What compiler is doing when we pass unnecessary parameters in scanf

What compiler is doing when we pass unnecessary parameters in scanf

[RAM_LOCK]

Hi,
In the second scenario what value is it printing when i have given extra
parameter in scanf?
Does it vary from compiler to compiler?

Scenario : I
-------------
root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> cat simple-interest.c
#include <stdio.h>

void main ()
{
        int p;
        float i=0;
        printf ("enter the principal amount\n");
        scanf ("%d",&p);
        i = (p*5*5)/100;
        printf ("Interterest is : %f\n",i);
}
root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
enter the principal amount
100
Interterest is : 25.000000


Scenario : II
-------------
> cat simple-interest.c
#include <stdio.h>

void main ()
{
        int p;
        float i=0;
        printf ("enter the principal amount\n");
        scanf ("p:%d",&p);
        i = (p*5*5)/100;
        printf ("Interterest is : %f\n",i);
}
root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
enter the principal amount
100
Interterest is : -9321198.000000

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Fwd: What compiler is doing when we pass unnecessary parameters in scanf

[Aneesh Bhasin]

----forgot to add list as cc------

Hi Kaushik,

any extra non-whitespace, non "%" characters are matched against the
input given from stdin - if they match, it is ignored and if it
doesn't match, no further input is taken.

In case of your second scenario, when you specify "p:%d" as string,
the program expects the input to be of the format "p:<some number>"
but since you give 100 as the input, no value is assigned to the
variable p (you can check the return value of scanf - its zero) and
hence, whatever junk value is in variable p, it is used for further
calculation.

With the second scenario, try giving the input as "p:100" and then p
should get value 100 as expected...

Hope that helps.
Aneesh

On Wed, Jul 29, 2009 at 7:48 PM, RAM_LOCK <Kaushik.Sarkar@netapp.com> wrote:
>
> Hi,
> In the second scenario what value is it printing when i have given extra
> parameter in scanf?
> Does it vary from compiler to compiler?
>
> Scenario : I
> -------------
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> cat simple-interest.c
> #include <stdio.h>
>
> void main ()
> {
>        int p;
>        float i=0;
>        printf ("enter the principal amount\n");
>        scanf ("%d",&p);
>        i = (p*5*5)/100;
>        printf ("Interterest is : %f\n",i);
> }
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
> enter the principal amount
> 100
> Interterest is : 25.000000
>
>
> Scenario : II
> -------------
> > cat simple-interest.c
> #include <stdio.h>
>
> void main ()
> {
>        int p;
>        float i=0;
>        printf ("enter the principal amount\n");
>        scanf ("p:%d",&p);
>        i = (p*5*5)/100;
>        printf ("Interterest is : %f\n",i);
> }
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
> enter the principal amount
> 100
> Interterest is : -9321198.000000
>
> --
> View this message in context: http://www.nabble.com/What-compiler-is-doing-when-we-pass-unnecessary-parameters-in-scanf-tp24719839p24719839.html
> Sent from the linux-c-programming mailing list archive at Nabble.com.
>
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Re: What compiler is doing when we pass unnecessary parameters in scanf

[Glynn Clements]

RAM_LOCK wrote:

> In the second scenario what value is it printing when i have given extra
> parameter in scanf?
> Does it vary from compiler to compiler?

>         scanf ("p:%d",&p);
>         i = (p*5*5)/100;
>         printf ("Interterest is : %f\n",i);
> }
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
> enter the principal amount
> 100
> Interterest is : -9321198.000000

The p: isn't matched in the input, so scanf() skips the following %d
conversion, leaving p unchanged. So p will contain garbage (i.e. 
whatever value happened to be stored in that memory location already).

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

Re: What compiler is doing when we pass unnecessary parameters in scanf

[shiva kumar]

behavior of this is undefined just do man scanf

RAM_LOCK wrote:
> 
> Hi,
> In the second scenario what value is it printing when i have given extra
> parameter in scanf?
> Does it vary from compiler to compiler?
> 
> Scenario : I
> -------------
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> cat simple-interest.c
> #include <stdio.h>
> 
> void main ()
> {
>         int p;
>         float i=0;
>         printf ("enter the principal amount\n");
>         scanf ("%d",&p);
>         i = (p*5*5)/100;
>         printf ("Interterest is : %f\n",i);
> }
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
> enter the principal amount
> 100
> Interterest is : 25.000000
> 
> 
> Scenario : II
> -------------
>> cat simple-interest.c
> #include <stdio.h>
> 
> void main ()
> {
>         int p;
>         float i=0;
>         printf ("enter the principal amount\n");
>         scanf ("p:%d",&p);
>         i = (p*5*5)/100;
>         printf ("Interterest is : %f\n",i);
> }
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
> enter the principal amount
> 100
> Interterest is : -9321198.000000
> 
> 

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RE: What compiler is doing when we pass unnecessary parameters in scanf

[Trevor Woollacott]

> -----Original Message-----
> From: linux-c-programming-owner@vger.kernel.org [mailto:linux-c-
> programming-owner@vger.kernel.org] On Behalf Of RAM_LOCK
> Sent: Wednesday, 29 July 2009 04:19 PM
> To: linux-c-programming@vger.kernel.org
> Subject: What compiler is doing when we pass unnecessary parameters in
> scanf
>
>
> Hi,
> In the second scenario what value is it printing when i have given
> extra
> parameter in scanf?
> Does it vary from compiler to compiler?
>
> Scenario : I
> -------------
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> cat simple-interest.c
> #include <stdio.h>
>
> void main ()
> {
>         int p;
>         float i=0;
>         printf ("enter the principal amount\n");
>         scanf ("%d",&p);
>         i = (p*5*5)/100;
>         printf ("Interterest is : %f\n",i);
> }
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
> enter the principal amount
> 100
> Interterest is : 25.000000
>
>
> Scenario : II
> -------------
> > cat simple-interest.c
> #include <stdio.h>
>
> void main ()
> {
>         int p;
>         float i=0;
>         printf ("enter the principal amount\n");
>         scanf ("p:%d",&p);
>         i = (p*5*5)/100;
>         printf ("Interterest is : %f\n",i);
> }
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
> enter the principal amount
> 100
> Interterest is : -9321198.000000
>
> --
> View this message in context: http://www.nabble.com/What-compiler-is-
> doing-when-we-pass-unnecessary-parameters-in-scanf-
> tp24719839p24719839.html
> Sent from the linux-c-programming mailing list archive at Nabble.com.
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-c-
> programming" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Hi

With scenario II your input of 100 is not matching, so you end up with 
whatever value was stored at &p before the scanf.
i.e.

(gdb) r
Starting program: ./a.out

Breakpoint 1, main () at test.c:6
6             float i=0;
(gdb) p p
$4 = -37284792
(gdb) s
7             printf ("enter the principal amount\n");
(gdb) s
enter the principal amount
8             scanf ("p:%d",&p);
(gdb) s
100
9             i = (p*5*5)/100;
(gdb) p p
$6 = -37284792
(gdb) s
10            printf ("Interterest is  : %f\n",i); }
(gdb) s
Interterest is : -9321198.000000

Here you can see p = -37284792 before and after your scanf
Then i = (-37284792*5*5)/100 = -9321198.000000

If you check the value returned by scanf, you can determine how many input 
values were successfully matched. In the case where you entered 100 in 
scenario I, scanf would have returned 1, and in scenario II it would have 
returned 0.

Regards,
Trevor 

RE: What compiler is doing when we pass unnecessary parameters in scanf

[Sarkar, Kaushik]

Thanks Trevor for your details explaination...
It is clear to me now... thanks again for clearing this concept :)



-----Original Message-----
From: Trevor Woollacott [mailto:trevorw@pharos-avantgard.com] 
Sent: Wednesday, July 29, 2009 9:32 PM
To: linux-c-programming@vger.kernel.org; Sarkar, Kaushik
Subject: RE: What compiler is doing when we pass unnecessary parameters
in scanf

> -----Original Message-----
> From: linux-c-programming-owner@vger.kernel.org [mailto:linux-c-
> programming-owner@vger.kernel.org] On Behalf Of RAM_LOCK
> Sent: Wednesday, 29 July 2009 04:19 PM
> To: linux-c-programming@vger.kernel.org
> Subject: What compiler is doing when we pass unnecessary parameters in
> scanf
>
>
> Hi,
> In the second scenario what value is it printing when i have given
> extra
> parameter in scanf?
> Does it vary from compiler to compiler?
>
> Scenario : I
> -------------
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> cat simple-interest.c
> #include <stdio.h>
>
> void main ()
> {
>         int p;
>         float i=0;
>         printf ("enter the principal amount\n");
>         scanf ("%d",&p);
>         i = (p*5*5)/100;
>         printf ("Interterest is : %f\n",i);
> }
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
> enter the principal amount
> 100
> Interterest is : 25.000000
>
>
> Scenario : II
> -------------
> > cat simple-interest.c
> #include <stdio.h>
>
> void main ()
> {
>         int p;
>         float i=0;
>         printf ("enter the principal amount\n");
>         scanf ("p:%d",&p);
>         i = (p*5*5)/100;
>         printf ("Interterest is : %f\n",i);
> }
> root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
> enter the principal amount
> 100
> Interterest is : -9321198.000000
>
> --
> View this message in context: http://www.nabble.com/What-compiler-is-
> doing-when-we-pass-unnecessary-parameters-in-scanf-
> tp24719839p24719839.html
> Sent from the linux-c-programming mailing list archive at Nabble.com.
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-c-
> programming" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Hi

With scenario II your input of 100 is not matching, so you end up with 
whatever value was stored at &p before the scanf.
i.e.

(gdb) r
Starting program: ./a.out

Breakpoint 1, main () at test.c:6
6             float i=0;
(gdb) p p
$4 = -37284792
(gdb) s
7             printf ("enter the principal amount\n");
(gdb) s
enter the principal amount
8             scanf ("p:%d",&p);
(gdb) s
100
9             i = (p*5*5)/100;
(gdb) p p
$6 = -37284792
(gdb) s
10            printf ("Interterest is  : %f\n",i); }
(gdb) s
Interterest is : -9321198.000000

Here you can see p = -37284792 before and after your scanf
Then i = (-37284792*5*5)/100 = -9321198.000000

If you check the value returned by scanf, you can determine how many
input 
values were successfully matched. In the case where you entered 100 in 
scenario I, scanf would have returned 1, and in scenario II it would
have 
returned 0.

Regards,
Trevor 

What compiler is doing when we pass unnecessary parameters in scanf

[RAM_LOCK]

Hi,
In the second scenario what value is it printing when i have given extra
parameter in scanf?
Does it vary from compiler to compiler?

Scenario : I
-------------
root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> cat simple-interest.c
#include 

void main ()
{
        int p;
        float i=0;
        printf ("enter the principal amount\n");
        scanf ("%d",&p);
        i = (p*5*5)/100;
        printf ("Interterest is : %f\n",i);
}
root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
enter the principal amount
100
Interterest is : 25.000000


Scenario : II
-------------
> cat simple-interest.c
#include 

void main ()
{
        int p;
        float i=0;
        printf ("enter the principal amount\n");
        scanf ("p:%d",&p);
        i = (p*5*5)/100;
        printf ("Interterest is : %f\n",i);
}
root@kaushik_Fedora11 ~/C/LET_US_C/ch-1> ./a.out
enter the principal amount
100
Interterest is : -9321198.000000

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