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  • 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

AthulMuralidhar
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    Perhaps this will help: https://docs.rs/volatile/latest/volatile/ It came from a web search on: `volatile rust` – Craig Estey Dec 13 '22 at 17:52
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    Rust doesn't have `volatile` types. Instead, it has [volatile read/write pointer operations](https://doc.rust-lang.org/stable/std/ptr/fn.read_volatile.html). You must use those or some wrapper around them, as mentioned by Craig. – PitaJ Dec 13 '22 at 17:55
  • i think the write pointer ops is what i was looking for, thx @PitaJ :) will check that one out and see if it works for my use case :) – AthulMuralidhar Dec 13 '22 at 18:03
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    __IO is simply `volatile` in STM32 HAL – 0___________ Dec 13 '22 at 21:46

1 Answers1

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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.

Clifford
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