malloc(0) returns 0x800 on FreeBSD 6.2 ?

Luigi Rizzo rizzo at
Mon Dec 11 10:05:53 PST 2006

i was debugging a program on FreeBSD 6, and much to my
surprise, i noticed that malloc(0) returns 0x800, as shown
by this program:

	> more a.c
	#include <stdio.h>
	int main(int argc, char *argv[])
		char *p = malloc(0);
		printf(" malloc 0 returns %p\n", p);
	> cc -o a a.c
	> ./a
	 malloc 0 returns 0x800

if you look at the source this is indeed clear - internally
the 0x800 is ZEROSIZEPTR and is set when a zero length is
passed to malloc() unless you have malloc_sysv set.

The thing is, i don't know if this behaviour is intentional or not,
but certainly is not documented -- the manpage documents
something totally different (in the section for the 'V'
MALLOC_OPTION, see below).

     V       Attempting to allocate zero bytes will return a NULL pointer
             instead of a valid pointer.  (The default behavior is to make a
             minimal allocation and return a pointer to it.)  This option is
             provided for System V compatibility.  This option is incompatible
             with the ``X'' option.

So what should we do with this ? Just fix the manpage or fix the
code ? This behaviour is likely to break quite a few things...


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