The library clause defines the logical names of design libraries, which are used by the design units. A library is a storage facility for previously analysed design units. In practice, this relates mostly to packages.

When a package is to be used in a design, it has to be made visible to the design. In order to specify it, the library clause (making the library visible) and use clause (making particular declarations visible) must be used. See use clause for more details.

There are two predefined libraries, which are used implicitly in every design: STD and WORK. The first of them contains standard packages STANDARD and TEXTIO. The other is a working library, where all user-created and analysed design units are stored.

User-specified packages are stored in the working library WORK.

Examples

library IEEE;
use IEEE.Std_Logic_1164.all;

Such declarations as in the above example must precede each design entity, which uses any of the declarations in the package Std_Logic_1164.

Important Notes

A library specified in a library clause of the primary design unit (entity, configuration or package) is visible in each secondary unit (architecture or package body) associated to it.
Library STD (containing packages STANDARD and TEXTIO) need not to be specified. Both packages are automatically included in every design unit.
Package Std_Logic_1164 is specified in the library IEEE. The library IEEE clause must be used in design units, which will use this package.
Library clause may contain more than one library names separated by commas.