User's Manual

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 HP-UX dynamic loader looks at the a.out
file to see what libraries the a.out file needs during execution.
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 HP-UX 10.0 release, however, this data copying is eliminated.
Data is accessed in the shared library itself. The code in the executable file references the shared
library data indirectly through a linkage pointer, in the same way that a shared library references
the data.
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 run-time
error and the program aborts. In PA-32 compatibility mode (with +compat), for example, suppose
that during development, a program is linked with the shared library liblocal.so in your current
working directory (say, /users/hyperturbo):
$ ld /usr/lib/hpux32/crt0.o prog.o -lc liblocal.so
In PA-32 mode, the linker records the path name of liblocal.so in the a.out file as
/users/hyperturbo/liblocal.so. When shipping this application to users, you must ensure
that (1) they have a copy of liblocal.so on their system, and (2) it is in the same location as
What are Shared Libraries? 91