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numbers of the libraries for which the program was compiled (that is, the version of the stub routines used), and the name of the file that contains each shared library. This is the file that ld.so will find when the program is executed. The first file in the list, by the way, linux-gate.so.1, is not a real shared library, but rather a so-called dynamic shared object provided by the kernel, a technicality that speeds up system calls into the kernel and provides other useful low-level things.

To use a shared library, the version of the stub routines (in the executable) must be compatible with the version of the shared libraries. Basically, a library is compatible if its major version number matches that of the stub routines. The major version number is the part right after the .so. In this case, libX11 (the most basic library used by the X Window System) is used with the major Version 6. The library file libX11.so.6 (which usually resides in /usr/X11R6/lib) might very well just be a symbolic link—for example, to libX11.so.6.2. This means that the library has the major version number 6 and the minor version number 2. Library versions with the same major version number are supposed to be interchangeable. This way, if a program was compiled with Version 6.0 of the stub routines, shared library Versions 6.1, 6.2, and so forth could be used by the executable. If a new version with the major version number 6 and the minor version number 3 were released (and thus had the filename libX11.so.6.3), all you would need to do to use this new version is change the symbolic link libX11.so.6 to point to the new version. The xterm executable would then automatically benefit from any bug fixes or similar that are included in the new version. In "More Fun with Libraries" in Chapter 21, we describe how to use shared libraries with your own programs.

The file /etc/ld.so.conf contains a list of directories that ld.so searches to find shared library files. An example of such a file is:

/usr/lib

/usr/local/lib

/usr/X11R6/lib

/opt/kde3/lib

ld.so always looks in /lib and /usr/lib, regardless of the contents of ld.so.conf. Usually, there's no reason to modify this file, and the environment variable LD_LIBRARY_PATH can add additional directories to this search path (e.g., if you have your own private shared libraries that shouldn't be used system-wide). However, if you do add entries to /etc/ld.so.conf or upgrade or install additional libraries on your system, be sure to use the ldconfig command, which will regenerate the shared library cache in /etc/ld.so.cache from the ld.so search path. This cache is used by ld.so to find libraries quickly at runtime without actually having to search the directories on its path. For more information, check the manual pages for ld.so and ldconfig.

Now that you understand how shared libraries are used, let's move on to upgrading them. The two libraries that are most commonly updated are libc (the standard C library) and libm (the math library). Because naming is a little bit special for these, we will look at another library here, namely libncurses, which "emulates" a graphical windowing system on the text console.

For each shared library, there are two separate files:

.a

This is the static version of the library. When a program is statically linked, routines are copied from this file directly into the executable, so the executable contains its own copy of the library routines.[*]

< library>.so.

This is the shared library image itself. When a program is dynamically linked, the stub routines from this file are copied into the executable, allowing ld.so to locate the shared library at runtime. When the program is executed, ld.so copies routines from the shared library into memory for use by the program. If a program is dynamically linked, the .a file is not used for this library.

For the libncurses library, you'll have files such as libncurses.a and libncurses.so.5.4. The .a files are generally kept in /usr/lib, and .so files are kept in /lib. When you compile a program,