STM32 - Pointers and sum - c

Question

I'm learning how to program STM32 Nucleo F446RE board using registers.
To know the position of a register, I take from datasheets the boundary address and the offset.
However, I cannot calculate the sum of them. I show an exmaple:
volatile uint32_t *GPIOA = 0x0; // Initialization of the boundary adress
GPIOA = (uint32_t*)0x40020000; // Boundary adress from datasheet
volatile uint32_t *GPIOA_ODR = 0x0; // Initialization of GPIOA_ODR register
GPIOA_ODR = GPIOA + (uint32_t*)0x14; // Calculation of GPIOA_ODR as the sum of the boundary adress and the offset (i.e. 0x14.
Line 5 gives me an error, do you know how to calculate it correctly?
Thank you very much in advance.

Answer 1

It is wrong. If you want to use this extremely inconvenient way:
#define GPIOA 0x4002000
#define ODR_OFFSET 0x14
#define GPIO_ODR (*(volatile uint32_t *)(GPIOA + ODR_OFFSET))
why #define not the pointer? It is just more compiler friendly and saves one memory read.
https://godbolt.org/z/LdLLVN
#define GPIOA 0x4002000
#define ODR_OFFSET 0x14
#define GPIO_ODR (*(volatile uint32_t *)(GPIOA + ODR_OFFSET))
volatile uint32_t *pGPIO_ODR = (volatile uint32_t *)(GPIOA + ODR_OFFSET);
void foo(uint32_t x)
{
GPIO_ODR = x;
}
void bar(uint32_t x)
{
*pGPIO_ODR = x;
}
and resulting code
foo:
ldr r3, .L3
str r0, [r3, #20]
bx lr
.L3:
.word 67117056
bar:
ldr r3, .L6
ldr r3, [r3]
str r0, [r3]
bx lr
.L6:
.word .LANCHOR0
pGPIO_ODR:
.word 67117076

Answer 2

The cast should be outside the constant value, in other words, you are adding GPIOA address + 14 to generate a new address. So the cast must be outside them:
GPIOA_ODR = (uint32_t*)(GPIOA + 0x14);

Answer 3

I tried but nothing. If I insert the GPIOA_ODR = (uint32_t*)(0x40020000 + 0x14); it works, instead if I insert GPIOA_ODR = (uint32_t*)(GPIOA + 0x14); it doesn't work.
Some other ideas?
Thank you very much for the answer. The complete code I'm using is the following:
int main(int argc, char* argv[])
{
/** RCC **/
/* RCC */
volatile uint32_t *RCC = 0x0;
RCC = (uint32_t*)0x40023800;
/* RCC_AHB1ENR */
volatile uint32_t *RCC_AHB1ENR = 0x0;
RCC_AHB1ENR = (uint32_t*)(0x40023800 + 0x30);
*RCC_AHB1ENR |= 0x1;
/** GPIOA **/
/* GPIOA */
volatile uint32_t *GPIOA = 0x0;
GPIOA = (uint32_t*)0x40020000;
/* GPIOA_MODER */
volatile uint32_t *GPIOA_MODER = 0x0;
GPIOA_MODER = (uint32_t*)(0x40020000 + 0x00);
*GPIOA_MODER |= 1 << 16;
*GPIOA_MODER &= ~(0 << 17);
/* GPIOA_ODR */
volatile uint32_t *GPIOA_ODR = 0x0;
GPIOA_ODR = (uint32_t*)(GPIOA + 0x14);
*GPIOA_ODR |= 1 << 8;
}
This code doens't work correctly because of the line GPIOA_ODR = (uint32_t*)(GPIOA + 0x14);. If I insert GPIOA_ODR = (uint32_t*)(0x40020000 + 0x14) it works correctly.