DDOEXIT") rm -f *.elf for f in $FILES; do for sf in "${STARTOPT[@]}"; do for sp in "${SPECS[@]}"; do for ex in "${EXIT[@]}"; do # bash substitutions, remove leading -specs= and trailing .specs specname=${sp:7} specname=${specname%.specs} outfile=`basename $f .c`${sf}-${specname}-${ex:2}.elf arm-none-eabi-gcc $ex $COMMONFLAGS $sf $sp -o $outfile $f 2>/dev/null && echo "Compiling $outfile succeeded" || echo "Compiling $outfile failed" done done done done arm-none-eabi-size *.elf ~~~ I silenced the error output, otherwise it gets too noisy. First, the following combinations fail: ~~~ Compiling main--.elf failed Compiling main--DOEXIT.elf failed Compiling main-nano-.elf failed Compiling main-nano-DOEXIT.elf failed ~~~ In these cases, linking fails because (shortened output) ~~~ x/arm-none-eabi/bin/ld: x/arm-none-eabi/lib/thumb/v7-m/nofp/libc.a(lib_a-exit.o): in function `exit': exit.c:(.text.exit+0x16): undefined reference to `_exit' x/arm-none-eabi/bin/ld: x/arm-none-eabi/lib/thumb/v7-m/nofp/libc_nano.a(lib_a-exit.o): in function `exit': exit.c:(.text.exit+0x1a): undefined reference to `_exit' ~~~ That is, when neither `-nostartfiles` nor `-specs=nosys.specs` is specified, the function `_exit` gets referenced without being defined. In every other case, the linker emits a warning, since the entry symbol is not defined: ~~~ cannot find entry symbol _start; defaulting to 0000000000008000 ~~~ So, let's look what we've got (sorted by the arm-none-eabi-size output): ~~~ text data bss dec hex filename 12 4 4 20 14 main-nostartfiles--.elf 12 4 4 20 14 main-nostartfiles-nano-.elf 12 4 4 20 14 main-nostartfiles-nosys-.elf 22 4 4 30 1e main-nostartfiles--DOEXIT.elf 22 4 4 30 1e main-nostartfiles-nano-DOEXIT.elf 22 4 4 30 1e main-nostartfiles-nosys-DOEXIT.elf 932 1096 68 2096 830 main-nosys-.elf 944 1096 68 2108 83c main-nosys-DOEXIT.elf ~~~ Looks interesting. Just using `nosys.specs` seems to add a lot of stuff. Let's look at the disassembly of the return vs. loop code. Since the specs file used makes no difference, use the first file of each group: ~~~ $ arm-none-eabi-objdump -S -d main-nostartfiles--.elf main-nostartfiles--DOEXIT.elf main-nostartfiles--.elf: file format elf32-littlearm Disassembly of section .text: 00008000
: static int zerobss; static int globalinit = 42; int main() { 8000: b480 push {r7} 8002: b083 sub sp, #12 8004: af00 add r7, sp, #0 int localvar = 21; 8006: 2315 movs r3, #21 8008: 607b str r3, [r7, #4] #ifdef DOEXIT return 0; #else while(1); 800a: e7fe b.n 800a main-nostartfiles--DOEXIT.elf: file format elf32-littlearm Disassembly of section .text: 00008000
: static int zerobss; static int globalinit = 42; int main() { 8000: b480 push {r7} 8002: b083 sub sp, #12 8004: af00 add r7, sp, #0 int localvar = 21; 8006: 2315 movs r3, #21 8008: 607b str r3, [r7, #4] #ifdef DOEXIT return 0; 800a: 2300 movs r3, #0 #else while(1); #endif } 800c: 4618 mov r0, r3 800e: 370c adds r7, #12 8010: 46bd mov sp, r7 8012: bc80 pop {r7} 8014: 4770 bx lr ~~~ No real surprises here. However: There is no code initializing the `.bss` section, not copying the `.data` section from flash memory to RAM (as we already could've guessed from looking at the code size). In these cases, writing the code manually would indeed be required. So, what does `nosys.specs` add? Let's just look at the symbol table of the looping example: ~~~ $ arm-none-eabi-objdump -t main-nosys-.elf main-nosys-.elf: file format elf32-littlearm SYMBOL TABLE: 00008000 l d .init 00000000 .init 0000800c l d .text 00000000 .text 00008388 l d .fini 00000000 .fini 00008394 l d .rodata 00000000 .rodata 00008398 l d .ARM.exidx 00000000 .ARM.exidx 000083a0 l d .eh_frame 00000000 .eh_frame 000183a4 l d .init_array 00000000 .init_array 000183ac l d .fini_array 00000000 .fini_array 000183b0 l d .data 00000000 .data 000187ec l d .bss 00000000 .bss 00000000 l d .comment 00000000 .comment 00000000 l d .debug_aranges 00000000 .debug_aranges 00000000 l d .debug_info 00000000 .debug_info 00000000 l d .debug_abbrev 00000000 .debug_abbrev 00000000 l d .debug_line 00000000 .debug_line 00000000 l d .debug_frame 00000000 .debug_frame 00000000 l d .debug_str 00000000 .debug_str 00000000 l d .ARM.attributes 00000000 .ARM.attributes 00000000 l df *ABS* 00000000 /opt/gcc-arm-none-eabi-8-2019-q3-update/bin/../lib/gcc/arm-none-eabi/8.3.1/thumb/v7-m/nofp/crti.o 00000000 l df *ABS* 00000000 /opt/gcc-arm-none-eabi-8-2019-q3-update/bin/../lib/gcc/arm-none-eabi/8.3.1/thumb/v7-m/nofp/crtn.o 00000000 l df *ABS* 00000000 exit.c 00000000 l df *ABS* 00000000 __call_atexit.c 0000802c l F .text 00000014 register_fini 00000000 l df *ABS* 00000000 crtstuff.c 000083a0 l O .eh_frame 00000000 00008040 l F .text 00000000 __do_global_dtors_aux 000187ec l .bss 00000001 completed.8885 000183ac l O .fini_array 00000000 __do_global_dtors_aux_fini_array_entry 00008064 l F .text 00000000 frame_dummy 000187f0 l .bss 00000018 object.8890 000183a8 l O .init_array 00000000 __frame_dummy_init_array_entry 00000000 l df *ABS* 00000000 /opt/gcc-arm-none-eabi-8-2019-q3-update/bin/../lib/gcc/arm-none-eabi/8.3.1/../../../../arm-none-eabi/lib/thumb/v7-m/nofp/crt0.o 00000000 l df *ABS* 00000000 main.c 00018808 l .bss 00000004 zerobss 000183b4 l O .data 00000004 globalinit 00000000 l df *ABS* 00000000 impure.c 000183c0 l O .data 00000428 impure_data 00000000 l df *ABS* 00000000 init.c 00000000 l df *ABS* 00000000 memset.c 00000000 l df *ABS* 00000000 atexit.c 00000000 l df *ABS* 00000000 fini.c 00000000 l df *ABS* 00000000 lock.c 00000000 l df *ABS* 00000000 __atexit.c 00000000 l df *ABS* 00000000 _exit.c 00000000 l df *ABS* 00000000 crtstuff.c 000083a0 l O .eh_frame 00000000 __FRAME_END__ 00000000 l df *ABS* 00000000 000183b0 l .fini_array 00000000 __fini_array_end 000183ac l .fini_array 00000000 __fini_array_start 000183ac l .init_array 00000000 __init_array_end 000183a4 l .init_array 00000000 __preinit_array_end 000183a4 l .init_array 00000000 __init_array_start 000183a4 l .init_array 00000000 __preinit_array_start 0001880c g O .bss 00000001 __lock___atexit_recursive_mutex 00018810 g O .bss 00000001 __lock___arc4random_mutex 000187e8 g O .data 00000004 __atexit_recursive_mutex 000082e0 g F .text 00000002 __retarget_lock_close 00018830 g .bss 00000000 _bss_end__ 000187ec g .bss 00000000 __bss_start__ 000183b0 g O .data 00000000 .hidden __dso_handle 00018814 g O .bss 00000001 __lock___env_recursive_mutex 00018818 g O .bss 00000001 __lock___sinit_recursive_mutex 00008394 g O .rodata 00000004 _global_impure_ptr 00008100 g F .text 00000048 __libc_init_array 00008080 g F .text 00000000 _mainCRTStartup 00008000 g F .init 00000000 _init 000082a4 g F .text 00000034 __libc_fini_array 0001881c g O .bss 00000001 __lock___malloc_recursive_mutex 000082fc g F .text 00000002 __retarget_lock_release_recursive 000082f4 g F .text 00000004 __retarget_lock_try_acquire_recursive 00018830 g .bss 00000000 __bss_end__ 000081e8 g F .text 000000b0 __call_exitprocs 00008080 g F .text 00000000 _start 000082f0 g F .text 00000004 __retarget_lock_try_acquire 00008300 g F .text 00000084 __register_exitproc 000082e4 g F .text 00000002 __retarget_lock_close_recursive 000082ec g F .text 00000002 __retarget_lock_acquire_recursive 000187ec g .bss 00000000 __bss_start 00008148 g F .text 000000a0 memset 000080f4 g F .text 0000000c main 000082dc g F .text 00000002 __retarget_lock_init_recursive 00018830 g .bss 00000000 __end__ 000082d8 g F .text 00000002 __retarget_lock_init 00008388 g F .fini 00000000 _fini 00008298 g F .text 0000000c atexit 000183b8 g O .data 00000004 _impure_ptr 000187ec g .data 00000000 _edata 00018830 g .bss 00000000 _end 00018820 g O .bss 00000001 __lock___at_quick_exit_mutex 0000800c g F .text 00000020 exit 000082e8 g F .text 00000002 __retarget_lock_acquire 000082f8 g F .text 00000002 __retarget_lock_release 00008384 g F .text 00000002 _exit 00018824 g O .bss 00000001 __lock___dd_hash_mutex 00018828 g O .bss 00000001 __lock___tz_mutex 00080000 g .comment 00000000 _stack 000183b0 g .data 00000000 __data_start 0001882c g O .bss 00000001 __lock___sfp_recursive_mutex ~~~ In the list, we see `_mainCRTStartup`. This is the code which is responsible for the tasks we want to be performed. We also see the `_exit` function defined (which gets referenced by the startup code). The disassembly doesn't really surprise, it's a simple infinite loop: ~~~ $ arm-none-eabi-objdump --disassemble=_exit main-nosys-.elf main-nosys-.elf: file format elf32-littlearm Disassembly of section .init: Disassembly of section .text: 00008384 <_exit>: 8384: e7fe b.n 8384 <_exit> Disassembly of section .fini: ~~~ I'll spare you with further disassembly output, since it's pretty long. ## Summary So, a quick recap: * *not* using `-nostartfiles` means the linker fails because `_exit` is not defined (except when using `nosys.specs` which contains a definition for that function). While this might sound contradicting at first, this is because the function is referenced by the startup code. You could simply define the function yourself, if it's the same as with the commercial Keil compiler, the signature should be `void _exit(int)`. Hint: Look for "retargeting". * When using `nosys.specs`, you can omit the `-nostartfiles` option, meaning `.bss` gets initialized and data copied to ROM. Note that the program will still not run on a microcontroller, like mentioned above, because you need the vector table for this. This will also require a custom linker script, which in turn means this linker script will have to define the symbols assumed by the startup code properly. Hint: using `-Wl,--verbose` with gcc enables verbose linker output, which also shows the default linker script. Some [StackOverflow answers][so1] also reference the option `-ffreestanding`. I found this makes no difference with my example, however. In short: Yes, you need to write the startup code that initializes `.bss` and copies `.data` from flash to RAM manually when doing bare-metal programming. *Update 2019-07-13*: Adding links Also, supplying `-nostdlib` has the same effect as `-nostartfiles` *for this particular example*. ## Links Regarding this topic, please also read "[From zero to main(): Bare metal C][zeromain]" on interrupt.memfault.com ([archive.org link][zeromainarch] in case website goes down). Another source that really made things "click" for me is "[Bare-metal C programming on ARM][bmarm]". Although the book handles the Cortex-A series, the section about startup code is really worth a read. The book is also [available as PDF][bmarmpdf] (again, [archive.org link][bmarch]). [1]: https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-rm/downloads [locm]: https://github.com/libopencm3/libopencm3 [cmsis]: https://github.com/ARM-software/CMSIS_5 [so1]: https://stackoverflow.com/a/51657692 [zeromain]: https://interrupt.memfault.com/blog/zero-to-main-1 [zeromainarch]: https://web.archive.org/web/20190515111558/https://interrupt.memfault.com/blog/zero-to-main-1 [bmarm]: https://github.com/umanovskis/baremetal-arm [bmarmpdf]: http://umanovskis.se/files/arm-baremetal-ebook.pdf [bmarch]: https://web.archive.org/web/20190713091239/http://umanovskis.se/files/arm-baremetal-ebook.pdf