malloc (0)

[Bhaskar]

Hey can any one tell me what malloc 0 does, it assigns space in memory
i know that but is it usable and how many bytes does it allocate ,
allocation of 0 bytes is difficult-to-digest concept and still in the
memory.

[Eric Sosman]

Joe Wright wrote:

Bhaskar wrote:

>Hey can any one tell me what malloc 0 does, it assigns space in memory
i know that but is it usable and how many bytes does it allocate ,
allocation of 0 bytes is difficult-to-digest concept and still in the
memory.

Try this.

#include <stdio.h>
#include <stdlib.h>
int main(void) {
void *vp = malloc(0);
printf("malloc( 0) returned %p\n", vp);
return 0;
}

Here at home,

malloc(0) returned 902a0

vp is non-null and so I assume that malloc did what I asked and
allocated 0 bytes successfully. Do you think I should free(vp) to avoid
a leak?

Not in this program, but in general yes! Otherwise, you
may well leak some extra "bookkeepin g" memory that malloc()
uses to keep track of each zero-length allocation. Try

#include <stdio.h>
#include <stdlib.h>
int main(void) {
unsigned long count = 0;
while (malloc(0) != NULL) {
if (++count == 0) {
printf ("More than %lu successful allocations"
" -- giving up.\n", (unsigned long)-1);
return 0;
}
}
printf ("%lu successful allocations before NULL\n",
count);
return 0;
}

"Here at home," I get

72960776 successful allocations before NULL

.... plus a lot of disk activity and some system alerts about
increasing the size of the page file. Strongly suggestive of
leakage, I'd say.

--
Eric Sosman
es*****@acm-dot-invalid.org

[Joe Wright]

Eric Sosman wrote:

Joe Wright wrote:

>Bhaskar wrote:

>>Hey can any one tell me what malloc 0 does, it assigns space in memory
i know that but is it usable and how many bytes does it allocate ,
allocation of 0 bytes is difficult-to-digest concept and still in the
memory.

Try this.

#include <stdio.h>
#include <stdlib.h>
int main(void) {
void *vp = malloc(0);
printf("malloc( 0) returned %p\n", vp);
return 0;
}

Here at home,

malloc(0) returned 902a0

vp is non-null and so I assume that malloc did what I asked and
allocated 0 bytes successfully. Do you think I should free(vp) to
avoid a leak?

Not in this program, but in general yes! Otherwise, you
may well leak some extra "bookkeepin g" memory that malloc()
uses to keep track of each zero-length allocation. Try

#include <stdio.h>
#include <stdlib.h>
int main(void) {
unsigned long count = 0;
while (malloc(0) != NULL) {
if (++count == 0) {
printf ("More than %lu successful allocations"
" -- giving up.\n", (unsigned long)-1);
return 0;
}
}
printf ("%lu successful allocations before NULL\n",
count);
return 0;
}

"Here at home," I get

72960776 successful allocations before NULL

... plus a lot of disk activity and some system alerts about
increasing the size of the page file. Strongly suggestive of
leakage, I'd say.

Leaks like a sieve. 73 million useless calls to malloc(0) and you
couldn't free() them if you wanted to. Crash and Burn.

--
Joe Wright
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---

[Mark McIntyre]

On 4 Jul 2006 12:20:41 -0700, in comp.lang.c , "Harald van D?k"
<tr*****@gmail. comwrote:

>Lew Pitcher wrote:

>The key phrase is "implementa tion defined".

....

>>As I read it, so long as your C compiler/environment's
implementati on documents it, malloc(0) could legitimately launch
Russian ICBM missles

>Unless either returning a null pointer by itself in a user function, or
calling malloc with a nonzero size, is allowed to launch missiles, no,
malloc(0) isn't allowed to launch missiles either.

Actually it is, but the implementation must define this behaviour. The
DS9K's C compiler provides a very similar feature.
--
Mark McIntyre

"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan

[Skarmander]

Mark McIntyre wrote:

On 4 Jul 2006 12:20:41 -0700, in comp.lang.c , "Harald van D?k"
<tr*****@gmail. comwrote:

>Lew Pitcher wrote:

>>The key phrase is "implementa tion defined".

...

>>As I read it, so long as your C compiler/environment's
implementatio n documents it, malloc(0) could legitimately launch
Russian ICBM missles

>Unless either returning a null pointer by itself in a user function, or
calling malloc with a nonzero size, is allowed to launch missiles, no,
malloc(0) isn't allowed to launch missiles either.

Actually it is, but the implementation must define this behaviour. The
DS9K's C compiler provides a very similar feature.

Although it hardly matters in this less than serious context, the DS9K
compiler would not be conforming for doing so. (Which would be odd to say
the least, since the DS9K manufacturer is usually very careful about this
sort of thing.)

N1124:
"3.4.1 implementation-defined behavior
unspecified behavior where each implementation documents how the choice is made"

"3.4.4 unspecified behavior
use of an unspecified value, or other behavior where this International
Standard provides two or more possibilities and imposes no further
requirements on which is chosen in any instance"

Even the most liberal interpretation of these statements requires that, if
the standard gives a choice between behaviors and states the actual behavior
is "implementa tion-defined", then the choice must be made from the behaviors
given and the choice itself must be documented. The implementation is not
allowed to implement additional or replacement behavior, not even if it
documents it.

The alternative would make "unspecifie d behavior" a synonym for "undefined
behavior", which is clearly not the intent.

S.

[Ben Pfaff]

"Kiru Sengal" <ki*********@gm ail.comwrites:

Bhaskar wrote:

>Hey can any one tell me what malloc 0 does, it assigns space in memory
i know that but is it usable and how many bytes does it allocate ,
allocation of 0 bytes is difficult-to-digest concept and still in the
memory.

I believe it returns NULL.

This is one possible implementation.
It may also return a unique address that may not be dereferenced.
--
int main(void){char p[]="ABCDEFGHIJKLM NOPQRSTUVWXYZab cdefghijklmnopq rstuvwxyz.\
\n",*q="kl BIcNBFr.NKEzjwC IxNJC";int i=sizeof p/2;char *strchr();int putchar(\
);while(*q){i+= strchr(p,*q++)-p;if(i>=(int)si zeof p)i-=sizeof p-1;putchar(p[i]\
);}return 0;}