Re: inside scanf [trap rep]

temper3243@xxxxxxxxx wrote:
On Jan 31, 11:59 am, CBFalconer <cbfalco...@xxxxxxxxx> wrote:
temper3...@xxxxxxxxx wrote:

Can someone explain me what is happening below ? Why is it printing
401380
$ cat scanf.c
#include<stdio.h>
int main()
{
int i;
scanf(" %d",&i);
printf("%x\n",i);
return 0;
}
prog@cygwin ~
$ ./a.exe
-1
ffffffff
prog@cygwin ~
$ ./a.exe
--1
401380
Why don't you give it an integer for input? Usual sin - failure to
check the return value from scanf.

Few more questions
1) Can a variable contain trap representation(trap rep) say a local
variable like int i;

Yes

2) Can a pointer contain trap rep ? like say int *p, can p be a trap
rep.

Yes

3) Can address of an uninitialized variable every contain trap rep ?
i.e can "&i" contain trap rep

No

4) Can we get to assign trap rep "on a 2's complement machine" ? i.e
int i= value_of_traprep;

No, as far as C is concerned, a variable containing a trap
representation doesn't have a value. Just attempting to do the above
initialization invokes undefined behavior.

printf(" %d",i); will it abort ;
5) On a 1's complement machine is trap rep always ~0 or is it
different.

There is no way a conforming program can know this.

When ~0 is trap rep,
what happens,
int i=~0,
printf(" %d",i); will it abort ;

6) How do i get what is the value for trap rep on my arch (i386) .

Not every platform has trap representations, you just have to be aware
that they *might*. Essentially, your questions come down to : "What
happens when I invoke undefined behavior?". The answer is: "anything or
nothing."


--
Clark S. Cox III
clarkcox3@xxxxxxxxx
.

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