Re: mmap
Re: mmap
- Subject: Re: mmap
- From: Louis Demers <email@hidden>
- Date: Tue, 9 Jul 2002 19:10:40 -0400
At 12:27 -0400 2002.07.09, Jim Magee wrote:
On Tuesday, July 9, 2002, at 12:32 AM, Louis Demers wrote:
trying to port inn news server 2.3.3 to osx and the standard
distribution seems to have problems with a mmap call. A static data
is mmap to a file and then porperly written (I dumped all 130180
bytes to a file and it's ok) but when unmapped, the file reverts to
all zeros. I checked and the file descriptor is opened with O_RDWR,
the mmap is done with PROT_WRITE|PROT_READ.
Did you set the MAP_SHARED flag?
GROUPheader = mmap(0, 131080, flag, MAP_SHARED, GROUPfd, 0)
and the file descriptor GROUPfd was opened O_RDWR | O_CREAT.
How large was the file when you mapped it,
Initially the file doesn't exist sot it is created with 0 bytes.
and how much of it did you map?
I imagine that the call to mmap mapped all 131080 and extended the file.
By "properly written" do you mean that you modified all the files
pages in memory with store instructions?
in c, various data was stored in the memory. at the address pointed
to by the retur nvalue of mmap.
How much of it was modified (up to but not beyond its current length)?
every 8 or 10 bytes was modifed.
Did you msync() your changes or just munmap()?
I tried msync() at various points before and after the close
(GROUPfd) but always before the munmap.
When you say you "dumped all 130180 bytes to a file and it's OK",
how did you do that?
bozo = open ("/usr/local/news/spool/overview/bozo" , O_RDWR | O_TRUNC , 0777);
syslog(L_FATAL, "LODE: create bozo in ov3:%d", bozo);
write (bozo , (void *)GROUPheader, 131080);
close(bozo);
Did you copy from the mapped file to another from outside the
program in question, or did you just issue a bunch of write() calls
from the mapped memory into the "dump file"?
Tried both: initially I did the second option, I called write() to
dump the memory into the dump file.
More info is needed?
continuing the tests, I single stepped and notice at some points the
mmap file has the proper data (did in a shell "od group.index") and I
saw the correct data. I executed the same command a second time and
oups the data is now all zeros ! as if the process to view the file
interfered. I can do a "cp group.index toto" to make a succesfull
snapshot of what the file should contain. If I do that a second time,
it too seems to wipe out the mmap file !
Any help would be appreciated. It seems very voodoo. I was once quite
active on Unix but never used mmap so my intuition can't guide me to
detect if the code is at fault or the implementation of mmap. It is
worth nothing that the code is reputed to work on the folowing OSes:
AIX 4.3
FreeBSD 2.2.x and up
HP-UX 10.20 and up
Linux 2.x (tested with libc 5.4 or glibc 2.0 and up)
OpenBSD 2.8 and up
SCO 5.0.4 (tested with gcc 2.8.1 and cc)
Solaris 2.5.x and up
UnixWare 7.1
UX/4800 R11 and up
--Jim
--
Louis Demers ing.
email@hidden
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| | >Re: mmap (From: Jim Magee <email@hidden>) |