Re: [HACKERS] mmap and MAP_ANON

Göran Thyni wrote:
>
> Bruce Momjian wrote:
> >
> > Would people tell me what platforms do NOT support the MAP_ANON flag to
> > the mmap() system call?  You should find it in the mmap() manual page.
> >
> > *BSD has it, but I am not sure of the others.  I am researching cache
> > size issues and the use of mmap vs. SYSV shared memory.
>
> SVR4 (at least older ones) does not support MMAP_ANON,
> but the recommended in W. Richards Stevens'
> "Advanced programming in the Unix environment" (aka the Bible part 2)
> is to use /dev/zero.
>
> This should be configurable with autoconf:
>
> <PSEUDO CODE>
>
> if (exists MAP_ANON) use it; else use /dev/zero
>
> ------------
>
> flags = MAP_SHARED;
> #ifdef HAS_MMAP_ANON
> fd = -1;
> flags |= MAP_ANON;
> #else
> fd = open('/dev/zero, O_RDWR);
> #endif
> area = mmap(0, size, PROT_READ|PROT_WRITE, flags, fd, 0);
>
> </PSEUDO CODE>

Ouch, hate to say this but:
I played around with this last night and
I can't get either of the above technics to work with Linux 2.0.33

I will try it with the upcoming 2.2,
but for now, we can't loose shmem without loosing
a large part of the users (including some developers).
flags = MAP_SHARED;

<PSEUDO CODE>
#ifdef HAS_WORKING_MMAP
#ifdef HAS_MMAP_ANON
fd = -1;
flags |= MAP_ANON;
#else
fd = open('/dev/zero, O_RDWR);
#endif
area = mmap(0, size, PROT_READ|PROT_WRITE, flags, fd, 0);
#else
id = shget(...);
area = shmat(...);
#endif
</PSEUDO CODE>

    not happy,
--
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Göran Thyni, sysadm, JMS Bildbasen, Kiruna