dev-cpp-users

[Dev-C++] stdin reset...

[<ca...@kr...>]

hi dev-cpp-users,

I try the following code:

<snip>
#include <stdio.h>

int main(int argc, char** argv)
{
    char buffer[5];
    while(1)
    {
        printf("prompt > ");
        scanf("%4[^\n]",buffer);
    
        printf("you typed: %s\n", buffer);
        printf("you typed: %d %d %d %d %d\n", buffer[0], buffer[1], buffer[2], buffer[3], buffer[4]);
    }

    return 0;
}
</snap>

but it obviously doesn't work.
I want to emulate a console behaviour.
so that whenever you press <ENTER>
-the just typed letters (only 4 of them) get placed in buffer (with a
terminating 0)
-buffer gets printed
-buffer gets cleared
-stdin gets cleared
-new prompt waiting for input ()

how can i do that?

gruss
/Christian                   mailto:ca...@kr...

---


I propose that the following character sequence for joke markers:

:-)

                             19-Sep-82 11:44    Scott E  Fahlman

Re: [Dev-C++] stdin reset...

[<ca...@kr...>]

> -stdin gets cleared
> how can i do that?

fflush(stdin);

just found out how.. thanks anyway!
 
gruss
/Christian                   mailto:ca...@kr...

---


I propose that the following character sequence for joke markers:

:-)

                             19-Sep-82 11:44    Scott E  Fahlman

RE: [Dev-C++] stdin reset...

[Ioannis V. <iv...@at...>]

> -----Original Message-----
> From: dev...@li...=20
> [mailto:dev...@li...] On Behalf=20
> Of Christian Sch=E4fer
> Sent: Friday, April 25, 2003 3:30 PM
> To: dev...@li...
> Subject: Re: [Dev-C++] stdin reset...
>=20
>=20
> > -stdin gets cleared
> > how can i do that?
>=20
> fflush(stdin);
>=20
> just found out how.. thanks anyway!


Which is not safe. getchar(), fgets(), etc, should be used instead,




Ioannis Vranos
=20
* Programming pages: http://www.noicys.freeurl.com
* Alternative URL 1: http://run.to/noicys
* Alternative URL 2: http://www.noicys.cjb.net


Anti-racism movement: http://www.positive-youthfoundation.org
Iraq Peace Pledge: http://www.peacepledge.org

Re[2]: [Dev-C++] stdin reset...

[<ca...@kr...>]

hi Ioannis,

>> > -stdin gets cleared
>> > how can i do that?
>> 
>> fflush(stdin);
> Which is not safe. getchar(), fgets(), etc, should be used instead,

and why is that?
I am very interested in code security issues but at least i want to
understand instead of obey them. so what exactly is considered
'unsafe'?

what I need is a secure way to read as much as there is from stdin
undtil '\n' into a char* buffer. I want that buffer to match the
needed length (I don't know how to do that, since I don't know the
exact needed length until i read it from stdin...) and I need to avoid
buffer overflows (of course).
I thought scanf() is a good way, since it reads only a specified
number of characters right? (ok, this is just a workaround, I guess..)

I don't think this is a unique problem so there must be a known way to
solve it, is there?
 
gruss
/Christian                   mailto:ca...@kr...

---


I propose that the following character sequence for joke markers:

:-)

                             19-Sep-82 11:44    Scott E  Fahlman

RES: Re[2]: [Dev-C++] stdin reset...

[Daniel K. O. <dan...@ya...>]

 >  >> fflush(stdin);
 >  > Which is not safe. getchar(), fgets(), etc, should be used instead,
 >
 >  and why is that?
 >  I am very interested in code security issues but at least i want to
 >  understand instead of obey them. so what exactly is considered
 >  'unsafe'?



The same reason you shouldn't fflush(2.5), fflush("some string") nor
fflush(a, b, c), you shouldn't fflush(stdin); the standard says that
"fflush()" is for output streams; not input streams, not strings, not
anything else; the non-standard way can work on a particular system, but it
can kill some lives on others.



 >  what I need is a secure way to read as much as there is from stdin
 >  undtil '\n' into a char* buffer. I want that buffer to match the
 >  needed length (I don't know how to do that, since I don't know the
 >  exact needed length until i read it from stdin...) and I need to avoid
 >  buffer overflows (of course).
 >  I thought scanf() is a good way, since it reads only a specified
 >  number of characters right? (ok, this is just a workaround, I guess..)


Try fgets(), then sscanf().


 >  I don't think this is a unique problem so there must be a known way to
 >  solve it, is there?




Suggestion (for all you C programmers):

The comp.lang.c answers to frequently asked questions:
http://www.eskimo.com/~scs/C-faq/top.html







Daniel K. O.

RE: Re[2]: [Dev-C++] stdin reset...

[Ioannis V. <iv...@at...>]

> -----Original Message-----
> From: Christian Sch=E4fer [mailto:ca...@kr...]=20
> Sent: Friday, April 25, 2003 11:55 PM
> To: dev...@li...; Ioannis Vranos
> Cc: dev...@li...
> Subject: Re[2]: [Dev-C++] stdin reset...
>=20
>=20
> and why is that?
> I am very interested in code security issues but at least i want to
> understand instead of obey them. so what exactly is considered
> 'unsafe'?
>=20
> what I need is a secure way to read as much as there is from stdin
> undtil '\n' into a char* buffer. I want that buffer to match the
> needed length (I don't know how to do that, since I don't know the
> exact needed length until i read it from stdin...) and I need to avoid
> buffer overflows (of course).
> I thought scanf() is a good way, since it reads only a specified
> number of characters right? (ok, this is just a workaround, I guess..)
>=20
> I don't think this is a unique problem so there must be a known way to
> solve it, is there?


fflush() is intended for output streams, not input. For example
fflush(stdin); does not work in GNU/Linux. So you can't expect
fflush(stdin); to be portable. You will have to use aither a plain
getchar(); call after a plain scanf() or create a clean function in the
style:


void clean_stdin()
{
  while(getchar()!=3D'\n')
    ;
}




Ioannis Vranos
=20
* Programming pages: http://www.noicys.freeurl.com
* Alternative URL 1: http://run.to/noicys
* Alternative URL 2: http://www.noicys.cjb.net


Anti-racism movement: http://www.positive-youthfoundation.org
Iraq Peace Pledge: http://www.peacepledge.org

Re[4]: [Dev-C++] stdin reset...

[<ca...@kr...>]

hi Ioannis,

> fflush() is intended for output streams, not input. For example
> fflush(stdin); does not work in GNU/Linux. So you can't expect
> fflush(stdin); to be portable. You will have to use aither a plain
> getchar(); call after a plain scanf() or create a clean function in the
> style:


> void clean_stdin()
> {
>   while(getchar()!='\n')
>     ;
> }

ok, I now understand why not to use fflush(). (I would have noticed
when porting to linux whats the final destination... ;)

so you mean, I scanf() whatever I need and then let getchar() go
through stdin again clearing everything until the first '\n', right?

ok then, that would solve the first halve of my problem.
for the second I open another thread. thanks a lot!

 
gruss
/Christian                   mailto:ca...@kr...

---


I propose that the following character sequence for joke markers:

:-)

                             19-Sep-82 11:44    Scott E  Fahlman

Re: [Dev-C++] stdin reset...

[philip M. <ph...@co...>]

You could have written:

        printf("prompt > ");
        scanf("%4s", buffer);
        while (fgetc(stdin) != '\n');

> I try the following code:
>
> <snip>
> #include <stdio.h>
>
> int main(int argc, char** argv)
> {
>     char buffer[5];
>     while(1)
>     {
>         printf("prompt > ");
>         scanf("%4[^\n]",buffer);
>
>         printf("you typed: %s\n", buffer);
>         printf("you typed: %d %d %d %d %d\n", buffer[0], buffer[1],
buffer[2], buffer[3], buffer[4]);
>     }
>
>     return 0;
> }
> </snap>

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