Initializing arrays of char

[Chris Torek]

In article <15674 at csli.Stanford.EDU> poser at csli.Stanford.EDU
(Bill Poser) writes:
>Regarding the assignment of "12345" to char x[5] ... [K&R 2 says]
>	...the number of characters in the string, NOT COUNTING
>	THE TERMINATING NULL CHARACTER, must not exceed the
>	size of the array. [emphasis mine]
>Can anyone explain [why the ending '\0' is not counted]?

This is a change in New (ANSI) C.  In Classic (K&R-1) C, a
double-quoted string in an initializer context%, when setting the
initial value of a character array, was treated uniformly as if it were
a bracketed initializer consisting of all the characters, including
the terminating NUL, in the string.  That is,

	char x[5] = "12345";

meant exactly the same thing as

	char x[5] = { '1', '2', '3', '4', '5', '\0' };

(and was therefore in error, having too many characters).

The X3J11 committee decided# that this was overly restrictive, and
relaxed the rule to `is equivalent to a bracketed initializer
consisting of all the characters, including the terminating NUL if it
fits'.  Thus

	char x[] = "12345";

means the same as

	char x[] = { '1', '2', '3', '4', '5', '\0' };

or

	char x[6] = { '1', '2', '3', '4', '5', '\0' };

but

	char x[5] = "12345";

now means the same as

	char x[5] = { '1', '2', '3', '4', '5' };

If the declaration is changed to

	char x[4] = "12345";

it is once again in error.

-----
% Note that here (in an initializer context) and as an argument to
  sizeof (e.g., `sizeof "abc"') are the only two places that a double
  quoted string does not undergo the usual `array degenerates into
  pointer' rule.  All other legal occurrences of a double-quoted string
  are in a value context, and therefore change from `array N of char'
  to `pointer to char', pointing to the first character in the string.

# This wording is not meant to imply judgement as to this decision.
  (When I do not take a stand on some aspect of the language I use
  weasel-wording like `seems to be' or merely present bare facts.)
  Since I use old compilers, I have not made up my mind on this.  I
  am leaning towards the `not a bad idea after all' faction.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 405 2750)
Domain:	chris at cs.umd.edu	Path:	uunet!mimsy!chris