DEVCPP + rewind(stdin)

[Olaf \El Blanco\]

Where is exactly neccesary to put "rewind(stdin)" ?
After a printf? Before a scanf?

I have a lot of problems with strings...
If somebody know any good document, please let me know where is...

Thank you.

[Flash Gordon]

Olaf "El Blanco" wrote:

Where is exactly neccesary to put "rewind(stdin)" ?
After a printf? Before a scanf?
I've never needed to put it anywhere. I have, on the other hand, needed
to use fflush(stdout) before waiting for input.
I have a lot of problems with strings...
If somebody know any good document, please let me know where is...

The comp.lang.c FAQ http://c-faq.com/
K&R2 (see the FAQ for the full reference).
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc

[Richard Heathfield]

Olaf "El Blanco" said:

Where is exactly neccesary to put "rewind(stdin)" ?
I've never done that in my life.

Think about what it would mean.

puts("Dear user: I know you have just spent the last twenty");
puts("minutes typing in all that stuff, and I thank you for");
puts("it - but now I'd like you to do it all again, EXACTLY");
puts("THE SAME as you did it last time.");

rewind(stdin);

I don't see that program lasting through to a second run.
After a printf? Before a scanf?
Neither.
I have a lot of problems with strings...
If somebody know any good document, please let me know where is...

The best way to capture text input is either a single character at a time,
ONLY, or a line at a time, ONLY. One or the other, and which one you choose
depends on what you're doing.

I've put a little essay together on the subject of capturing text data a
line at a time, and you can find it at:

<http://www.cpax.org.uk/prg/writings/fgetdata.php>

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)

[Keith Thompson]

Richard Heathfield <in*****@invali d.invalid> writes:

Olaf "El Blanco" said:

Where is exactly neccesary to put "rewind(stdin)" ?

I've never done that in my life.

Think about what it would mean.

puts("Dear user: I know you have just spent the last twenty");
puts("minutes typing in all that stuff, and I thank you for");
puts("it - but now I'd like you to do it all again, EXACTLY");
puts("THE SAME as you did it last time.");

rewind(stdin);

I don't see that program lasting through to a second run.

[...]

If stdin is coming from an interactive device, rewind()ing it doesn't
make much sense. But if it's coming from, say, a disk file, then
rewind() *might* be a sensible thing to do.

However, the source from which stdin receives its input is generally
outside the control of the program. The rewind() function has its
uses, but if you're going to need to rewind() something, it should
probably be a named file (opened with fopen()), not stdin.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Richard Tobin]

In article <ln************ @nuthaus.mib.or g>,
Keith Thompson <ks***@mib.or g> wrote:

However, the source from which stdin receives its input is generally
outside the control of the program. The rewind() function has its
uses, but if you're going to need to rewind() something, it should
probably be a named file (opened with fopen()), not stdin.

I find this unconvincing. The program has no control over whether
stdin is rewindable, but equally it has no control over whether stdin
is a file containing the right data, or even data in the right format.
It has no control over whether it's a file that it makes sense to open
in text mode. And on the other hand on many modern operating systems
it has no control over whether a named file is rewindable, especially
if the file name is not built in to the program. About the only way
to be sure a file is rewindable is for the program to create it
itself!

If you want to make your program robust against non-rewindable (or
non-fseekable) files, whether named or not, you need to use some
mechanism outside the C standard. For example in unix you might
test a file with stat() and copy it to a temporary file if necessary.

-- Richard

[Keith Thompson]

ri*****@cogsci. ed.ac.uk (Richard Tobin) writes:

In article <ln************ @nuthaus.mib.or g>,
Keith Thompson <ks***@mib.or g> wrote:

However, the source from which stdin receives its input is generally
outside the control of the program. The rewind() function has its
uses, but if you're going to need to rewind() something, it should
probably be a named file (opened with fopen()), not stdin.

I find this unconvincing. The program has no control over whether
stdin is rewindable, but equally it has no control over whether stdin
is a file containing the right data, or even data in the right format.
It has no control over whether it's a file that it makes sense to open
in text mode. And on the other hand on many modern operating systems
it has no control over whether a named file is rewindable, especially
if the file name is not built in to the program. About the only way
to be sure a file is rewindable is for the program to create it
itself!

If you want to make your program robust against non-rewindable (or
non-fseekable) files, whether named or not, you need to use some
mechanism outside the C standard. For example in unix you might
test a file with stat() and copy it to a temporary file if necessary.

I suppose it's a matter of style. There's nothing illegal about
rewind(stdin), but in practice it's more likely to fail than
rewind(some_fil e_I_opened_expl icitly).

I would consider it poor style to attempt to call rewind(stdin). In
normal usage, my expectation is that a program will read its standard
input from beginning to end, and will not attempt to do any
re-positioning.

I was about to mention that you should, of course, always check
whether rewind() was successful -- but I see that you can't do that.
rewind(stream) is equivalent to (void)fseek(str eam, 0L, SEEK_SET),
except that it clears the error indicator for the stream. In other
words, it deliberately ignores any errors.

Why was rewind() defined this way?

Based on that, I think I'd avoid using rewind() at all.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Richard Tobin]

In article <ln************ @nuthaus.mib.or g>,
Keith Thompson <ks***@mib.or g> wrote:

In
normal usage, my expectation is that a program will read its standard
input from beginning to end, and will not attempt to do any
re-positioning.

I suppose expectations vary :-) I regard standard input as a natural
way to provide a program's primary input, regardless of how it is used
(I usually follow the unix convention of using stdin if no file name
is given on the command line).

Rewinding standard input in C has a long history. Here is an amusing
example predating the modern standard i/o library:

http://minnie.tuhs.org/UnixTree/V5/u...s1/goto.c.html

-- Richard

[CBFalconer]

Keith Thompson wrote:

Richard Heathfield <in*****@invali d.invalid> writes:

Olaf "El Blanco" said:

Where is exactly neccesary to put "rewind(stdin)" ?

I've never done that in my life.

Think about what it would mean.

puts("Dear user: I know you have just spent the last twenty");
puts("minutes typing in all that stuff, and I thank you for");
puts("it - but now I'd like you to do it all again, EXACTLY");
puts("THE SAME as you did it last time.");

rewind(stdin);

I don't see that program lasting through to a second run.

[...]

If stdin is coming from an interactive device, rewind()ing it
doesn't make much sense. But if it's coming from, say, a disk
file, then rewind() *might* be a sensible thing to do.

However, the source from which stdin receives its input is
generally outside the control of the program. The rewind()
function has its uses, but if you're going to need to rewind()
something, it should probably be a named file (opened with
fopen()), not stdin.

And if the systems works the way I think it should work, then
rewind will fail when stdin is interactive, and work when stdin has
been redirected to a disk file.

--
"If you want to post a followup via groups.google.c om, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers." - Keith Thompson
More details at: <http://cfaj.freeshell. org/google/>
Also see <http://www.safalra.com/special/googlegroupsrep ly/>

[Keith Thompson]

CBFalconer <cb********@yah oo.com> writes:

Keith Thompson wrote:

[...]

If stdin is coming from an interactive device, rewind()ing it
doesn't make much sense. But if it's coming from, say, a disk
file, then rewind() *might* be a sensible thing to do.

However, the source from which stdin receives its input is
generally outside the control of the program. The rewind()
function has its uses, but if you're going to need to rewind()
something, it should probably be a named file (opened with
fopen()), not stdin.

And if the systems works the way I think it should work, then
rewind will fail when stdin is interactive, and work when stdin has
been redirected to a disk file.

Except that rewind() has no mechanism for telling you that it failed.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Jordan Abel]

On 2006-05-24, Keith Thompson <ks***@mib.or g> wrote:

Richard Heathfield <in*****@invali d.invalid> writes:

Olaf "El Blanco" said:

Where is exactly neccesary to put "rewind(stdin)" ?

I've never done that in my life.

Think about what it would mean.

puts("Dear user: I know you have just spent the last twenty");
puts("minutes typing in all that stuff, and I thank you for");
puts("it - but now I'd like you to do it all again, EXACTLY");
puts("THE SAME as you did it last time.");

rewind(stdin);

I don't see that program lasting through to a second run.

[...]

If stdin is coming from an interactive device, rewind()ing it doesn't
make much sense. But if it's coming from, say, a disk file, then
rewind() *might* be a sensible thing to do.

However, the source from which stdin receives its input is generally
outside the control of the program. The rewind() function has its
uses, but if you're going to need to rewind() something, it should
probably be a named file (opened with fopen()), not stdin.

You could attempt to rewind (or seek) and complain loudly when it fails.