I’m writing code for a specific stm32 chip with memory mapped peripherals. I happen to know that one of the GPIO ports, GPIOA, is controlled by several 32 bit ints located starting at address 0x40018000. How can I express this information at compile time?

My initial attempt looked like this:

struct Gpio {
    volatile uint32_t crl;
    volatile uint32_t crh;
    // ...
};

constinit Gpio& GPIOA = *reinterpret_cast<Gpio*>(0x40018000);

This seems like a sensible way to say “The reference GPIOA refers to an object located at memory address 0x40018000

Unfortunately this does not work:

error: 'constinit' variable 'GPIOA' does not have a constant initializer
error: 'reinterpret_cast' from integer to pointer

What options are available to me? How can I spell a specific numeric memory address at compile time?

Prior research:

• Is it possible to have a pointer literal?
  • It is not possible to have a pointer literal.
• How to place a variable at a given absolute address in memory (with GCC)
  • Globals can be placed at specific addresses using linker scripts.
  • This happens at link time, not compile time.