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From: Charlie Conklin <cc@dolphinet.co.uk>
Newsgroups: comp.unix.bsd.freebsd.misc
Subject: mmap problems
Date: Sun, 03 Nov 1996 10:56:33 +0000
Organization: Dolphin Internet Services
Lines: 64
Message-ID: <327C7A61.41C67EA6@dolphinet.co.uk>
References: <54o9cc$cci@uuneo.neosoft.com> <54udvs$j1q@anorak.coverform.lan>
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Hi All,
I am having a fairly mysterious problem with using an mmap'ed file
as shared memory. I am doing this in Apache, and have used their
method of shared memory (like used for the scoreboard)...
Here is the situation...
I have a function that allocates some section of shared memory as
follows:
static my_stats *new_my_stats()
{
my_stats *stats;
caddr_t m;
m = mmap((caddr_t)0, sizeof(my_stats),
PROT_READ | PROT_WRITE, MAP_ANON | MAP_SHARED, -1, 0);
if (m == (caddr_t)-1) {
perror("mmap");
fprintf(stderr, "httpd: Could not mmap memory\n");
exit(1);
}
stats = (my_stats *)m;
... various initialization ...
return stats;
}
This is called at startup time:
static my_stats global_stats = new_my_stats();
This structure is then used. Over a period of time, slots in my
structure are used up. At some point, we need to allocate more
memory, and something like this takes place:
global_stats->next_stats = new_my_stats()
Which forms a linked list of these bits of shared memory. Note
that this second call of new_my_stats will be done from a child
process of the original server process.
Here is the problem...
I seem to get intermittent SIGSEGV's when traversing the second
link of the list. Sometimes it works, sometimes it doesn't. I
have also verified that as far as I can tell, the pointer values
are OK. In other words, the pointer value for global_stats->next
is the same for a success as for a failure! But nonetheless,
sometimes, dereferencing that pointer results in a seg fault.
I thought for a while that it was dieing because I was not doing
any write access locking for the shared memory, so I added that
in, but still have the problem. I can make the problem go away
however by allocating enough shared memory that I do not have
to make a linked list (i.e. allocate another block of shared
memory), but this is not a good solution and I would like to
understand the problem.
If anyone has any ideas about this, I would appreciate hearing
them.
Thanks...
- Charlie Conklin cc@dolphinet.co.uk