Re: assigning pointer to NULL

santosh wrote, On 31/01/08 08:20:
Ian Collins wrote:

Joachim Schmitz wrote:
user923005 wrote:
On Jan 30, 1:01 am, r...@xxxxxxxxxxxxxxxxxxxxxx (Richard Bos) wrote:
<snip>
P.S.
I almost always set freed pointers to null myself (the exception
being in C++ because it is pointless to set a deleted class member
pointer to null in a destructor.)
It helps with a very small class of problems (but does have one bad
side effect -- double frees are less likely to be detected)
But they won't harm (i.e. possibly crash the program) anymore either.

But the effect can be detrimental on a system (which includes most
desktop environments) where the allocator can detect duplicate frees.

For that reason, I never set a freed pointer to NULL.

Same here on the basis that, in C at least, the programmer *must* always
be aware of which pointers have valid or invalid values at all points
in his program. Anything short of this is going to lead to bugs. From
this P.O.V. setting dangling pointers to NULL is redundant.

In some situations setting pointers to NULL can be useful and used to indicate whether it is valid or not. For example, I have some code of this general form...

char *ptr = NULL;

/* do stuff */

if (cond1) {
ptr = malloc(something);
/* do stuff with ptr */
if (cond2) {
free(ptr);
ptr = NULL;
}
}
/* do stuff not using ptr */
if (cond3) {
/* do stuff using ptr */
free(ptr);
}


Note that the code deliberately has only one pointer to the malloc'd block.
--
Flash Gordon
.

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