buggy_double_use_of _scanf

buggy_double_use_of _scanf

[simon]

hello,

I have observe a strange scanf behaviour...
when using two successive scanf... the second receive a return character

for example :

int a;
char b;

scanf ("%d", &a);
fflush (stdin);
scanf ("%c", &b);
fprintf (stdout, "a : %d\nb : %c\n", a, b);

what's the problem ?
is scanf using a buggy internal buffer ?

simon

Re: buggy_double_use_of _scanf

[Glynn Clements]

simon wrote:

> I have observe a strange scanf behaviour...
> when using two successive scanf... the second receive a return character
> 
> for example :
> 
> int a;
> char b;
> 
> scanf ("%d", &a);
> fflush (stdin);
> scanf ("%c", &b);
> fprintf (stdout, "a : %d\nb : %c\n", a, b);
> 
> what's the problem ?

What makes you think that there is a problem?

What's the input? If it's a decimal number followed by newline, the
first scanf() will return the parsed number, the second will return
the newline.

-- 
Glynn Clements <glynn@gclements.plus.com>

Re: buggy_double_use_of _scanf

[Fábio Russo]

----- Original Message -----
From: "Glynn Clements" <glynn@gclements.plus.com>
To: "simon" <simon.guinot@laposte.net>
Cc: "linux-C-programming" <linux-c-programming@vger.kernel.org>
Sent: Tuesday, October 26, 2004 6:59 PM
Subject: Re: buggy_double_use_of _scanf


>
> simon wrote:
>
> > I have observe a strange scanf behaviour...
> > when using two successive scanf... the second receive a return character
> >
> > for example :
> >
> > int a;
> > char b;
> >
> > scanf ("%d", &a);
> > fflush (stdin);
> > scanf ("%c", &b);
> > fprintf (stdout, "a : %d\nb : %c\n", a, b);
> >
> > what's the problem ?
>
> What makes you think that there is a problem?
>
> What's the input? If it's a decimal number followed by newline, the
> first scanf() will return the parsed number, the second will return
> the newline.


The problem, I beleve is in the fflush function. I Have the same behaviour
with this source code,
but when I put a additional scanf in the line, all works fine -;)

Now I ask:
Why the fflush function did not remove the new line caracter when it should
?

it´s all folks !!!

        Russo.


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RE: buggy_double_use_of _scanf

[Huber, George K RDECOM CERDEC STCD SRI]

Hi,

Russo wrote:

[... snip ...]

>The problem, I beleve is in the fflush function. I Have the same behaviour
>with this source code, but when I put a additional scanf in the line, all 
>works fine -;)

>Now I ask:
>Why the fflush function did not remove the new line caracter when it
should?

quoteth the man page:

'The function fflush forces a write of all user-space buffered data for
 the given output or update stream via the stream's underlying write 
 function.'

Thus fflush only causes user-space buffered data to write their contents to 
the kernel-space buffers (which can be forced to update to the storage
medium
via a call to sync or fsync).  fflush has no effect when called on stdin - 
you are reading data not writing it.

George

Re: buggy_double_use_of _scanf

[Glynn Clements]

Fábio Russo wrote:

> > > I have observe a strange scanf behaviour...
> > > when using two successive scanf... the second receive a return character
> > >
> > > for example :
> > >
> > > int a;
> > > char b;
> > >
> > > scanf ("%d", &a);
> > > fflush (stdin);
> > > scanf ("%c", &b);
> > > fprintf (stdout, "a : %d\nb : %c\n", a, b);
> > >
> > > what's the problem ?
> >
> > What makes you think that there is a problem?
> >
> > What's the input? If it's a decimal number followed by newline, the
> > first scanf() will return the parsed number, the second will return
> > the newline.
> 
> 
> The problem, I beleve is in the fflush function. I Have the same behaviour
> with this source code,
> but when I put a additional scanf in the line, all works fine -;)
> 
> Now I ask:
> Why the fflush function did not remove the new line caracter when it should
> ?

fflush() should not have removed any characters from the buffer. The
purpose of fflush() is to ensure that any data in *output* buffers is
written to the underlying descriptor.

If your input consists of lines, you should probably be using fgets()
to read each line then using sscanf() to parse the text, rather than
using scanf() (or fscanf()) directly.

For a start, using fgets() makes it much easier to recover from
errors. If you call scanf(), and it indicates that it didn't read
everything which you wanted it to, it can be unclear as to exactly
what it did read and what is likely to be left in the buffer.

-- 
Glynn Clements <glynn@gclements.plus.com>
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