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Date: Sun,  7 Nov 2010 17:22:13 +0100
From: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
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To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
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Subject: Re: [9fans] pcc limitation?
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On 07.11.2010 15:46, erik quanstrom wrote:
>> writearr should point to a one-member const unsigned char array, and the
>> zeroth element of that array has the value JEDEC_WREN.
>>
> =
> it's not clear to me that c99 allows one to declare an unnamed array
> and assign a pointer to it in this way except if the array is a char* or
> wchar_t*.
>
> i think the cleanest approach to solving your problem is
> to define writeattr as an array, not a uchar*.
>

The big problem with defining it as an array inside struct spicmd is
that writearr has variable length. writearr is a command sent to a SPI
chip by a SPI controller. writearr can have any length of 1-1056 bytes.
There's also an analogous readarr in struct spi_command (not mentioned
in the example to keep it brief) and that one can have any length
between 0 and 16777217 (2^24+1) bytes. One variable-length array at the
end of a struct is possible, but an array of structs with a variable
length array in the struct won't work since you can't compute the offset
of individual members of the outer arrray.

The only solution I can see is this:

#define JEDEC_WREN              0x06
#define JEDEC_SE                0x20
struct spi_command {
        const unsigned char *writearr;
};
int main(int argc, char *argv[])
{
	const unsigned char writearr_helper1[] = { JEDEC_WREN };
	const unsigned char writearr_helper2[] = { JEDEC_SE, 0x01, 0x23, 0x45 };
        struct spi_command cmds[] = {
        {
                .writearr       = writearr_helper1,
        },{
                .writearr       = writearr_helper2,
        }};
        /* ... */
}


That solution wastes variables and is generally pretty ugly.
I still hope we can find a better solution.

Regards,
Carl-Daniel

--
http://www.hailfinger.org/