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From:   David Laight <David.Laight@ACULAB.COM>
To:     "'Maciej W. Rozycki'" <macro@orcam.me.uk>,
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Subject: RE: [PATCH v3] serial: make uart_console_write->putchar()'s character
 an unsigned char
Thread-Topic: [PATCH v3] serial: make uart_console_write->putchar()'s
 character an unsigned char
Thread-Index: AQHYLl5tVpWrjMz0S0GPyaCV/1nc9aysw9yAgADDqR+AAACPoA==
Date:   Thu, 3 Mar 2022 11:44:42 +0000
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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.

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'.)

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.
It also makes defining EOF troublesome!

[1] The C compiler for a DSP had this 'feature'.

	David

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