It obviously saves on compilation time if these commonly-used procedures can be kept in compiled form as object modules. Almost all operating systems allow a collection of object modules to be stored in an object library (sometimes known as a pre-compiled or relocatable-code library). This is a file containing a collection of object modules together with an index which allows them to be extracted easily. Object libraries are not only more efficient but also easier to use as there is only one file-name to specify to the linker. The linker can then work out for itself which modules are needed to satisfy the various CALL statements and function references encountered in the preceding object modules. Object libraries also simplify the management of a procedure collection and may reduce the amount of disc space needed. There are usually simple ways of listing the contents of an object library, deleting modules from it, and replacing modules with new versions.
All Fortran systems come with a system library which contains the object modules for various intrinsic functions such as SIN, COS, and SQRT. This is automatically scanned by the linker and does not have to be specified explicitly.
Software is often available commercially in the form of procedure libraries containing modules which may be linked into any Fortran program. Those commonly used cover fields such as statistics, signal processing, graphics, and numerical analysis.