Hexadecimal Escape Sequence

[Walter Murray]

Steve Hosgood writes:

> I recently discovered in K&R Edition 2 (ANSII C) that the hex escape
> sequence will accept any number of valid hex characters after the "\x".
> This means that the printf statement:-
> 	printf("\x1bfred");	/* i.e "<ESC>fred" */
> ..suddenly failed in a program when we updated from Microsoft 4.0 to
> 5.1 recently.

> According to K&R2, (my only reference), MSC5.1 interprets this correctly
> as "<hex BF>red", and 4.0 was wrong to limit itself to 2 hex chars following
> the \x. It seems that an infinite number of hex characters may follow the
> \x sequence, though what happens if the result fails to fit in a char is
> undefined.

A Standard-conforming compiler is required to produce a diagnostic if
the value of a hexadecimal escape sequence doesn't fit in an unsigned
char.

> Is this what you'd call "expected behaviour"?

It's for the benefit of implementations where a char is more than
eight bits.

> After all, the octal escape sequence limits itself to 3 characters...

True.  Hexadecimal escape sequences are different.

> If it IS correct, how do you write "<ESC>fred" using a hex escape? I
> ended up having to use the octal escape in the end, which seems rather
> an inelegant method.

You use adjacent string literals.  You can safely write
   printf("\x1b" "fred");
or
   #define ESC "\x1b"
   printf(ESC "fred");
This works because escape sequences are converted to single characters
prior to the concatenation of adjacent string literals.

Walter
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