From:	CSBVAX::MRGATE!@KL.SRI.Com:R022DB3L@VB.CC.CMU.EDU@SMTP 18-SEP-1987 22:07
To:	EVERHART
Subj:	Re: 2 C Questions

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Date: Thu, 18 Sep 87 00:09:35 EDT
From: Mithrandir <R022DB3L@VB.CC.CMU.EDU>
Subject: Re: 2 C Questions
To: info-vax@KL.SRI.COM

For Richard Steinberger's query:

>        I wrote a short main routine (see below) to test a function. One
> of the inputs is an integer number, and the next input is a filename.
> The problem is that the scanf() that reads the number apparently leaves
> a LF character in a buffer that is then read by the code that is
> expecting the filename.       ..... 
> 					      Why does scanf leave a LF
> character, or am I misinterpreting what's going on?  Are there solutions
> other than using a getchar() call after a scanf() that preceeds a gets()
> or getchar() call?  ....

I've never actually programmed in Vax C, but just from my general C work,
I would expect this to be the case, according to the definition of SCANF.
Basically, your call to scanf is causing an I/O request, to which you
enter the string size and press RETURN.  Thus, your input buffer contains
the number followed by a newline.

You're asking scanf to read a decimal input field into an integer
variable.  According to scanf (or at least the definition of it that I
have handy), an "input field" is:

    * All characters up to **(but not including)** the next whitespace
      character
    * All characters up to the first one that cannot be converted under
      the current format specification (such as an 8 or 9 under octal
      format)
    * Up to 'n' characters where 'n' is the specified field width.

"Whitespace" is defined as one of the characters blank ( ), tab (\t) or
**newline (\n)**.

Therefore, when you do a scanf("%d",&variable), scanf simply parses through
the input until it hits the whitespace character (the newline).  The number
it reads gets assigned into the variable, and according to the first rule
above, the whitespace character is not parsed, and therefore stays in the input
buffer for the next call to scanf (or any function accessing the standard
input stream).

As far as a solution, how about just changing your scanf to:

	scanf("%d\n",&variable)

(Actually, according to scanf, if there is any whitespace in the format
 string, it will scan over as much whitespace as necessary in the input at
 that point, so I suppose technically, you could use a space or \t in
 place of the \n above since they're all whitespace, but the \n seems logical).

I do presume however, that even the above code would run into problems if the
user were to input a string like "10    20{RETURN}", since the scanf would
stop at the space, and you'd still have the rest of the string in the buffer
for the next scanf.  Perhaps an even better solution would be:

	scanf("%d%*[^\n]\n",&variable)

What this code will do is read an integer, followed by a string which will
be ignored (due to the *), the string consisting of the series of characters
in the input which aren't a newline.  The [] construct is a character search
set specifier - since the first character is ^, it's a inverted set, matching
any characters not listed.  Effectively, this will skip over any extra
characters on the input up to the newline, and then the \n in the format
string will read the newline out of the buffer.

Please note that I haven't actually had the time to try out these function
calls, but I used function calls very similar this summer, so if they
aren't quite perfect, play around a bit with the concept... :-)


-- David Bolen				Arpanet:  R022DB3L@VB.CC.CMU.EDU
   Carnegie-Mellon University		Bitnet :  R022DB3L@CMCCVB
   Pittsburgh, PA  15213