faq why use p=NULL?

[fcvcnet]

Hi,
This is c code , but the question is same to c++ £¨new/delete£©.

char* p = malloc(10);
strcpy(p, "hello");
printf("%s\n", p);
free(p);
// free just add p to the free list of memory, and do nothing else.
// Is there any use of p? If we never use p why not just nullify p in
free()?
p = NULL; // If we do not use p after call free , we can delete this line
..

And someone write p = NULL is not a good idea for Unix.

Thanks.

[Peter Jansson]

"fcvcnet" <fc*****@163.co m> skrev i meddelandet
news:e2******** **@news.cn99.co m...

Hi,
This is c code , but the question is same to c++ £¨new/delete£©.

char* p = malloc(10);
strcpy(p, "hello");
printf("%s\n", p);
free(p);
// free just add p to the free list of memory, and do nothing else.
// Is there any use of p? If we never use p why not just nullify p in
free()?
p = NULL; // If we do not use p after call free , we can delete this line .

And someone write p = NULL is not a good idea for Unix.

Thanks.

Hello,

This does not answer your question byt why replace the code with the
following?
printf("%s\n"," hello");

Regards,
Peter Jansson
http://www.jansson.net/

[fcvcnet]

Hi,
Do you mean "byt why not replace the code with the following?" ?
If it is , I think use "p" instead of "hello" is simple.
And I think if use "hello" , the compiler will allocate a chunck of memory
to hold "hello".

[fcvcnet]

Hi,
Can somebody introduce any book about c++ with memory, I have so many
question about this part.
Thank you very much.

[dan2online]

fcvcnet wrote:

Hi,
This is c code , but the question is same to c++ £¨new/delete£©.

char* p = malloc(10);
malloc returns void * pointer, so the statement is changed into:
char *p = (char *)malloc(10);
free(p);
// free just add p to the free list of memory, and do nothing else.
// Is there any use of p? If we never use p why not just nullify p in
free()?
p = NULL; // If we do not use p after call free , we can delete thisline
. you can wrap the free function that has this ability, but the argument
is the reference to the pointer.

void free_wrapper (void **pp)
{
free ( *pp );
*pp = NULL;
}
And someone write p = NULL is not a good idea for Unix.

Why? I think NULL is a cleaner way to stand for Null pointer than 0 or
(void *)0 at least.
But for C++, it is a good point to introduce null object design
pattern instead of NULL.

[Ian Collins]

fcvcnet wrote:

Hi,
This is c code , but the question is same to c++ £¨new/delete£©.

char* p = malloc(10); Don't use malloc, use new.
strcpy(p, "hello");
printf("%s\n", p); Why not printf("hello\n "); ?
free(p);
// free just add p to the free list of memory, and do nothing else.
// Is there any use of p? If we never use p why not just nullify p in
free()? Because you pass the pointer by value, so free() can't change the value
of the pointer.
p = NULL; // If we do not use p after call free , we can delete this line
..

And someone write p = NULL is not a good idea for Unix.

Who and where?

--
Ian Collins.

[fcvcnet]

yes, I reference from
http://users.actcom.co.il/~choo/lupg...x-memory.html#
c_alloc

char* p = malloc(10);
And someone write p = NULL is not a good idea for Unix.
Why? I think NULL is a cleaner way to stand for Null pointer than 0 or
(void *)0 at least.
But for C++, it is a good point to introduce null object design
pattern instead of NULL.
sorry , I am wrong , I made a mistake of this.

[Peter Jansson]

"fcvcnet" <fc*****@163.co m> skrev i meddelandet
news:e2******** **@news.cn99.co m...

Hi,
Do you mean "byt why not replace the code with the following?" ?
If it is , I think use "p" instead of "hello" is simple.
And I think if use "hello" , the compiler will allocate a chunck of memory
to hold "hello".

Hi,

Yes, I did mean "...but why not replace..."

The compiler will also allocate a chunck of memory to hold "hello" for the
following part of your code:
strcpy(p, "hello");

Regards,
Peter Jansson
http://www.jansson.net/

[fcvcnet]

Thanks.

Because you pass the pointer by value, so free() can't change the value
of the pointer.

now I know.

And someone write p = NULL is not a good idea for Unix.

Who and where?

sorry for that I made a mistake.

BTW, Can I edit/delete the news I have had posted for there is a mistake I
made and that bring much more misunderstand?

[persenaama]

>BTW, Can I edit/delete the news I have had posted for there is a mistake I

made and that bring much more misunderstand?

No.

Regarding the p=NULL issue, if the pointer object goes out of scope it
indeed is redundant. The language doesn't guard against this kind of
stuff: you have to keep track manually what is freed and what isn't
(likewise, what is allocated and what isn't). This is why it's a good
idea generally to zero pointers to memory that is already freed (to
avoid freeing the same memory twice or more).

But, if the pointer goes out of scope, knock yourself out and not zero
it.