Date: Thu, 23 Nov 2000 14:37:59 +0000 From: Konstantin Chuguev <Konstantin.Chuguev@dante.org.uk> To: Peter Pentchev <roam@orbitel.bg> Cc: Dmitry Sychov <accelware@accelware.com>, hackers@FreeBSD.ORG Subject: Re: dlopen() Message-ID: <3A1D2BC7.65AB61AA@dante.org.uk> References: <001001c05552$45913f80$0e61bec3@dima> <3A1D1E52.95A9E626@dante.org.uk> <20001123154748.C7746@ringworld.oblivion.bg>
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Peter Pentchev wrote:
> > > Is it safe to remove the *.so file after it is loaded
> > > into the process space and addresses to its
> > > functions are gotten?
> >
> > It's safe to remove it as soon as it's been opened.
> > The file will still exist in the filesystem, only there won't be any
> > references to it from any directories, so you won't be able to open it
> > by its name. The file will be really removed from the file system when
> > you close your file descriptor.
>
> So the original poster's question is better translated as 'does the dynamic
> loader ever close and reopen a file after the initial loading, so it could
> accidentally open the new version of the library and use the old addresses'?
>
> I'd guess the answer is 'no' - the dynamic loader most probably opens
> the file once, and mmap()'s the needed functions; I might very well be wrong
> though.
>
This is how I think it works (don't have any FreeBSD box nearby):
For all dynamically loaded modules:
dlopen() opens the file (gets a file descriptor) and mmaps it (possibly
excluding the debug and some other sections). dlclose() unmaps and closes it.
For shared libraries:
__init() code of the binary executable dlopen()'s all the libraries, then goes
through the table of external symbols, lookups them with dlsym(RTLD_NOW) in the
libraries and replaces stub function pointers with the pointers to real
functions. The latter may be optimised somehow.
__fini() code dlclose()'s all the libraries.
But the best way is to look at the source code, of course ;-)
--
* * Konstantin Chuguev - Application Engineer
* * Francis House, 112 Hills Road
* Cambridge CB2 1PQ, United Kingdom
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