On Wed, Mar 23, 2011 at 10:25 AM, Leelakrishna A <amudala.krishna@gmail.com> wrote:
Hi All, I have few doubts regarding static and shared (dynamic) libraries linking. Can anybody please clarify it.
Hello Leelakrishna, This list is about Linux kernel development, not user-space application development. There is a special mailing list for that purpose that is: linux-c-programming@vger.kernel.org. But I will try to answer your questions:
There are two kinds of libraries - static libary(.a file) and shared library (.so file). Also there are two types of library linkages static linkage and dynamic linkage.
Yes, static linkage is used with static libraries and dynamic linkage is used with shared (dynamic) libraries. You can't static link a shared library nor dynamic link a static library. Although there are tools that create a pseudo-static binary, but is more a hack than a normal behaviour (i have never used those tools so I can't tell you how well they work).
So Static library can be linked to an executable in either ways (Statically and dynamically). in the same way shared libraries can be linked to executable in both the ways (Statically and dynamically).
As I said you can only static link a static library. For example the command: gcc -static main.c -lhello Will create an executable statically linked with the static version of the library libhello (libhello.a) and not the shared (dynamic) version of the library (libhello.so).
Here my doubt is what is the use if we statically linked the shared library to the executable binary (by this way the shared library is part of the executable) and what is the point in calling it as shared libary if it can be linked statically to the executable file.
The shared library never is part of the executable. Linkage can happen in three different moments: at compile time (statically), at load time (if you dynamical linked your program to a shared library) or at run time (dynamic loading) by requesting the dynamic linker to load and link shared libraries. Only when you static link a static library with a program, the library becomes part of the executable. If you use dynamical linking your binary has information that tells the loader that it has to load the shared library and resolve symbols upon execution while dynamic loading allows you to load a shared library and call a particular function even when you didn't dynamic link your program against the library. There is a small but powerful API to do dyamic loading (dlopen, dlsym, dlerror, dlclose). It is used mostly for programs that implements plug-ins and to do function interposition (also known as function hijacking).
If we linked the shared library to the executable file using dynamic linkage, executable file will pick the library at run time.
It is not the executable who pick the library but the loader.
Here my doubt is to pick up the library at run time who will load the library into the memory and how will the executable know about the address of the function in the library to invoke the function. Thanks in advance, Leela krishna.
It is the loader who pick the library upon execution and bind the binary symbols with the ones found in the shared library. Which library it will pick depends on your configuration (i.e: the value in your LD_LIBRARY_PATH var and ld.so.conf file). The picked library doesn't even has to be the same that was used to dynamic link at compile time. To see what libraries the loader will pick for a binary execute the ldd command. $ ldd binary You can also force to load a specific shared library before any other (including libc) by setting LD_PRELOAD to the library path. Hope its helps. Best regards, ----------------------------------------- Javier MartÃnez Canillas (+34) 682 39 81 69 PhD Student in High Performance Computing Computer Architecture and Operating System Department (CAOS) Universitat Autònoma de Barcelona Barcelona, Spain