what is the equivalent of __IO in rust?

Question

• with the given example from here: What is the use of __IO & static keywords in C?, I am working on converting some C files that are using microcontrollers to rust

• I would like to know the equivalent of this volatile type in rust

• i am aware of unsafe code in rust: is the __IO alternative an example of this?

• I'm using a STM32f4XX type HAL

Answer 1

All the I/O qualifier macros are defined in core_cm4.h (normally included indirectly via stm324xx.h) thus:

#ifdef __cplusplus
  #define   __I     volatile             /*!< Defines 'read only' permissions */
#else
  #define   __I     volatile const       /*!< Defines 'read only' permissions */
#endif
#define     __O     volatile             /*!< Defines 'write only' permissions */
#define     __IO    volatile             /*!< Defines 'read / write' permissions */

as you can see they are simply aliases for standard C qualifiers and not themselves C keyword or even compiler extensions - they are part of the CMSIS.

I am no Rust expert, but it seems that there is no direct equivalent in Rust, in that you cannot simply qualify a variable to ensure all accesses are explicit, so memory-mapped I/O uses a different approach. There is an mmio crate and read_volatile/write_volatile and also a Volatile wrapper, all of which may be relevant to this issue.