Hi All,
As per the standard what is the result of passing NULL to both malloc and free?
Regards,
Mohan.
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On Fri, 6 Feb 2004 20:33:12 +0000 (UTC), mo************@ msn.com
(Mohanasundaram ) wrote:
Hi All,
As per the standard what is the result of passing NULL to both malloc and free?
Wow, do you think you've cross-posted to enough groups to guarantee a
good sampling of responses to select from?
I'll reply to just the clc post (thanks, Agent, for giving me the
conscious option)
Passing 0 to free() is a no-op.
Passing it to malloc will return a pointer to a zero-length block of
memory with which you may do absolutely nothing (and don't forget to
free it).
-leor
Regards,
Mohan.
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Leor Zolman
BD Software le**@bdsoft.com www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html
Leor Zolman wrote: On Fri, 6 Feb 2004 20:33:12 +0000 (UTC), mo************@ msn.com
(Mohanasundaram ) wrote:
Hi All,
As per the standard what is the result of passing NULL to both malloc
and free?
<snip>
Passing 0 to free() is a no-op.
Passing it to malloc will return a pointer to a zero-length block of
memory with which you may do absolutely nothing (and don't forget to
free it).
It might, instead, legitimately return NULL.
Note that some implementations don't handle malloc(0) well. At least one
implementation of my acquaintance crashes when you try to malloc(0).
Consequently, *even though* the Standard endorses malloc(0), some wise
programmers like to check that their malloc request is not for 0 bytes (if
there is a risk of this) before actually calling malloc.
--
Richard Heathfield : bi****@eton.pow ernet.co.uk
"Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
K&R answers, C books, etc: http://users.powernet.co.uk/eton
"Leor Zolman" <le**@bdsoft.co m> wrote in message
news:8b******** *************** *********@4ax.c om... On Fri, 6 Feb 2004 20:33:12 +0000 (UTC), mo************@ msn.com
(Mohanasundaram ) wrote:
Hi All,
As per the standard what is the result of passing NULL to both
malloc and free?
Passing 0 to free() is a no-op.
Passing it to malloc will return a pointer to a zero-length block of
memory with which you may do absolutely nothing (and don't forget to
free it).
The OP asked about passing NULL to malloc, not 0. malloc(NULL) may or may
not be a compilation error. In case it isn't, it behaves as you described.
Peter
On Fri, 6 Feb 2004 21:27:57 +0000 (UTC), Richard Heathfield
<do******@addre ss.co.uk.invali d> wrote: Leor Zolman wrote:
On Fri, 6 Feb 2004 20:33:12 +0000 (UTC), mo************@ msn.com
(Mohanasundaram ) wrote:
Hi All,
As per the standard what is the result of passing NULL to both malloc
and free?
<snip>
Passing 0 to free() is a no-op.
Passing it to malloc will return a pointer to a zero-length block of
memory with which you may do absolutely nothing (and don't forget to
free it).
It might, instead, legitimately return NULL.
Thanks. I'm still learning how to read the Standard... in this case I
looked at the description for malloc without realizing there was more
"general" info about the allocation function on the previous page that
made this point.
-leor
Leor Zolman
BD Software le**@bdsoft.com www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html
On Fri, 6 Feb 2004 21:37:24 -0000, "Peter Pichler" <pi*****@pobox. sk>
wrote: "Leor Zolman" <le**@bdsoft.co m> wrote in message
news:8b******* *************** **********@4ax. com... On Fri, 6 Feb 2004 20:33:12 +0000 (UTC), mo************@ msn.com
(Mohanasundaram ) wrote: >
>Hi All,
>
> As per the standard what is the result of passing NULL to both malloc and free?
Passing 0 to free() is a no-op.
Passing it to malloc will return a pointer to a zero-length block of
memory with which you may do absolutely nothing (and don't forget to
free it).
The OP asked about passing NULL to malloc, not 0. malloc(NULL) may or may
not be a compilation error. In case it isn't, it behaves as you described.
That was a "C++ mentality" issue -- using 0 to avoid the debate over
the implications of using NULL ;-)
I'm now trying to understand how malloc(NULL) would draw a compile
error... something perhaps about NULL being an
"implementa tion-defined null pointer constant" that refuses to convert
to a size_t? Help...
Thanks,
-leor
Peter
Leor Zolman
BD Software le**@bdsoft.com www.bdsoft.com -- On-Site Training in C/C++, Java, Perl & Unix
C++ users: Download BD Software's free STL Error Message
Decryptor at www.bdsoft.com/tools/stlfilt.html
"Leor Zolman" <le**@bdsoft.co m> wrote in message
news:nd******** *************** *********@4ax.c om...
I'm now trying to understand how malloc(NULL) would draw a compile
error... something perhaps about NULL being an
"implementa tion-defined null pointer constant" that refuses to convert
to a size_t? Help...
NULL may be #defines as (void*)0 in C.
Mohanasundaram wrote: As per the standard what is the result of passing NULL to both malloc and free?
There are separate standards for C and C++. We tried to
make them agree as much as possible where the facilities
are mainly "C". So I'll reply only from the point of
view of the current C standard (C99+TC1).
malloc() is passed an integer argument, not a pointer.
So I will asume that you meant realloc(). When the
pointer argument in a call to realloc() is null, the
function acts just like a call to malloc() with the same
size argument.
If the argument to free() is a null pointer, no action
is taken.
Some older (non standards compliant) implementations of
these functions will not behave as the standard specifies,
so for maximal portability you might prefer to avoid
relying on that behavior. This is easy to do:
if (p == NULL)
q = malloc(n);
else
q = realloc(p, n);
if (r != NULL)
free(r);
Peter Pichler wrote:
"Leor Zolman" <le**@bdsoft.co m> wrote in message
news:8b******** *************** *********@4ax.c om... On Fri, 6 Feb 2004 20:33:12 +0000 (UTC), mo************@ msn.com
(Mohanasundaram ) wrote:
Hi All,
As per the standard what is the result of passing NULL to both
malloc and free?
Passing 0 to free() is a no-op.
Passing it to malloc will return a pointer to a zero-length block of
memory with which you may do absolutely nothing (and don't forget to
free it).
The OP asked about passing NULL to malloc, not 0. malloc(NULL) may or may
not be a compilation error. In case it isn't, it behaves as you described.
May or may not? NULL is 0 or ((void *)0) and because of C89 prototyping
will arrive at malloc as 'size_t 0' graranteed. What 'compilation error'
were you thinking about?
--
Joe Wright http://www.jw-wright.com
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---
On Sat, 07 Feb 2004 02:22:22 GMT, Joe Wright
<jo********@ear thlink.net> wrote in comp.lang.c: Peter Pichler wrote:
"Leor Zolman" <le**@bdsoft.co m> wrote in message
news:8b******** *************** *********@4ax.c om... On Fri, 6 Feb 2004 20:33:12 +0000 (UTC), mo************@ msn.com
(Mohanasundaram ) wrote:
>
>Hi All,
>
> As per the standard what is the result of passing NULL to both
malloc and free?
Passing 0 to free() is a no-op.
Passing it to malloc will return a pointer to a zero-length block of
memory with which you may do absolutely nothing (and don't forget to
free it).
The OP asked about passing NULL to malloc, not 0. malloc(NULL) may or may
not be a compilation error. In case it isn't, it behaves as you described.
May or may not? NULL is 0 or ((void *)0) and because of C89 prototyping
will arrive at malloc as 'size_t 0' graranteed. What 'compilation error'
were you thinking about?
There is no automatic conversion from "void *" to size_t. If an
implementation defines NULL as (void *)0, and there is a prototype for
malloc() in scope identifying the type of the argument as size_t, the
compiler must issue a diagnostic for the constraint violation of
attempting to convert a pointer (of any type) to an integer type
without a cast.
--
Jack Klein
Home: http://JK-Technology.Com
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