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Re: arm-gcc, Cortex-M0+, uint64_t and alignment

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On 2026-01-20, pozz <pozzugno@gmail.com> wrote:
> Il 20/01/2026 17:41, Grant Edwards ha scritto:
>> On 2026-01-20, David Brown <david.brown@hesbynett.no> wrote:
>> 
>>> You can't reduce the alignment of a struct or its elements by adding an
>>> __aligned_ attribute to the struct itself or any of its fields.  The
>>> best you can do on the struct itself is __attribute__((packed)).  But
>>> that can come with disadvantages, and inefficient use.
>> 
>> Yep making a structure aligned is an excellent way to introduce subtle
>> bugs that happen when somebody, somewhere passes a pointer to one of
>> those structure fields to some library function. 

Aargh, my bad. I meant that making a strucutre _packed_ is an
excellent way to introduce subtle bugs that happen when somebody,
somewhere passes a pointer to one of those structure fields to some
library function.

> However, as long as the application runs on Cortex-M0+, the aligned 
> version shouldn't introduce issues, should it?

A non-packed structure should always be OK.

A packed structure will work fine as long as it's being accessed by
code that "knows" it's packed.  You can pass a pointer to packed
struct to a function as long as it's declared in that function as a
pointer to a packed struct: the compiler will generate extra code to
deal with accesses to values that are misaligned due to the
packing. However, passing a pointer to an packed field structure
(e.g. to a uint64_t) to a function where it was declared as a normal
"uint64_t *p" can cause failures on ARM targets. It will work OK on
X86. I think it used to work OK on m68k also. IIRC SPARC failed in
similar ways to ARM.

--
Grant

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arm-gcc, Cortex-M0+, uint64_t and alignment pozz <pozzugno@gmail.com> - 2026-01-20 13:26 +0100
  Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-20 17:07 +0100
    Re: arm-gcc, Cortex-M0+, uint64_t and alignment Grant Edwards <invalid@invalid.invalid> - 2026-01-20 16:41 +0000
      Re: arm-gcc, Cortex-M0+, uint64_t and alignment pozz <pozzugno@gmail.com> - 2026-01-20 18:09 +0100
        Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-20 18:44 +0100
          Re: arm-gcc, Cortex-M0+, uint64_t and alignment pozz <pozzugno@gmail.com> - 2026-01-21 09:11 +0100
            Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-21 10:02 +0100
              Re: arm-gcc, Cortex-M0+, uint64_t and alignment pozz <pozzugno@gmail.com> - 2026-01-21 15:58 +0100
                Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-21 17:13 +0100
                Re: arm-gcc, Cortex-M0+, uint64_t and alignment pozz <pozzugno@gmail.com> - 2026-01-21 17:57 +0100
                Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-22 10:03 +0100
        Re: arm-gcc, Cortex-M0+, uint64_t and alignment Grant Edwards <invalid@invalid.invalid> - 2026-01-20 17:48 +0000
      Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-20 18:41 +0100
        Re: arm-gcc, Cortex-M0+, uint64_t and alignment Grant Edwards <invalid@invalid.invalid> - 2026-01-20 18:10 +0000
          Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-20 22:32 +0100
            Re: arm-gcc, Cortex-M0+, uint64_t and alignment Grant Edwards <invalid@invalid.invalid> - 2026-01-21 03:38 +0000
              Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-21 08:54 +0100
    Re: arm-gcc, Cortex-M0+, uint64_t and alignment pozz <pozzugno@gmail.com> - 2026-01-20 17:55 +0100
      Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-20 22:24 +0100

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