malloced union member

[rohit]

Hi,

Iam confused as to when is the memory freed in this program.

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

union test{
char *i;
char *ch;
};

int main()
{
union test test;
test.ch = malloc(6);
strcpy(test.ch, "hello");
printf("test.i == %s test.ch == %s test.i == [%p] test.ch ==
[%p]\n",test.i,test .ch,test.i,test .ch);
free(test.i);
printf("test.i == %s test.ch == %s test.i == [%p] test.ch ==
[%p]\n",test.i,test .ch,test.i,test .ch);
free(test.ch);
printf("test.i == %s test.ch == %s test.i == [%p] test.ch ==
[%p]\n",test.i,test .ch,test.i,test .ch);

return 0;
}

And the output I get from my solaris box is :
test.i == hello test.ch == hello test.i == [209b8] test.ch == [209b8]
test.i == hello test.ch == hello test.i == [209b8] test.ch == [209b8]
test.i == hello test.ch == hello test.i == [209b8] test.ch == [209b8]

regards
rohitash

[Richard Bos]

"Stephen L." <sd*********@ca st-com.net> wrote:

Richard Bos wrote:

"Stephen L." <sd*********@ca st-com.net> wrote:

rohit wrote:
> printf("test.i == %s test.ch == %s test.i == [%p] test.ch ==
> [%p]\n",test.i,test .ch,test.i,test .ch);
> free(test.ch);

You got lucky here. You've `free()'d the same
pointer value _twice_. You've corrupted your heap.
You don't know this. It invokes undefined behaviour; this _may_ mean
corrupting his heap, but it may also mean ignoring the statement,
crashing with a segmentation fault, or mailing his resume to ...

man malloc(3c) for details.

Your particular system's manual pages are immaterial. This is
comp.lang.c, not comp.lang.steph en-l-s-system.manpage. c. Just because
_your_ system defines that using free() on an already free()d pointer
corrupts "your heap", that doesn't mean you can assume all systems do.
Posters who have identified this behavioral
aspect of `free()' have usually noted it by stating
that their (next) `malloc()'/`free()' core dumps.
Note: _usually_.
"Undefined results will occur if the space assigned by
malloc() is overrun or if some random number is
passed to free()."
_Exactly_. _Undefined_ results. _Not_ "the heap will be corrupted".
It _doesn't_ say, "and BTW, the heap is fine."
Neither does it say "and the heap will be corrupted", but that is what
you claimed would happen. It _may_ happen. It _may_ also mail a love
letter to your supervisor.
page(s) which would bring me to any of the
conclusions you've arrived at - why are posters
trying to DEFINE undefined behavior?
I don't know. _You_ are the one trying to do so, by claiming that it
_will_ corrupt the heap. I merely pointed out that undefined behaviour
means that you don't even know that this is true.
But providing garbage answers like "mailing his
resume to ..." really hurt the credibility of
the group as a whole, IMHO.

I'm so sorry. Could you please provide a quote from the Standard (note:
_not_ your system-specific man pages) which shows me the error of my
ways, by stating that undefined behaviour may _not_ result in a resume
being mailed?

Richard