21.1 Overview of Libraries
  Libraries are files that can contain object modules, text
  modules, and shareable images. There are two types of
  libraries: system-supplied and user-supplied. System-
  supplied libraries are provided by the OpenVMS system,
  and user-supplied libraries are libraries that you create.

  Shareable images are similar to libraries; they contain code
  that can be shared by other programs. However, shareable
  images contain executable code rather than object code.

  If you have routines that are used in many programs, plac-
  ing the routines in object module libraries or shareable image
  libraries lets you access them at link time. You do not need
  to include the routines in the source code, thus shortening
  compilation time and conserving disk space.

  If you have routines that are used simultaneously by many
  different programs, placing the routines in installed shareable
  images can improve performance at run time, conserve main
  physical memory, and reduce paging I/O because one copy of
  the executable code is shared by all users.

  Object module libraries, shareable image libraries, and share-
  able images can be accessed in the BASIC environment as
  well as at DCL command level. When you link programs at
  DCL command level, these libraries can contain object code
  created by any native mode compiler or assembler.

  When you run a program in the BASIC environment, you
  can access:

  .
        Object libraries containing only BASIC object code (object
        code from SUB, FUNCTION, and PICTURE subpro-
        grams)
  .
        Shareable image libraries that contain BASIC or non-
        BASIC object code (such as a transfer vector written in
        VAX MACRO)

  Only BASIC subprograms can be loaded into the environment
  with the LOAD command.

  For a more thorough understanding of libraries and share-
  able images, see the OpenVMS Linker Utility Manual and
  the Guide to Creating Modular Library Procedures . See the
  OpenVMS System Manager's Manual for more information
  about installing shareable images. For information about text
  libraries, see Chapter 17.