Hi,
I just want to get this right.
A call to realloc() will return NULL on error and the original memory
is left untouched,
both when requesting a larger or a smaller size that the original,
right?
But a call to realloc() with size set to zero is equivalent to free(),
with returns void.
Does that mean that a call to realloc() can fail when shinking memory
except when
shrinking it to zero in which case it will always succeed?
Thanks!
31  12752  ba******@hushma il.com wrote:
Hi,
I just want to get this right.
A call to realloc() will return NULL on error and the original memory
is left untouched,
both when requesting a larger or a smaller size that the original,
right?
But a call to realloc() with size set to zero is equivalent to free(),
with returns void.
Does that mean that a call to realloc() can fail when shinking memory
except when
shrinking it to zero in which case it will always succeed?
Yes, realloc() can fail when shrinking an allocation, or
when growing it, or even when leaving its size unchanged.
When reallocating to zero size, the situation is complicated.
C90 says "If size is zero and ptr is not a null pointer, the
object it points to is freed." Since free() cannot fail (given
an argument that doesn't invoke undefined behavior), it follows
that realloc(...,0) cannot fail.
But C99 has no such text, and makes no special case for size
zero. All we're told is that realloc(...,0) either fails or it
returns a pointer to an object of size zero. Nothing I can find
in C99 forbids realloc(...,0) to fail, so presumably it can.
--
Eric Sosman es*****@acm-dot-org.invalid ba******@hushma il.com wrote:
Hi,
I just want to get this right.
A call to realloc() will return NULL on error
The word failure might be more appropriate in this context than the
word error.
and the original memory is left untouched,
both when requesting a larger or a smaller size that the original, right?
Yes.
But a call to realloc() with size set to zero is equivalent to free(),
Semantically yes.
with returns void.
No, it returns an indeterminate value.
Does that mean that a call to realloc() can fail when shinking memory except when
shrinking it to zero in which case it will always succeed?
No. realloc is not guaranteed to suceed even when the new size is zero.
On Apr 1, 9:06 pm, Eric Sosman <esos...@acm-dot-org.invalidwrot e:
banan...@hushma il.com wrote:
Hi,
I just want to get this right.
A call to realloc() will return NULL on error and the original memory
is left untouched,
both when requesting a larger or a smaller size that the original,
right?
But a call to realloc() with size set to zero is equivalent to free(),
with returns void.
Does that mean that a call to realloc() can fail when shinking memory
except when
shrinking it to zero in which case it will always succeed?
Yes, realloc() can fail when shrinking an allocation, or
when growing it, or even when leaving its size unchanged.
When reallocating to zero size, the situation is complicated.
C90 says "If size is zero and ptr is not a null pointer, the
object it points to is freed." Since free() cannot fail (given
an argument that doesn't invoke undefined behavior), it follows
that realloc(...,0) cannot fail.
But C99 has no such text, and makes no special case for size
zero. All we're told is that realloc(...,0) either fails or it
returns a pointer to an object of size zero. Nothing I can find
in C99 forbids realloc(...,0) to fail, so presumably it can.
Thank you for your reply!
But I wonder, if realloc(...,0) fails in C99, how can I know that?
How can I know if a returned NULL means that realloc() failed,
or if it is the pointer to the new memory, empty, which is NULL.
(That is what I got on my compiler anyway)
On Apr 1, 9:07 pm, "santosh" <santosh....@gm ail.comwrote:
banan...@hushma il.com wrote:
Hi,
I just want to get this right.
A call to realloc() will return NULL on error
The word failure might be more appropriate in this context than the
word error.
and the original memory is left untouched,
both when requesting a larger or a smaller size that the original, right?
Yes.
But a call to realloc() with size set to zero is equivalent to free(),
Semantically yes.
with returns void.
No, it returns an indeterminate value.
What? My manpage says "void free(void *ptr);",
doesn't free return void? By that I meant, free doesn't return
anything.
Does that mean that a call to realloc() can fail when shinking memory except when
shrinking it to zero in which case it will always succeed?
No. realloc is not guaranteed to suceed even when the new size is zero.
And as I wrote in the other post, I wonder how I can know that.
<ba******@hushm ail.comha scritto nel messaggio
news:11******** *************@y 66g2000hsf.goog legroups.com...
Does that mean that a call to realloc() can fail when shinking memory
except when
shrinking it to zero in which case it will always succeed?
No. realloc is not guaranteed to suceed even when the new size is zero.
And as I wrote in the other post, I wonder how I can know that.
Can't you just use free()? ba******@hushma il.com wrote:
On Apr 1, 9:07 pm, "santosh" <santosh....@gm ail.comwrote:
banan...@hushma il.com wrote:
Hi,
I just want to get this right.
A call to realloc() will return NULL on error
The word failure might be more appropriate in this context than the
word error.
and the original memory is left untouched,
both when requesting a larger or a smaller size that the original, right?
Yes.
But a call to realloc() with size set to zero is equivalent to free(),
Semantically yes.
with returns void.
No, it returns an indeterminate value.
What? My manpage says "void free(void *ptr);",
doesn't free return void? By that I meant, free doesn't return
anything.
You're correct. But realloc with a new size of zero either returns a
null pointer, upon failure, or returns an indeterminate value.
Does that mean that a call to realloc() can fail when shinking memory except when
shrinking it to zero in which case it will always succeed?
No. realloc is not guaranteed to suceed even when the new size is zero.
And as I wrote in the other post, I wonder how I can know that.
If the reallocation to zero size suceeds, an indeterminate pointer
value is returned, if it fails, a null pointer value is returned and
the original block is unchanged. ba******@hushma il.com writes:
But I wonder, if realloc(...,0) fails in C99, how can I know that?
I asked on comp.std.c recently. In C99, realloc returns NULL if and
only if it fails. You'll see it if you read the C99 text without
expecting compatibliity with the C89 realloc(,0) behavior.
How can I know if a returned NULL means that realloc() failed,
or if it is the pointer to the new memory, empty, which is NULL.
Just avoid realloc(, 0).
In theory, you can check '#if __STDC_VERSION_ _ >= 199901L' to see if you
have a C99 compiler. In practice, that doesn't tell you if you have a
C99 _library_: You might have a compiler like gcc which has both C99 and
C89 mode, but uses the system C library in either case.
--
Regards,
Hallvard
On Apr 1, 10:17 pm, "santosh" <santosh....@gm ail.comwrote:
banan...@hushma il.com wrote:
On Apr 1, 9:07 pm, "santosh" <santosh....@gm ail.comwrote:
banan...@hushma il.com wrote:
Hi,
I just want to get this right.
A call to realloc() will return NULL on error
The word failure might be more appropriate in this context than the
word error.
and the original memory is left untouched,
both when requesting a larger or a smaller size that the original, right?
Yes.
But a call to realloc() with size set to zero is equivalent to free(),
Semantically yes.
with returns void.
No, it returns an indeterminate value.
What? My manpage says "void free(void *ptr);",
doesn't free return void? By that I meant, free doesn't return
anything.
You're correct. But realloc with a new size of zero either returns a
null pointer, upon failure, or returns an indeterminate value.
Does that mean that a call to realloc() can fail when shinking memory except when
shrinking it to zero in which case it will always succeed?
No. realloc is not guaranteed to suceed even when the new size is zero.
And as I wrote in the other post, I wonder how I can know that.
If the reallocation to zero size suceeds, an indeterminate pointer
value is returned, if it fails, a null pointer value is returned and
the original block is unchanged.
In that case there is something strange here, if I compile this in gcc
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
char *ptr = malloc(42);
if (realloc(ptr, 0) == NULL)
printf("realloc failed\n");
return 0;
}
I always get "realloc failed" printed, in both c89 and c99 mode.
I wouldn't expect it everytime as it does here.
Anyway, I will use free() instead, to avoid confusion.
Thanks for the help!
On 1 Apr 2007 11:50:05 -0700, ba******@hushma il.com wrote:
>Hi,
I just want to get this right.
A call to realloc() will return NULL on error and the original memory
is left untouched,
both when requesting a larger or a smaller size that the original,
right?
It returns NULL if the space cannot be allocated. Whether it is due
to an error or not is something else.
>But a call to realloc() with size set to zero is equivalent to free(),
with returns void.
No it is not. Read the last sentence of 7.20.3 again.
>Does that mean that a call to realloc() can fail when shinking memory
except when
shrinking it to zero in which case it will always succeed?
No. Success is not guaranteed.
>
Thanks!
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