RE: [Dev-C++] Question about pointers (beginner)

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

Since ptr_int is unitialized at the first assign (*ptr_int = 456), it is
completely random - ur op to the compiler - to decide what happens.

In most cases, the ptr_int will be piont at whatever value happens to be
stored at that location in the stack when main() is called. For some
compilers - such as MSVC - a compile build might initialize auto
(stack-based) variables to magic values, just to more easily follow and
detect these magic values at other parts of the program. It works
reasonably well but not perfect since the magic values are not reserved,
but normal integer values and might be used for real in some programs.

Because ptr_int is uninitialized, sometimes the program works, sometimes
it gives wrong answers, sometimes it explodes big time.

/Per W

On Tue, 27 May 2003, Christian Schmidt wrote:

> I tried the code below with a standard DevCPP and MSVC++.
>
> DevCPP just compiles and runs it and gives 456 for *ptr_int even before its
> initialized.
> MSVC++ 6 compiles it but gives an warning that the pointer hasn't been
> initialized. When I try to run it, it crashes.
>
> The code:
>
> #include <stdio.h>
>
> main ( )
> {
>   int x, *ptr_int;
>
>   *ptr_int = 456;
>
>   printf ("The value of ptr_int is %d\n",ptr_int);
>   printf ("The value of ptr_int is %p\n",ptr_int);
>   printf ("The value of *ptr_int is %d\n",*ptr_int);
>   printf ("The value of &ptr_int is %p\n",&ptr_int);
>
>   x = 123;
>
>   ptr_int = &x;
>
>   printf ("The value of ptr_int is %p\n",ptr_int);
>   printf ("The value of *ptr_int is %d\n",*ptr_int);
>   printf ("The value of x is %d\n",x);
>   printf ("The value of &x is %p\n",&x);
>
> }
>
> > -----Original Message-----
> > From: dev...@li...
> > [mailto:dev...@li...]On Behalf Of Khalid
> > El-Kary
> > Sent: dinsdag 27 mei 2003 21:14
> > To: Dev...@li...
> > Subject: Re: [Dev-C++] Question about pointers (beginner)
> >
> >
> > Did you try that code with any compiler, the line *ptr_int = 456; should
> > give a segmentation fault!
> > given that there's isn't a code you missed from the book :-)
> >
> > >From my book:
> > >
> > >Given x = 123 and ptr_int = &x after that *ptr_int = 456 has
> > been executed.
> > >What is the value of x?
> > >
> > >I thought it was 123 as the pointer *ptr_int is a null pointer and can't
> > >have any value.
> > >
> > >This is how I see it:
> > >
> > >main ( )
> > >{
> > >int x, *ptr_int;
> > >
> > >*ptr_int = 456; /*This pointer is a null pointer at this moment right? */
> > >
> > >x =123;
> > >ptr_int = &x; /* As x is just set to 123 and the statement
> > *ptr_int = 456;
> > >was executed before I would think *ptr_int would be 123 */
> > >
> > >}
> > >
> > >----------
> > >
> > >But the books says the value of *ptr_int at the end is 456. How come??
> > >
> > >Thanks for your time,
> > >
> > >Christian
> > >
> > >
> > >
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