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转自：http://unix.stackexchange.com/questions/5719/linux-gnu-gcc-ld-version-scripts-and-the-elf-binary-format-how-does-it-wor/10317#10317

First of all, ELF is the specification use by Linux for executable files (programs), shared libraries, and also object files which are the intermediate files found when compiling software. Object files end in .o, shared libraries end with .so followed by zero or more digits separated by periods, and executable files don't have any extension normally.

There are typically three forms to name a shared library, the first form simply ends in .so. For example, a library called readline is stored in a file called libreadline.so and is located under one of /lib, /usr/lib, or /usr/local/lib normally. That file is located when compiling software with an option like -lreadline. -l tells the compiler to link with the following library. Because libraries change from time to time, it may become obsolete so libraries embed something called a SONAME. The SONAME for readline might look like libreadline.so.2 for the second version major version of libreadline. There may also be many minor versions of readline that are compatible and do not require software to be recompiled. A minor version of readline might be named libreadline.so.2.14. Normally libreadline.so is just a symbolic link to the most recent major version of readline, libreadline.so.2 in this case. libreadline.so.2 is also a symbolic link to libreadline.so.2.14 which is actually the file being used.

The SONAME of a library is embedded inside the library file itself. Somewhere inside the file libreadline.so.2.14 is the string libreadline.so.2. When a program is compiled and linked with readline, it will look for the file libreadline.so and read the SONAME embedded in it. Later, when the program is actually executed, it will load libreadline.so.2, not just libreadline.so, since that was the SONAME that was read when it was first linked. This allows a system to have multiple incompatible versions of readline installed, and each program will load the appropriate major version it was linked with. Also, when upgrading readline, say, to 2.17, I can just install libreadline.so.2.17 alongside the existing library, and once I move the symbolic link libreadline.so.2 from libreadline.so.2.13 to libreadline.so.2.17, all software using that same major version will now see the new minor update to it.