Re: [boost] [Interprocess] Possible bug in shared memory on FreeBSD 8.0

Subject: Re: [boost] [Interprocess] Possible bug in shared memory on FreeBSD 8.0
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2009-08-11 02:55:31

Manish Vachharajani escribió:
> I've been happily using Boost.Interprocess to create shared memory
> regions under FreeBSD 7.2, however, I am having issues on FreeBSD 8.0
> pre-release.  I can reproduce the problem with Boost 1.39.0 and the
> svn trunk.  The source of the problem appears to be that
> Boost.Interprocess calls lseek on a file descriptor returned by
> shm_open, but the results of such an lseek are undefined.  Below, I
> explain further how I came to this conclusion.

You are right, it's undefined, but it usually works. However, I can't 
find a way to obtain the size of a shared memory segment using POSIX, if 
you could fin a way to do it it would be really nice.

> I've confirmed that FreeBSD 7.2 allowed such seeks but 8.0 does not by
> looking at the kernel source code.  For what it's worth AIX 6.1 does
> not allow such seeks either (see
> http://publib.boulder.ibm.com/infocenter/systems/index.jsp?topic=/com.ibm.aix.basetechref/doc/basetrf1/lseek.htm).

Interesting, maybe that's the reason this has not appeared sooner.

> Could someone advise me on what the correct fix for this bug is?  I'd
> be glad to implement it, but I'm not sure what the right fix ought to
> be.

No idea at all, maybe the fix is particular to each system, maybe we 
could open the segment as a normal file in the filesystem (if shared 
memory is mounted somewhere, I don't know if that's the case) and try to 
obtain its size. "ls" command displays the size, so there must be a why 
to obtain the size of a segment.

Best,

Ion