Re: [avr-libc-dev] New pgm_read_ptr() macro?

[David Brown]

Hi,

Are there any opinions about whether a macro such as the one I gave 
below should be part of avrlibc?  It's not as clear-cut as Dean's 
pgm_read_ptr() macro, which follows the existing patterns in the 
headers.  Macros like mine are more like template programming than the 
fixed-size macros, and thus are a slight change in philosophy.

If the idea of a generic pgm_read(&x) macro does appeal, then the same 
tricks can be used for other accesses such as wrapping an access in 
"volatile", reading and writing eeprom, and atomically accessing data.

mvh.,

David


> How about a more general solution based on typeof and sizeof?
>
> #define pgm_read(v) ((typeof(*v)) \
> (__builtin_choose_expr(sizeof(*v) == 1, pgm_read_byte(v), \
> __builtin_choose_expr(sizeof(*v) == 2, pgm_read_word(v), \
> __builtin_choose_expr(sizeof(*v) == 4, pgm_read_dword(v), \
> ({ char error[((sizeof(*v) == 1) || (sizeof(*v) == 2) || \
> (sizeof(*v) == 4)) ? 1 : -1]; error[0]; }) \
> )))))
>
>
> The macro above looks a bit ugly, but it's not too hard to follow.
> Basically, we are using sizeof(*v) to figure out the size of the data
> you are wanting, and using it to pick the correct pgm_read_xxx function
> for the data size. We use __builtin_choose_expr instead of ?: to avoid
> any unwanted expression evaluations or side effects, as well as to allow
> different sized return values. The "char error[...]" part is a way to
> force a compile-time error if the macro is used with something whose
> size is not 1, 2 or 4 bytes.
>
> The result is that you can write:
>
> char x;
> char* PROGMEM FlashPointer = &x;
> char* FlashPtr = pgm_read(&FlashPointer);
>
> You can equally write:
>
> static const PROGMEM uint8_t x8 = 8;
> static const PROGMEM uint16_t x16 = 16;
> static const PROGMEM uint32_t x32 = 32;
>
> void test(void) {
> volatile uint8_t y8 = pgm_read(&x8);
> volatile uint16_t y16 = pgm_read(&x16);
> volatile uint32_t y32 = pgm_read(&x32);
> }
>
>
> Now your pgm_read's are independent of the type, assuming they have a
> compatible size.
>
>
> Personally, my preference would be to change the semantics to remove the
> "&", so that you would write:
>
> char* FlashPtr = pgm_read(FlashPointer);
>
> This is just a small change to the macro, but I think it's neater - you
> are reading an object from flash, rather than reading data from an
> address in flash. But such a change would require a name change to avoid
> accidents.
>
> mvh.,
>
> David
>
>
>
>
>
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