dev-cpp-users

[Dev-C++] scanf with gets

[<and...@su...>]

Dear programmers,

I´m having to use the fflush command in the following code:

#include <stdio.h>
#include <stdlib.h>

int main()
{
   char name[10];
   int a, b, c;

   printf("Enter a-->");
   scanf("%d", &a);
   printf("Enter some name-->");
   fflush(stdin);
   gets(name);
   printf("Enter com b-->");
   scanf("%d", &b);
   printf("The sum of %d with %d is %d\n", a, b, a+b);
   printf("And the name typed was %s\n", name);
   system("PAUSE");
   return 0;
}

The reason is that if I don´t put it, the program doesn´t wait for the name
to be typed via keyboard. It passes directly to the other operator, b,
indicating that the input buffer is loaded with something. I made some
experiments with short int (hd), but nothing solved.

1-Can anyone explain what is happening?
2-What is the solution for don´t use the fflush?

Regards
Andre

Re: [Dev-C++] scanf with gets

[Ricardo R. <ri...@un...>]

scanf ( ) doesnot read the 'return' char,
then it stays in the input buffer until the next reading.
Two options for the next sentence:
(1) If you read in the next sentence with gets( )
the 'return' char is transformed in the
empty char string:  " " = '\0' .
(2) If you read a char in the next sentence with scanf( )
the 'return' char it is transformed in the 'line jump'='\n'.

In consequence, if you dont want any strange/side effect in your input
the best option I know after an entry with scanf( ) it's to free the 
input buffer:

             fflush(stdin).


Any other explanation is wellcome,
but I hope it can help you,
Ricardo.

Re: [Dev-C++] scanf with gets

[Per W. <pw...@ia...>]

But fflush() is not portable for input streams!

It is only indended for flushing output streams. On a Linux, it would
return EOF and set errno to EBADF if the stream isn't opened for output.

/Per W

On Fri, 11 Apr 2003, Ricardo L=F3pez Ruiz wrote:

> scanf ( ) doesnot read the 'return' char,
> then it stays in the input buffer until the next reading.
> Two options for the next sentence:
> (1) If you read in the next sentence with gets( )
> the 'return' char is transformed in the
> empty char string:  " " =3D '\0' .
> (2) If you read a char in the next sentence with scanf( )
> the 'return' char it is transformed in the 'line jump'=3D'\n'.
>
> In consequence, if you dont want any strange/side effect in your input
> the best option I know after an entry with scanf( ) it's to free the
> input buffer:
>
>              fflush(stdin).
>
>
> Any other explanation is wellcome,
> but I hope it can help you,
> Ricardo.
>
>
>
> -------------------------------------------------------
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RE: [Dev-C++] scanf with gets

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

> -----Original Message-----
> From: dev...@li... 
> [mailto:dev...@li...] On Behalf 
> Of Andrι Macαrio Barros
> Sent: Friday, April 11, 2003 8:34 AM
> To: Dev-Cpp-Users
> Subject: [Dev-C++] scanf with gets
> 
> 
> Dear programmers,
> 
> I´m having to use the fflush command in the following code:
> 
> #include <stdio.h>
> #include <stdlib.h>
> 
> int main()
> {
>    char name[10];
>    int a, b, c;
> 
>    printf("Enter a-->");
>    scanf("%d", &a);
>    printf("Enter some name-->");
>    fflush(stdin);

The  behaviour of the above is undefined (e.g. the airplane may crash).
fflush() is intended to be used on output streams.

You can simply do:

getchar();



>    gets(name);


Extremelly Dangerous for buffer overflow (that is you must trust the user
very well, and he user must know the internals of your code). A better one:

fgets(name,10,stdin);





>    printf("Enter com b-->");
>    scanf("%d", &b);
>    printf("The sum of %d with %d is %d\n", a, b, a+b);
>    printf("And the name typed was %s\n", name);
>    system("PAUSE");
>    return 0;
> }
> 
> The reason is that if I don´t put it, the program doesn´t 
> wait for the name
> to be typed via keyboard. It passes directly to the other operator, b,
> indicating that the input buffer is loaded with something. I made some
> experiments with short int (hd), but nothing solved.
> 
> 1-Can anyone explain what is happening?



scanf() leaves the '\n' on the stream. Then gets() was reading and
interpreting it as if you entered an empty string.



> 2-What is the solution for don´t use the fflush?


getchar(); or something. You can even do a function in the style:


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



And use it. If you are going to use it with stream redirection in the style


myprogram <whatever.txt


for example,

you have to get EOF into account.





Ioannis Vranos
 
* 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: [Dev-C++] scanf with gets

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

> -----Original Message-----
> From: dev...@li...=20
> [mailto:dev...@li...] On Behalf=20
> Of Andr? Macario Barros
> Sent: Friday, April 11, 2003 8:34 AM
> To: Dev-Cpp-Users
> Subject: [Dev-C++] scanf with gets
>=20
>=20
> Dear programmers,
>=20
> I=B4m having to use the fflush command in the following code:
>=20
> #include <stdio.h>
> #include <stdlib.h>
>=20
> int main()
> {
>    char name[10];
>    int a, b, c;
>=20
>    printf("Enter a-->");
>    scanf("%d", &a);
>    printf("Enter some name-->");
>    fflush(stdin);

The  behaviour of the above is undefined (e.g. the airplane may crash).
fflush() is intended to be used on output streams.

You can simply do:

getchar();



>    gets(name);


Extremelly Dangerous for buffer overflow (that is you must trust the =
user
very well, and he user must know the internals of your code). A better =
one:

fgets(name,10,stdin);





>    printf("Enter com b-->");
>    scanf("%d", &b);
>    printf("The sum of %d with %d is %d\n", a, b, a+b);
>    printf("And the name typed was %s\n", name);
>    system("PAUSE");
>    return 0;
> }
>=20
> The reason is that if I don=B4t put it, the program doesn=B4t=20
> wait for the name
> to be typed via keyboard. It passes directly to the other operator, b,
> indicating that the input buffer is loaded with something. I made some
> experiments with short int (hd), but nothing solved.
>=20
> 1-Can anyone explain what is happening?



scanf() leaves the '\n' on the stream. Then gets() was reading and
interpreting it as if you entered an empty string.



> 2-What is the solution for don=B4t use the fflush?


getchar(); or something. You can even do a function in the style:


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



And use it. If you are going to use it with stream redirection in the =
style


myprogram <whatever.txt


for example,

you have to get EOF into account.





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: [Dev-C++] scanf with gets

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

> -----Original Message-----
> From: dev...@li...=20
> [mailto:dev...@li...] On Behalf=20
> Of Andr? Macario Barros
> Sent: Friday, April 11, 2003 8:34 AM
> To: Dev-Cpp-Users
> Subject: [Dev-C++] scanf with gets
>=20
>=20
> Dear programmers,
>=20
> I=B4m having to use the fflush command in the following code:
>=20
> #include <stdio.h>
> #include <stdlib.h>
>=20
> int main()
> {
>    char name[10];
>    int a, b, c;
>=20
>    printf("Enter a-->");
>    scanf("%d", &a);
>    printf("Enter some name-->");
>    fflush(stdin);

The  behaviour of the above is undefined (e.g. the airplane may crash).
fflush() is intended to be used on output streams.

You can simply do:

getchar();



>    gets(name);


Extremelly Dangerous for buffer overflow (that is you must trust the =
user
very well, and he user must know the internals of your code). A better =
one:

fgets(name,10,stdin);





>    printf("Enter com b-->");
>    scanf("%d", &b);
>    printf("The sum of %d with %d is %d\n", a, b, a+b);
>    printf("And the name typed was %s\n", name);
>    system("PAUSE");
>    return 0;
> }
>=20
> The reason is that if I don=B4t put it, the program doesn=B4t=20
> wait for the name
> to be typed via keyboard. It passes directly to the other operator, b,
> indicating that the input buffer is loaded with something. I made some
> experiments with short int (hd), but nothing solved.
>=20
> 1-Can anyone explain what is happening?



scanf() leaves the '\n' on the stream. Then gets() was reading and
interpreting it as if you entered an empty string.



> 2-What is the solution for don=B4t use the fflush?


getchar(); or something. You can even do a function in the style:


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



And use it. If you are going to use it with stream redirection in the =
style


myprogram <whatever.txt


for example,

you have to get EOF into account.





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