The problem of sharing resources between processes was briefly discussed in Chapter 3. Two requirements were identified as being essential: mutual exclusion and condition synchronisation. This chapter discusses various ways in which these requirements can be met in Ada without having to encapsulate the resource in a server task and without having to use the rendezvous. Ada gives direct support to protected data by the protected object feature, the discussion of which is the main focus of this chapter. However, the language does also support the notions of atomic and volatile data, which are covered in Section 7.13.

Protected objects

A protected object in Ada encapsulates data items and allows access to them only via protected subprograms or protected entries. The language guarantees that these subprograms and entries will be executed in a manner that ensures that the data is updated under mutual exclusion. Consequently, they are rather like monitors found in previous concurrent programming languages (as described in Chapter 3).

A protected unit may be declared as a type or as a single instance; it has a specification and a body.