Re: threads and kernel

["Steve Graegert" <graegerts@gmail.com>]

On 10/23/07, vibi <vibi_sreenivasan@cms.com> wrote:
> On Tue, 2007-10-23 at 07:14 +0200, Steve Graegert wrote:
> > On 10/23/07, vibi <vibi_sreenivasan@cms.com> wrote:
> > >  > "The init code used to set up the main program doesn't care about threads,
> > >
> > >  because it assumes that they won't be used anyway."
> > >
> > > when is init code linked to the program ,during the compile time or
> > > during the run time?
> >
> > It's being added by the linker at compile time.
> >
> >       \Steve
>
> the application you gave earlier is linked without any knowledge of
> multi-threading
> so
>
> > cc test.c
> > ./a.out
>
> would fail
>
> but you also said
>
> > cc test.c
> > LD_PRELOAD=/usr/lib/libpthread.so ./a.out
>
> will not fail
>
> in both the cases same init code is added at compile time and you also
> said that init code determines whether a program is multi-threaded.
>
> So i am a little bit confused because how at run time the init code is
> changed.

Linkage takes place at compile time and run time.  When starting a
program its execution environment is setup properly including loading
and linking in all libraries (including those specified on the command
line with LD_PRELOAD and at compile time).

The LD_PRELOAD environment variable allows you to load additional
shared libraries at program startup.  The loader ld.so.1, loads the
specified shared libraries as if the program had been linked
explicitly with the shared libraries in LD_PRELOAD before any other
dependents of the program.  At startup time, the loader implicitly
loads one or more libraries, if found, specified in the LD_PRELOAD
environment. It uses the same load order and symbol resolution order
as if the library had been explicitly linked as the first library in
the link line when building the executable.

As a result, at startup the program is multi-threaded when
LD_PRELOAD=/usr/lib/libpthread.so is specified, but if libpthread is
neither specified at compile time nor __before__ startup of the main
program (i.e. LD_PRELOAD) the execution environment is unchanged (not
threaded at all).

This is possible because the init code of the libraries is executed at
before control is transfered to the main program, thus the program is
multi-threaded although libpthread has no been specified at compile
time and no thread-related function calls are made during runtime.

	\Steve

--

Steve Grägert
DigitalEther.de
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