On Wed, Oct 26, 2005 at 10:18:38AM +0200, Tshepang Lekhonkhobe wrote: >Hello, >I wanted to avoid some searching headache by asking this question here: >I hear that the advantage of shared libraries is that there's only one >copy of that library in memory and I assume that the library is loaded >in memory only the first moment that a dependant package is run. But >who's reponsible for making sure whose turn it is to use that library >in case of several dependant packages requiring its use? Is it the >kernel? Now, I'm no expert on this, so if anyone can elaborate more please do so. There are two levels of sharing of libraries. They can be shared in the sense that two binary executables load the same file from disk, and they can be shared in the sense that two processes share the memory pages where a library is loaded. I assume the latter is more interesting for you. (IIRC you get the former type by not using -fPIC when compiling the object files.) When the kernel loads the executable binary from disk it first performs some stuff related to the ELF data in the binary, then it hands things over to the runtime linker, /lib/ld-linux.so. The runtime linker maps share libraries to the process memory, performs some relocations and then returns execution to the program's entry point. Each ELF binary consists of sections, roughly they can be separated into executable sections (containing code) and data sections. When two processes share a loaded library they share the code sections, while the data sections are per-process. The resource they share is read-only, hence no risk of them stepping on each other's toes. /M -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus@therning.org http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. early every electrical engineer believes deep in his heart that he is a better at writing computer programs than any computer programmer, and can show as proof the fact that he has written a number of small applications, each of which was done quickly, easily, and exactly met his needs.
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