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Message-Id: <1A568R.RVXRB53P9YYQ1@crapouillou.net>
Date: Thu, 03 Mar 2022 12:27:37 +0000
From: Paul Cercueil <paul@...pouillou.net>
To: David Laight <David.Laight@...LAB.COM>
Cc: "'Maciej W. Rozycki'" <macro@...am.me.uk>,
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Subject: RE: [PATCH v3] serial: make uart_console_write->putchar()'s character
an unsigned char
Hi David,
Le jeu., mars 3 2022 at 11:44:42 +0000, David Laight
<David.Laight@...LAB.COM> a écrit :
> From: Maciej W. Rozycki
>> Sent: 03 March 2022 11:31
> ..
>> It does, but, oh dear, it's a "solution" to a problem we have
>> created in
>> the first place. Why do we ever want to have signed characters in
>> the TTY
>> layer, and then to vary between platforms? It's asking for
>> portability
>> issues.
>
> C 'char' is signed because the pdp/11 byte load sign extended.
That's incorrect. The C standard does say that "the implementation
shall define char to have the same range, representation, and behavior
as either signed char or unsigned char".
C 'char' is signed on x86 (and MIPS and Sparc etc.). It is unsigned on
ARM, PowerPC and Risc-V among others.
> I guess some ABI use unsigned char to avoid issues with all
> the functions that take/return an int parameter that is
> either a 'char' cast to 'unsigned char' or EOF.
>
> EOF is usually (-1) - but doesn't have to be.
> But it needs to be different from any value obtained
> by casting a 'char' to 'unsigned char'.
> (But that may only need to be all characters, not all values of
> 'char'.)
Is the putchar() callback ever going to be called with EOF? I don't
think so.
> Then you get the requirement that:
> sizeof (int) >= sizeof (short) >= sizeof (char)
> which means that it is perfectly valid for all 3 to be the same size
> [1].
> In that case 'unsigned char' promotes to 'unsigned int'
> which probably breaks some code.
We're talking about Linux here. Ints are 32-bit.
Cheers,
-Paul
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