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From: chris@mimsy.umd.edu (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: How to validate input?
Message-ID: <28132@mimsy.umd.edu>
Date: 30 Nov 90 14:10:52 GMT
References: <2195.2754fcc2@iccgcc.decnet.ab.com>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 32

In article <2195.2754fcc2@iccgcc.decnet.ab.com>
browns@iccgcc.decnet.ab.com (Stan Brown) writes:
[regarding *scanf("%o%c"...) with input beginning with `8']
>The standard says sscanf stops converting when it finds an invalid
>character, so at first blush we expected num_fields to be 0 because the
>first character in the string isn't an octal digit.  But working through
>the definition of %o, which is in terms of strtoul( ), and in working
>through the definition of the latter, we find that a "base part" of no
>characters is considered valid.

I got a different answer when I wrote my `scanf' innards: only a few
formats are allowed to be `empty', and none of the numeric conversion
formats are included in those few.  So while "" is a `proper' octal
number to strtoul(), it is not a `proper' octal number to %o and %o
must stop with a matching failure, causing the sscanf call to return 0.

>    2. What is the best way to accomplish what we're trying to
>       accomplish, i.e. to check quickly that the user typed a valid
>       octal number and nothing else?

I prefer

	char *cp; unsigned long value;

	value = strtoul(buf, &cp, 8);
	if (cp == buf || *cp != '\0')
		... input value was invalid ...;

but I believe the sscanf described in the parent article should also work.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 405 2750)
Domain:	chris@cs.umd.edu	Path:	uunet!mimsy!chris