What flags are needed in arm-none-eabi-gcc to produce FDPIC ELF binary?

Question

what flags do i need to add to:

arm-none-eabi-gcc test.c -o test -mcpu=cortex-m4 -Wall -Os

to get FDPIC binary ?

I made many experiments, and still i receive not a valid format. I know about existence of linker flags:

--oformat elf32-littlearm-fdpic

but i don't know how to pass them properly.

Spliting compiling and linking into separate steps:

arm-none-eabi-gcc -c test.c -o test.o -mcpu=cortex-m4 -Wall -Os
arm-none-eabi-ld test.o --oformat elf32-littlearm-fdpic -pie -o test

Result in that no newlib is added and warning about start is produced:

arm-none-eabi-ld: warning: cannot find entry symbol _start; defaulting to 0000000000000074

Who can help me ? What is the correct commands (flags) ?

I want to start my program on "bare" linux kernel (i have already working kernel on that stm32). I deliver it in initramfs compiled in kernel

EDIT:

arm-none-eabi-gcc test.c -o test -mcpu=cortex-m4 -Wall -fpic -fpie -Os -Xlinker "--oformat elf32-littlearm-fdpic"

Results:

/usr/lib/gcc/arm-none-eabi/10.3.1/../../../arm-none-eabi/bin/ld: unrecognized option '--oformat elf32-littlearm-fdpic'
/usr/lib/gcc/arm-none-eabi/10.3.1/../../../arm-none-eabi/bin/ld: use the --help option for usage information
collect2: error: ld returned 1 exit status
make: *** [Makefile:5: all] Error 1

---

MOREOVER this tends to work (but i want to use start files):

Makefile:

all:
    arm-none-eabi-as -mcpu=cortex-m4 init.S -o init.o
    arm-none-eabi-gcc -c -fpic -Os -mcpu=cortex-m4 -fpie -Wall init.c -o init-c.o
    arm-none-eabi-ld init.o init-c.o --oformat elf32-littlearm-fdpic -pie -o init --thumb-entry=_start

init.c:

#define USART1_DR (*((volatile unsigned int *)0x40011004))

int main()
{
    volatile int i;

    USART1_DR = 'A';
    
    for (i = 0; i < 1000000; i++) {
    }

    USART1_DR = 'B';

    while (1) {}
    return 0;
}

init.S:

.syntax unified
.cpu cortex-m4
.thumb
.global _start

_start:
bl main
b .

I mean compilation commands. I know my code snippet works only because STM32 has no MMU (and i configured kernel not to use MPU).

Answer 1

arm-none-eabi-gcc is for building things that run on bare-metal, with no OS involved. From a quick search FDPIC seems to be an ABI for loading shared libraries in platforms with no MMU.

If I'm not mistaken, you can't load shared libraries in bare-metal environments, thus maybe the problem is that you're building with the wrong compiler?