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Re: [Chicken-hackers] [PATCH] Send C_DEBUGGER_REPLY_GET_BYTES responses

From: Peter Bex
Subject: Re: [Chicken-hackers] [PATCH] Send C_DEBUGGER_REPLY_GET_BYTES responses as unsigned char values
Date: Sat, 5 Mar 2016 14:16:40 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Fri, Mar 04, 2016 at 02:03:14PM +1300, Evan Hanson wrote:
> This allows the client to consider the stream as a written u8vector,
> without needing to mask off the extra bits of any elements outside the
> byte range.
> ---
>  dbg-stub.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> diff --git a/dbg-stub.c b/dbg-stub.c
> index 96227de..bbfa3c5 100644
> --- a/dbg-stub.c
> +++ b/dbg-stub.c
> @@ -381,7 +381,7 @@ send_event(int event, C_char *loc, C_char *val, C_char 
> *cloc, int cln)
>          send_string(rw_buffer);
>          for(n = 0; n < reply; ++n) {
> -          sprintf(rw_buffer, " %lu", (unsigned long)((char 
> *)C_data_pointer(x))[ n ]);
> +          sprintf(rw_buffer, " %u", ((unsigned char *)C_data_pointer(x))[ n 
> ]);

Shouldn't this be printed as a character?
+          sprintf(rw_buffer, " %c", ((unsigned char *)C_data_pointer(x))[ n ]);

Does it even matter?

Actually, now I'm *really* confused by C's vararg handling and printf.
The manual (and the C11 draft standard) says:

  c  If no 'l' length modifier is present, the int argument is converted
      to an unsigned char, and the resulting character is written.

Does this mean an unsigned char argument to a varags function is always
widened to "int"?  How else can it know how much to take off the stack?

I can't find anything about this in the spec, and while it makes sense
in practice if you consider alignment of the rest of the arguments, I
don't really see how this follows from the spec.

And if it really is the case that characters are always widened to
integers, why bother to have a %c format specifier in the first place?


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