shared mapping of /dev/zero

[Denis Zaitsev <zzz@cd-club.ru>]

I'm trying to do something like the anonymous mapping of the same
memory region thru a different virtual addresses.  I.e.:

    char *a, *b;
    int z= open("/dev/zero", O_RDWR);
    a= mmap(NULL, 0x2000, PROT_READ|PROT_WRITE, MAP_SHARED, z,0);
    b= mmap(a+0x1000, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_FIXED, z,0);

And what I have as the result, are two mapping as if they was created
with MAP_ANONYMOUS flag, so they do not share the same space (of file
desc z, at offset 0), as required.  So, mapping of /dev/zero is just
the synonum for the anonymous mapping...  Is this the correct
behaviour, and if so, what is the way to do the trick?

Thanks in advance...