I am trying to write a simple "Hello, World!" firmware for Cortex-M0 CPU. The goal is to correctly initialize and shutdown C++ runtime so that global constructors are called before main() and global destructors are called after main(). So far I managed to get exactly half of it working - global ctors run correctly, but global destructors don't. I use gcc on Windows with the Newlib, which is supposed to initialize the runtime.
struct Test {
Test() // This is invoked correctly.
{}
~Test() // This is not invoked at all.
{}
};
Test test;
int main()
{
return 0;
}
extern void __libc_init_array();
extern void __libc_fini_array();
void reset_handler() // This is the entry point.
{
__libc_init_array(); // Test::Test().
asm volatile( "bl main" ); // Invoke 'main'.
__libc_fini_array(); // Expect Test::~Test() but nothing happens.
}
I did quite a bit of research already, and it seems that for ARM compiler should generate a section called .init_array, for global constructors, and .fini_array, for global destructors, where it put pointer to the function to be invoked. Then, the linker will merge the sections from all units together, and __libc_init_array will "walk" through that section and call corresponding functions.
This is the relevant part of the linker script:
/* Initialization functions which run before main(),
such as global constructors. */
.init_array : ALIGN( 4 ) {
/* preinit data */
PROVIDE_HIDDEN ( __preinit_array_start = . );
KEEP( *( .preinit_array ) )
PROVIDE_HIDDEN( __preinit_array_end = . );
. = ALIGN(4);
/* init data */
PROVIDE_HIDDEN ( __init_array_start = . );
KEEP( *( SORT( .init_array.* ) ) )
KEEP(*(.init_array))
PROVIDE_HIDDEN ( __init_array_end = . );
} > flash
/* Finalization functions which run after main(),
such as global destructors. */
.fini_array : ALIGN( 4 ) {
/* finit data */
PROVIDE_HIDDEN (__fini_array_start = .);
KEEP ( *( .fini_array.* ) )
KEEP ( *( .fini_array ) )
PROVIDE_HIDDEN (__fini_array_end = .);
} > flash
What worries me, however, is that when I dump the object file, main.o, not even an executable, I can only see the .init_array section, but no .fini_array:
Sections:
Idx Name Size VMA LMA File off Algn
0 .group 00000008 00000000 00000000 00000034 2**2
CONTENTS, READONLY, GROUP, LINK_ONCE_DISCARD
1 .group 00000008 00000000 00000000 0000003c 2**2
CONTENTS, READONLY, GROUP, LINK_ONCE_DISCARD
2 .text 0000022c 00000000 00000000 00000044 2**2
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
3 .data 00000000 00000000 00000000 00000270 2**0
CONTENTS, ALLOC, LOAD, DATA
4 .bss 00000001 00000000 00000000 00000270 2**2
ALLOC
5 .text._ZN4TestC2Ev 00000012 00000000 00000000 00000270 2**1 << Test::Test()
CONTENTS, ALLOC, LOAD, READONLY, CODE
6 .text._ZN4TestD2Ev 00000012 00000000 00000000 00000282 2**1 << Test::~Test()
CONTENTS, ALLOC, LOAD, READONLY, CODE
7 .init_array 00000004 00000000 00000000 00000294 2**2 << WHERE IS .fini_array???
CONTENTS, ALLOC, LOAD, RELOC, DATA
8 .debug_info 000012d9 00000000 00000000 00000298 2**0
CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
9 .debug_abbrev 000003a1 00000000 00000000 00001571 2**0
CONTENTS, READONLY, DEBUGGING, OCTETS
10 .debug_aranges 00000030 00000000 00000000 00001912 2**0
CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
11 .debug_ranges 00000020 00000000 00000000 00001942 2**0
CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
12 .debug_line 000002d2 00000000 00000000 00001962 2**0
CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
13 .debug_str 000009e4 00000000 00000000 00001c34 2**0
CONTENTS, READONLY, DEBUGGING, OCTETS
14 .comment 0000004d 00000000 00000000 00002618 2**0
CONTENTS, READONLY
15 .debug_frame 0000054c 00000000 00000000 00002668 2**2
CONTENTS, RELOC, READONLY, DEBUGGING, OCTETS
16 .ARM.attributes 0000002c 00000000 00000000 00002bb4 2**0
CONTENTS, READONLY
This is how I invoke the compiler:
arm-none-eabi-c++.exe -o main.elf --verbose -mcpu=cortex-m0 -mthumb --specs=nano.specs --entry reset_handler -T./../src/firmware.ld -ggdb -O0 -Wall -Wextra -Wpedantic -Werror ../src/main.cpp
And this is the log:
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 7a1ab17ae8404f635d46188cccacd8be
COLLECT_GCC_OPTIONS='-o' 'main.elf' '-v' '-mcpu=cortex-m0' '-mthumb' '-specs=nano.specs' '-e' 'reset_handler' '-T' './../src/firmware.ld' '-ggdb' '-O0' '-Wall' '-Wextra' '-Wpedantic' '-Werror' '-mfloat-abi=soft' '-march=armv6s-m'
c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/../../../../arm-none-eabi/bin/as.exe -v -march=armv6s-m -mfloat-abi=soft -meabi=5
GNU assembler version 2.34.0 (arm-none-eabi) using BFD version (GNU Arm Embedded Toolchain 9-2020-q2-update) 2.34.0.20200428
COMPILER_PATH=c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/;c:/id/gcc/bin/../lib/gcc/;c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/../../../../arm-none-eabi/bin/
LIBRARY_PATH=c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/thumb/v6-m/nofp/;c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/../../../../arm-none-eabi/lib/thumb/v6-m/nofp/;c:/id/gcc/bin/../arm-none-eabi/lib/thumb/v6-m/nofp/;c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/;c:/id/gcc/bin/../lib/gcc/;c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/../../../../arm-none-eabi/lib/;c:/id/gcc/bin/../arm-none-eabi/lib/
COLLECT_GCC_OPTIONS='-o' 'main.elf' '-v' '-mcpu=cortex-m0' '-mthumb' '-specs=nano.specs' '-e' 'reset_handler' '-T' './../src/firmware.ld' '-ggdb' '-O0' '-Wall' '-Wextra' '-Wpedantic' '-Werror' '-mfloat-abi=soft' '-march=armv6s-m'
c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/collect2.exe -plugin c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/liblto_plugin-0.dll -plugin-opt=c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/lto-wrapper.exe -plugin-opt=-fresolution=C:\Users\MAXID~1\AppData\Local\Temp\ccpOTkFN.res -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lg_nano -plugin-opt=-pass-through=-lc_nano -plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lc_nano --sysroot=c:\id\gcc\bin\../arm-none-eabi -X -o main.elf -e reset_handler c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/thumb/v6-m/nofp/crti.o c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/thumb/v6-m/nofp/crtbegin.o c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/../../../../arm-none-eabi/lib/thumb/v6-m/nofp/crt0.o -Lc:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/thumb/v6-m/nofp -Lc:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/../../../../arm-none-eabi/lib/thumb/v6-m/nofp -Lc:/id/gcc/bin/../arm-none-eabi/lib/thumb/v6-m/nofp -Lc:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1 -Lc:/id/gcc/bin/../lib/gcc -Lc:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/../../../../arm-none-eabi/lib -Lc:/id/gcc/bin/../arm-none-eabi/lib C:\Users\MAXID~1\AppData\Local\Temp\cc1lU5Gg.o -lstdc++_nano -lm --start-group -lgcc -lg_nano -lc_nano --end-group --start-group -lgcc -lc_nano --end-group c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/thumb/v6-m/nofp/crtend.o c:/id/gcc/bin/../lib/gcc/arm-none-eabi/9.3.1/thumb/v6-m/nofp/crtn.o -T ./../src/firmware.ld
COLLECT_GCC_OPTIONS='-o' 'main.elf' '-v' '-mcpu=cortex-m0' '-mthumb' '-specs=nano.specs' '-e' 'reset_handler' '-T' './../src/firmware.ld' '-ggdb' '-O0' '-Wall' '-Wextra' '-Wpedantic' '-Werror' '-mfloat-abi=soft' '-march=armv6s-m'
To be fair, I have zero idea of what could be wrong - I tried the default linker script, .fini_array did not appear. The only idea which comes to mind is that my compiler was built with some kind of a flag which disabled the .fini_array section???
Any ideas will be much appreciated!
UPDATE: after further investigation, it seems that the destructors have more to do with __cxa_atexit... Will investigate further.
UPDATE2: Thanks to this wonderful discussion here https://forum.osdev.org/viewtopic.php?f=13&t=36728 I added -fno-use-cxa-atexit, and the .fini_array section appeared!
Ok, so for those who are interested, there are two ways for gcc to generate calls to global destructors. One way is via __cxa_atexit. The compiler will inject a piece of code into the global constructor, which uses __cxa_atexit to register the destructor of that object. This way, when exit is called, the runtime "knows" which destructors to invoke. This complexity is needed to deal with loading/unloading of shared libraries.
However, it is also possible to pass a -fno-cxa-atexit flag, and the compiler will put the calls to global destructors into .fini_array. The Newlib then uses this section to invoke the destructors. More can be found here: https://forum.osdev.org/viewtopic.php?f=13&t=36728