realloc(p, 0) - C / C++

copx

Is realloc(p, 0) a valid operation or does it cause undefined behaviour?
Can you still free(p) after this?
Even if it's valid do you think compiler writers actually thought of this
case?

Nov 14 '05 #1

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1034

Peter Nilsson

copx wrote:

Is realloc(p, 0) a valid operation
Yes.

"If size is zero and ptr is not a null pointer, the object
it points to is freed."

Use a reference like N869 if you don't have the standard.
or does it cause undefined behaviour?
Can you still free(p) after this?
No.
Even if it's valid do you think compiler writers actually
thought of this case?

No, but standard library authors _must_, if they want to claim
compliance.

--
Peter

Nov 14 '05 #2

copx

"Peter Nilsson" <ai***@acay.com.au> schrieb im Newsbeitrag
news:11**********************@f14g2000cwb.googlegr oups.com...

copx wrote:
Is realloc(p, 0) a valid operation

Yes.

"If size is zero and ptr is not a null pointer, the object
it points to is freed."

Use a reference like N869 if you don't have the standard.
or does it cause undefined behaviour?
Can you still free(p) after this?

No.
Even if it's valid do you think compiler writers actually
thought of this case?

No, but standard library authors _must_, if they want to claim
compliance.

Thanks for the info!

copx

Nov 14 '05 #3

Christopher Benson-Manica

copx <in*****@invalid.com> spoke thus:

Is realloc(p, 0) a valid operation or does it cause undefined behaviour?
It is valid.

http://www.eskimo.com/~scs/C-faq/q7.30.html
Can you still free(p) after this?
No.
Even if it's valid do you think compiler writers actually thought of this
case?

Probably most modern compiler writers did. YMMV.

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cyberspace.org | don't, I need to know. Flames welcome.

Nov 14 '05 #4

Keith Thompson

Christopher Benson-Manica <at***@nospam.cyberspace.org> writes:

copx <in*****@invalid.com> spoke thus:
Is realloc(p, 0) a valid operation or does it cause undefined behaviour?

It is valid.

http://www.eskimo.com/~scs/C-faq/q7.30.html
Can you still free(p) after this?

No.

That depends on how you invoke it.

If you just call

realloc(p, 0);

and ignore the returned value, p will be a pointer to deallocated
memory, and free(p) (or any ues of p) will invoke undefined behavior.

If you call it like this:

p = realloc(p, 0);

then the value of p will be either a null pointer or a valid pointer;
in either case, you can safely call free(p).

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

Nov 14 '05 #5

Christopher Benson-Manica

Keith Thompson <ks***@mib.org> spoke thus:

That depends on how you invoke it.

True; I just figured OP was asking about the case where you can't,
since he didn't seem to be checking the return value of realloc().

--
Christopher Benson-Manica | I *should* know what I'm talking about - if I
ataru(at)cyberspace.org | don't, I need to know. Flames welcome.

Nov 14 '05 #6

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