GDB don't stop on breakpoints and continue

Question

I am a beginner in embedded programming so to learn I'm trying to work with a minimalist program. I have the following program I try to execute.
My environment:

• arm-none-eabi-gcc (15:9-2019-q4-0ubuntu1) 9.2.1 20191025 (release) [ARM/arm-9-branch revision 277599]
• gdb-multiarch GNU gdb (Ubuntu 9.1-0ubuntu1) 9.1
• Open On-Chip Debugger 0.10.0
• STM32F3Discovery - ARM Cortex-M4

main.c

int main(void)
{
}


__attribute__((section(".isr_vector"))) void Reset_Handler(void)
{

  char * my_pointer = (char *)0x20000000;
  *my_pointer = 42;
  main();
  while(1) {

  }
}

stm32.ld

MEMORY
{
    FLASH : ORIGIN = 0x08000000, LENGTH = 256K
    RAM : ORIGIN = 0x20000000, LENGTH = 48K
}

start = Reset_Handler;
ENTRY(Reset_Handler)

SECTIONS
{
    .isr_vector :
    {
    *(.isr_vector)
    } >FLASH
    .text :
    {
    *(.text)
    } >FLASH
}

Everything is compiled using the following command: arm-none-eabi-gcc -mthumb -march=armv7e-m -mcpu=cortex-m4 -nostdlib -Tstm32.ld -g -o main.elf main.c.
This program does nothing, it's just a try to make it running and write something in memory.

Using OpenOCD and GDB I load the program on the board, and here is what I get with layout asm in GDB.

|  >0x8000000 <Reset_Handler>       push    {r7, lr}                                                                          │
│   0x8000002 <Reset_Handler+2>     sub     sp, #8                                                                            │
│   0x8000004 <Reset_Handler+4>     add     r7, sp, #0                                                                        │
│   0x8000006 <Reset_Handler+6>     mov.w   r3, #536870912  ; 0x20000000                                                      │
│   0x800000a <Reset_Handler+10>    str     r3, [r7, #4]                                                                      │
│   0x800000c <Reset_Handler+12>    ldr     r3, [r7, #4]                                                                      │
│   0x800000e <Reset_Handler+14>    movs    r2, #42 ; 0x2a                                                                    │
│   0x8000010 <Reset_Handler+16>    strb    r2, [r3, #0]                                                                      │
│   0x8000012 <Reset_Handler+18>    bl      0x8000018 <main>                                                                  │
│   0x8000016 <Reset_Handler+22>    b.n     0x8000016 <Reset_Handler+22>                                                      │
│   0x8000018 <main>                push    {r7}                                                                              │
│   0x800001a <main+2>              add     r7, sp, #0                                                                        │
│   0x800001c <main+4>              movs    r3, #0                                                                            │
│   0x800001e <main+6>              mov     r0, r3                                                                            │
│   0x8000020 <main+8>              mov     sp, r7                                                                            │
│   0x8000022 <main+10>             pop     {r7}                                                                              │
│   0x8000024 <main+12>             bx      lr

My program seems to be loaded at the beginning of the FLASH memory, and I start on Reset_Handler, but I cannot run step by step, I'm not even sure whether the program is running. If I add a breakpoint or use step I get:

(gdb) b 11
Breakpoint 1 at 0x8000006: file main.c, line 11.
(gdb) continue
Continuing.
Note: automatically using hardware breakpoints for read-only addresses.

I've read the following questions / answers but that didn't help. I guess my program's linking is invalid or I may not do anything without initializing something.

• GDB step over function (next) doesn't seem to work
• gdb can't access memory address error
• https://www.sourceware.org/gdb/onlinedocs/gdb/Variables.html

Note: I am able to run a very minimalist assembly program in the board and debug it, but the embedded C & linking part are quite new.

Answer 1

Your code is fundamentally wrong.

First of all

__attribute__((section(".isr_vector"))) void Reset_Handler(void)

is wrong. The beginning of the flash is the default location of the vector table not the handlers code.

Vector table has a specific format and it has to be followed:

first 4 bytes is an initial stack pointer value

second 4 bytes is the address of the reset handler routine code.

the next 32 bits words are addresses of other exception and interrupt handlers.

MEMORY
{
    FLASH : ORIGIN = 0x08000000, LENGTH = 256K
    RAM : ORIGIN = 0x20000000, LENGTH = 48K
}

start = Reset_Handler;
ENTRY(Reset_Handler)

_RAM_END = ORIGIN(RAM_START) + 48K; 

SECTIONS
{
    .isr_vector :
    {
    *(.isr_vector)
    } >FLASH
    .text :
    {
    *(.text)
    } >FLASH
}

then the C file.

extern uint32_t _RAM_END;

void Reset_Handler(void);

void __attribute__((section(".isr_vector"))) (*vector_tabler[])(void) = {(void (*)(void))&_RAM_END, Reset_Handler};

int main(void)
{
}


void Reset_Handler(void)
{

  char * my_pointer = (char *)0x20000000;
  *my_pointer = 42;
  main();
  while(1) {

  }
}

BTW I would rather suggest to read the documentation before you start to do very low level stuff. I advice to use STM32 provided startup files and linker scripts before you get enough knowledge and experience to modify (or write new ones from the scratch).