On Tue, 2007-10-23 at 08:58 +0200, Steve Graegert wrote: > On 10/23/07, vibi <vibi_sreenivasan@xxxxxxx> wrote: > > On Tue, 2007-10-23 at 07:14 +0200, Steve Graegert wrote: > > > On 10/23/07, vibi <vibi_sreenivasan@xxxxxxx> 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 > - To unsubscribe from this list: send the line "unsubscribe linux-c-programming" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
