Showing posts from 2015

Ada 2012 Type Invariants and Predicates

Constrained Data Types and Subtypes The Ada language has always had a limited ability to specify type invariants through the definition of range constraints on scalar data types. Along with giving the programmer the ability to specify range constraints, the Ada language has always provided some pre-defined subtypes of the Integer data type. For instance, the subtype Natural is defined as
subtype Natural is Integer range 0..Integer’Last;
This definition specifies that Natural is an integer with a constrained range of values, in this case the smallest valid Natural value is 0 and the highest is the maximum valid Integer value. Similarly, Ada provides the pre-defined subtype Positive, defined as
subtype Positive is Integer range 1..Integer’Last;
Type Integer is a signed data type, allowing both positive and negative values, but subtype Positive is an integer that can only contain a positive value. While range specifications are very useful, they are also highly limited in their usefulnes…

Shared Resource Design Patterns

Summary Many applications are constructed of groups of cooperating threads of execution. Historically this has frequently been accomplished by creating a group of cooperating processes. Those processes would cooperate by sharing data. At first, only files were used to share data. File sharing presents some interesting problems. If one process is writing to the file while another process reads from the file you will frequently encounter data corruption because the reading process may attempt to read data before the writing process has completely written the information. The solution used for this was to create file locks, so that only one process at a time could open the file. Unix introduced the concept of a Pipe, which is effectively a queue of data. One process can write to a pipe while another reads from the pipe. The operating system treats data in a pipe as a series of bytes. It does not let the reading process access a particular byte of data until the writing process has comple…