6.1 Input and Output

XSB's I/O is based on ISO-style streams, although it also supports older DEC-10 style file handling. The use of streams provides a unified interface to a number of different classes of sources and sinks. Currently these classes include textual and binary files, console input and output, pipes, and atoms; in the future sockets and urls may be handled under the stream interface. When streams are opened, certain actions may occur depending on the class of the source or sink and on the wishes of the user. For instance when a file F is opened for output mode, an existing file F may be truncated (in write mode) or not (in append mode). In addition, various operations may or may not be valid depending on the class of stream. For instance, repositioning is valid for an atom or file but not a pipe or console.

XSB provides several default I/O streams, which make it easier for a user to embed XSB in other applications. These streams include the default input and output streams. They also include the standard error stream, to which XSB writes all error messages. By default the standard error stream is the same as the standard output stream, but it can be redirected either by UNIX shell-style I/O redirection or by the predicates file_reopen/4 and file_clone/3. Similarly there is the standard warning stream (to which all system warnings are written), the standard debugging stream (to which debugging information is written), and the standard feedback stream (for interpreter prompts, yes/no answers, etc). All of these streams are aliased by default to standard output, and can be redirected by the predicates the predicates file_reopen/4 and file_clone/3.

Streams may also be aliased: the default input and output streams can be denoted by user_in and user_out and they refer to the process'es standard input and standard output streams ^6.1.

Streams are distinguished by their class - whether they are file or atom, etc.; as well as by various properties. These properties include whether a stream is positionable or not and whether a (file) stream is textual or binary.

• Console The default streams mentioned above are console streams, which are textual and not repositionable.
• File A file stream corresponds to an operating system file and is repositionable. On Windows, binary files and textual files differ, while on UNIX they are the same.
• Atom XSB can read from an atom, just as it can from a file. Atoms are considered to be textual and repositionable. Writing to atoms via streams is not currently available in XSB, although the predicate term_to_atom/[2,3] contains much of the functionality that such streams would provide.

• Pipe XSB can also open pipes either directly, or as part of its ability to spawn processes. When made into streams, pipes are textual and not repositionable.