Figure 6 Linking with an Archive Library
What are Shared Libraries?
Like an archive library, a shared library contains object code. However, ld treats shared libraries quite differently from archive libraries. When linking an object file with a shared library, ld does not copy object code from the library into the a.out file; instead, the linker simply notes in the a.out file that the code calls a routine in the shared library. An a.out file that calls routines in a shared library is known as an incomplete executable.
The Dynamic Loader
When an incomplete executable begins execution, the
The kernel activates the dynamic loader for an a.out.The dynamic loader then loads and maps any required shared libraries into the process's address space - known as attaching the libraries. A program calls shared library routines indirectly through a linkage table that the dynamic loader fills in with the addresses of the routines. By default, the dynamic loader places the addresses of shared library routines in the linkage table as the routines are called - known as deferred binding. Immediate binding is also possible - that is, binding all required symbols in the shared library at program startup. In either case, any routines that are already loaded are shared.
Consequently, linking with shared libraries generally results in smaller a.out files than linking with archive libraries. Therefore, a clear benefit of using shared libraries is that it can reduce disk space and virtual memory requirements.
NOTE: In prior releases, data defined by a shared library was copied into the program file at link time. All references to this data, both in the libraries and in the program file, referred to the copy in the executable file. With the
Default Behavior When Searching for Libraries at Run Time
If the dynamic loader cannot find a shared library from the list by default, it generates a
$ ld /usr/lib/hpux32/crt0.o prog.o
In
What are Shared Libraries? 91