Re: threads and kernel

[vibi <vibi_sreenivasan@cms.com>]

On Tue, 2007-10-23 at 08:58 +0200, Steve Graegert wrote:
> 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

so init code is just like a stub which is resolved while setting up the
execution environment of the program,ie linkage resolution at load time.

am i understanding correctly.

thanks very much for your help
regards 
vibi sreenivasan
 
> 
> --
> 
> Steve Grägert
> DigitalEther.de
> 

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