HP Fortran Programmer's Guide (B3908-90031; September 2011)

Compiling and linking
Linking HP Fortran programs
Chapter 2 83
Linking to shared libraries
Many HP Fortran libraries as well as HP-UX libraries exist in both shared and archive versions, as indicated
by the library extension name (.sl or .a or .sl on Itanium ). For example, there are both shared and
archive versions of the HP Fortran runtime library, /usr/lib/libcl.sl and /usr/lib/libcl.a.
The difference between a shared library and an archive library is that the linker does not actually link the
code in a shared library with your program. Instead, any references that your program makes to entities in
the shared library are resolved at load-time, when the library is loaded into the executable program’s address
space. By contrast, code in the archive library is copied to the executable program file.
The advantages of linking shared libraries are:
• The executable is smaller than it would be if linked with an archive file because the executable file is
incomplete—it doesn’t include code from the library.
• Using shared libraries ensures that you always get the most recent version of the library. If you link
with an archive version, you get the version that was available at link-time. If, later on, you want a
more recent version of the library, you must re-link your program with that library.
The disadvantage of linking with a shared library is that it creates a dependency between the library and the
program; both the shared library and the program must always be installed together on the same system. By
contrast, linking with an archive library makes the executable program independent of the library to which it
was linked. Also, programs that make frequent calls to library routines may run more slowly when linked to
shared libraries.
By default, the linker selects the shared version of a library, if one is available; otherwise, it selects the
archive version.
NOTE For libF90, libU77, and libIO77 (Itanium only), archive libraries are selected by
default (see +sharedlibF90, +sharedlibU77, and +sharedlibIO77 options).
To force the linker to select archive libraries, specify the -Wl,-a,archive option on the f90 command
line. f90 passes the arguments to the -Wl option (-a and archive) to the linker. This option must appear
before the names of any libraries also specified on the command line. The following command line compiles
prog.f90 and links it with the archive versions of the default libraries as well as with the math library (as
specified by the -lm option):
$ f90 -Wl,-a,archive prog.f90 -lm
For information about the linker’s -a option, see the ld(1) man page. For more information about shared
libraries, see “Creating shared libraries” on page 90.