IS this a proper way of freeing memory with free()

[Raman]

Hi All,

Here is a small Code,

int main(void)
{
char *p=(char *) malloc(100);
strcpy(p,"Test1 234567890");
p=p+10;
free(p);
/*** Is here a memory Leak, because p is now
pointing 10 location past to the start of allocated memory
****/

/** some stuff with p again**/


}
Thanks and Regards,
Raman Chalotra

[Ian Collins]

Raman wrote:

Hi All,

Here is a small Code,

int main(void)
{
char *p=(char *) malloc(100);

How many times must people here have to say "do not cast the return
value of malloc"? Does anyone read the archive before they post?

strcpy(p,"Test1 234567890");
p=p+10;
free(p);
/*** Is here a memory Leak, because p is now
pointing 10 location past to the start of allocated memory
****/

It's completely undefined behaviour. Your toilet might explode. Don't
do it.

--
Ian Collins.

[Christopher Layne]

Raman wrote:

char *p=(char *) malloc(100);

char *p = malloc(100 * sizeof *p);

Don't cast the return value from malloc().

strcpy(p,"Test1 234567890");
p=p+10;
free(p);
/*** Is here a memory Leak, because p is now
pointing 10 location past to the start of allocated memory
****/

Completely invalid. free() expects a pointer originally returned from one of
the other allocation functions (calloc, malloc, realloc).

NAME
free - free allocated memory

SYNOPSIS
#include <stdlib.h>

void free(void *ptr);

DESCRIPTION
The free() function shall cause the space pointed to by ptr to be
deallocated; that is, made available for further allocation. If ptr is a null
pointer, no action shall occur. Otherwise, if the argument does not match a
pointer earlier returned by the calloc(), malloc(), or realloc() function, or
if the space has been deallocated by a call to free() or realloc(), the
behavior is undefined.

Any use of a pointer that refers to freed space results in undefined
behavior.

[Christopher Layne]

Ian Collins wrote:

> char *p=(char *) malloc(100);

How many times must people here have to say "do not cast the return
value of malloc"? Does anyone read the archive before they post?

Basically forever. As long as it's still in books that people read - they'll
keep doing it.

[Ian Collins]

Christopher Layne wrote:

Ian Collins wrote:

>> char *p=(char *) malloc(100);

How many times must people here have to say "do not cast the return
value of malloc"? Does anyone read the archive before they post?

Basically forever. As long as it's still in books that people read - they'll
keep doing it.

So maybe book burning isn't such a bad idea after all.

--
Ian Collins.

[David T. Ashley]

"Raman" <ra***********@ gmail.comwrote in message
news:11******** *************@s 48g2000cws.goog legroups.com...

Hi All,

Here is a small Code,

int main(void)
{
char *p=(char *) malloc(100);
strcpy(p,"Test1 234567890");
p=p+10;
free(p);

Problems:

a)You don't test the return value from malloc() to be sure malloc() is able
to grant the memory. There can be some reasons why not ... you should
always test.

b)You may only free() pointers returned by malloc() and friends. The
behavior is undefined. Most likely it will wreck(*) the state of the
runtime library.

(*)The reason for this is that memory allocation is written to be efficient,
and assumes a correct client.

--
David T. Ashley (dt*@e3ft.com)
http://www.e3ft.com (Consulting Home Page)
http://www.dtashley.com (Personal Home Page)
http://gpl.e3ft.com (GPL Publications and Projects)

[CBFalconer]

Raman wrote:

>
int main(void)
{
char *p=(char *) malloc(100);
strcpy(p,"Test1 234567890");
p=p+10;
free(p);
/*** Is here a memory Leak, because p is now pointing 10
location past to the start of allocated memory ****/

/** some stuff with p again**/
}

Is here undefined behaviour, because you freed a pointer not
created by malloc. Incidentally, the cast is not required, and bad
because you suppressed the error report from the compiler, due to
failure to #include <stdlib.h>.

--
<http://www.cs.auckland .ac.nz/~pgut001/pubs/vista_cost.txt>

"A man who is right every time is not likely to do very much."
-- Francis Crick, co-discover of DNA
"There is nothing more amazing than stupidity in action."
-- Thomas Matthews

[CBFalconer]

Ian Collins wrote:

Christopher Layne wrote:

>Ian Collins wrote:

>>> char *p=(char *) malloc(100);

How many times must people here have to say "do not cast the return
value of malloc"? Does anyone read the archive before they post?

Basically forever. As long as it's still in books that people read
- they'll keep doing it.

So maybe book burning isn't such a bad idea after all.

Are there any reliable estimates as to the number of wood stove
fires annually kindled by BullSchildt books? I have the impression
the value has been dropping for several years. This may be
correlated with the disappearance of Herbs from the C book market.

--
<http://www.cs.auckland .ac.nz/~pgut001/pubs/vista_cost.txt>

"A man who is right every time is not likely to do very much."
-- Francis Crick, co-discover of DNA
"There is nothing more amazing than stupidity in action."
-- Thomas Matthews

[Ryan Ply]

Ian Collins <ia******@hotma il.comwrites:

Raman wrote:

Hi All,

Here is a small Code,

int main(void)
{
char *p=(char *) malloc(100);

How many times must people here have to say "do not cast the return
value of malloc"? Does anyone read the archive before they post?

strcpy(p,"Test1 234567890");
p=p+10;
free(p);
/*** Is here a memory Leak, because p is now
pointing 10 location past to the start of allocated memory
****/

It's completely undefined behaviour. Your toilet might explode. Don't
do it.

Its explicitly stated in the book "The C Programming Language" by
Ritchie and Kernighan Second Edition. Thats good enough for me. How
many compilers are *that* strict about the standard anyway? I still
remember a few years ago when one of the versions of gcc (can't remember
specifically) would actually give me a warning if I didn't cast it.

Thats my thoughts on the matter.

Ryan -

[Richard Heathfield]

Ryan Ply said:

Ian Collins <ia******@hotma il.comwrites:

>Raman wrote:

Hi All,

Here is a small Code,

int main(void)
{
char *p=(char *) malloc(100);

How many times must people here have to say "do not cast the return
value of malloc"? Does anyone read the archive before they post?

strcpy(p,"Test1 234567890");
p=p+10;
free(p);
/*** Is here a memory Leak, because p is now
pointing 10 location past to the start of allocated memory
****/

It's completely undefined behaviour. Your toilet might explode. Don't
do it.

Its explicitly stated in the book "The C Programming Language" by
Ritchie and Kernighan Second Edition. Thats good enough for me.

Yes, but K&R remembered to #include <stdlib.h>, didn't they? And in any
case, he wasn't talking about the cast. he was talking about the free().

See K&R's errata page, where they admit that the cast isn't such a great
idea after all (although of course it isn't exactly *wrong* provided you
remember <stdlib.h>).

How
many compilers are *that* strict about the standard anyway? I still
remember a few years ago when one of the versions of gcc (can't remember
specifically) would actually give me a warning if I didn't cast it.

It was probably telling you, indirectly, that you forgot to #include
<stdlib.h- casting malloc, however, does *not* solve the problem gcc was
warning you about; it merely hides the warning message.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at the above domain, - www.