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 -- 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
