Structure of the SDDS Header

The first line of a data set must be of the form ``SDDSn'', where n is the integer SDDS version number. This document describes version 1.

The SDDS header consists of a series of namelist-like constructs, called namelist commands. These constructs differ from FORTRAN namelists in that the SDDS routines scan each construct, determine which it is, and use the data appropriately. There are six namelist commands recognized under Version 1. Each is listed below along with the data type and default values.

For each command, an example of usage is given. Several styles of entering the namelist commands are exhibited. I suggest that the user choose a style that makes it easy to pick out the beginning of each command. Note that while each namelist command may occupy one or more lines, no two commands may occupy portions of the same line.

Any field value containing an ampersand must be enclosed in double quotes, as must string values containing whitespace characters.

Another character with special meaning is the exclamation point, which introduces a comment. An exclamation point anywhere in a line indicates that the remainder of the line is a comment and should be ignored. A literal exclamation point is obtained with the sequence \!, or by enclosing the exclamation point in double quotes.

The commands are briefly described in the following list, and described in detail in the following subsections:

• description -- Specifies a data set description, consisting of informal and formal text descriptions of the data set.
• column -- Defines an additional column for the tabular-data section of the data pages.
• parameter - Defines an additional parameter data element for the data pages.
• array -- Defines an additional array data element for the data pages.
• include -- Directs that header lines be read from a named file. Rarely used.
• data -- Defines the data mode (ASCII or binary) along with layout parameters, and is always the last command in the header.

The column, parameter, and array commands have a name field that is used to identify the data being defined. Each type of data has a separate ``name-space'', so that one may, for example, use the same name for a column and a parameter in the same file. This is discouraged, however, because it may produce unexpected results with some programs. Names may contain any alphanumeric character, as well as any of the following: @ : # + - % . _ $ & / . The first letter of a name may not be a digit.