4.4.1 Tradeoffs for Static Libraries

There are three main issues to keep in mind regarding static, as compared to dynamic, libraries and linking:

• Static libraries are more self-contained but less adaptable.

  If you bind an a.out executable file statically, the library routines it needs become part of the executable binary. However, if it becomes necessary to update a static library routine bound into the a.out executable, the entire a.out file must be relinked and regenerated to take advantage of the updated library. With dynamic libraries, the library is not part of the a.out file and linking is done at runtime. To take advantage of an updated dynamic library, all that is required is that the new library be installed on the system.

• The “elements” in a static library are individual compilation units, .o files.

  Since a single compilation unit (a source file) can contain more than one subprogram, these routines when compiled together become a single module in the static library. This means that all the routines in the compilation unit are loaded together into the a.out executable, even though only one of those subprograms was actually called. This situation can be improved by optimizing the way library routines are distributed into compilable source files. (Still, only those library modules actually referenced by the program are loaded into the executable.)

• Order matters when linking static libraries.

  The linker processes its input files in the order in which they appear on the command line—left to right. When the linker decides whether or not to load an element from a library, its decision is determined by the library elements that it has already processed. This order is not only dependent on the order of the elements as they appear in the library file but also on the order in which the libraries are specified on the compile command line.

Example: If the Fortran program is in two files, main.f and crunch.f, and only the latter accesses a library, it is an error to reference that library before crunch.f or crunch.o:

demo% f95 main.f -lmylibrary crunch.f -o myprog

(Incorrect)

demo% f95 main.f crunch.f -lmylibrary -o myprog    

(Correct)