NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/9076

Internet Message Format | 1996-08-05 | 4.9 KB

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From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: Q:  gets, getch, and stdin
Date: Wed, 06 Mar 96 22:28:27 GMT
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In article <[e-mail]> [e-mail] "Chris Redekop u" writes:

>
>Hi.  I'm trying to teach myself C with the help of 'Teach Yourself C In
>21 Days'.  Everything was going fine until day 14, 'Working with the
>Screen, Printer, and Keyboard'.

Are you aware that Screen, Printer, and Keyboard are not things that the
C language supports. All it knows about are files and 'streams' which you
can open to files to read and write data. stdin, stdout and stderr are
examples of streams that are opened for you when the program starts.
Your particular system can contrive to attach Screen etc. to streams
effectively simulating files but knowing this may help you understand how the
I/O works.

> On page 319 (in case anyone reading this
>has the book), it talked about how using scanf can leave unwanted
>characters in stdin.  It offered this function as a way to clear stdin:

The facilities that scanf provides simply doesn't make it particularly
suitable for line based input - it doesn't treat '\n' specially, just like
any other white-space.

>void clear_kb(void)
>{
>char junk[80];
>gets(junk);
>}

This is evil - gets() is historical baggage that you should never use.
The problem is that you can't tell it the size of the buffer you passed it
so if it encounters a line that is longer than the buffer it will overwrite
the end of it and corrupt anything that happens to be there.

>
>I thought to myself, 'That might work, but it uses a whole array to do
>it.  Instead of clearing stdin by reading a whole string, I should be
>able to clear it one chartacter at a time, by using getch().  Then I
>would only need one local variable of type char (or int).'  So I tried this:

getch() is not a standard C function so its behaviour is not well defined
and could vary from implementation to implementation. That happens in
practice: under Unix getch() is part of the Curses library and works
differently in relation to other functions that does the version you
typically find under DOS. If your book didn't make it clear that getch()
is non-standard that's the 2nd (or is that the 3rd?) black mark.

>void clear_kb(void)
>{
>char ch;
>
>while ((ch = getch()) != '\r')
>        ;
>}
>
>This didn't work, so after I tried a few different things, I found that
>this works:
>
>void clear_kb(void)
>{
>char ch;
>
>while ((ch = getchar()) != '\n')
>        ;
>}

getchar() is a standard C library function however it returns int. There
is a good reason for this because as well as any valid character (which
is returned as a positive value) it can return EOF (which is a negative
value) which is distinct from any valid character. It is important to test
for EOF since you don't necessarily know where your input is coming from
(e.g. stdin may have been redirected to a file by whatever ran your
program).

>This is what confuses me.  I can't understand why the third function
>works, and not the second.  As well, the third function does not output
>the cleared characters to the screen, even though getchar provides input
>with echo.

I couldn't say what getch() does because it is not well defined.

getchar() is not defined to produce any sort of echo. However it, as well
as any other standard function reading characters from an input stream,
must get characters from the OS. In the case of stdin this is usually
connected to the keyboard when the program starts. The OS will typically
arrange to perform some rudimentary command line editing before making
the complete line available to the program, e.g. when you press <return>.
That line editing may well involve echoing characters to the screen but
all of this is performed before the line is passed to the C program.
The C library in general and getchar() in particular performs no echoing.

The correct way to approach line based input is to read the line as a whole
into a buffer using fgets(). fgets() allows you to specify the size of
the buffer you pass it so doesn't suffer the same defect as gets(). Also
remember it leaves the trailing '\n' at the end of the string. Once you
have the line in the buffer you can pick it apart with your own code or
a range of standard library functions e.g. atoi(), strtol(), sscanf(),
strtok() etc.

I suggest you look through the FAQ which provides much information
in these areas. You can ftp it from rtfm.mit.edu under
/pub/usenet/comp.lang.c.

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
-----------------------------------------