- This clause describes the package Text_IO, which provides facilities for
input and output in human-readable form. Each file is read or written
sequentially, as a sequence of characters grouped into lines, and as a
sequence of lines grouped into pages. The specification of the package is
given below in subclause A.10.1.
- The facilities for file management given above, in subclauses A.8.2 and
A.8.3, are available for text input-output. In place of Read and Write,
however, there are procedures Get and Put that input values of suitable types
from text files, and output values to them. These values are provided to the
Put procedures, and returned by the Get procedures, in a parameter Item.
Several overloaded procedures of these names exist, for different types of
Item. These Get procedures analyze the input sequences of characters based
on lexical elements (see Section 2) and return the corresponding values; the
Put procedures output the given values as appropriate lexical elements.
Procedures Get and Put are also available that input and output individual
characters treated as character values rather than as lexical elements.
Related to character input are procedures to look ahead at the next character
without reading it, and to read a character ``immediately'' without waiting
for an end-of-line to signal availability.
- In addition to the procedures Get and Put for numeric and enumeration
types of Item that operate on text files, analogous procedures are provided
that read from and write to a parameter of type String. These procedures
perform the same analysis and composition of character sequences as their
counterparts which have a file parameter.
- For all Get and Put procedures that operate on text files, and for many
other subprograms, there are forms with and without a file parameter. Each
such Get procedure operates on an input file, and each such Put procedure
operates on an output file. If no file is specified, a default input file or
a default output file is used.
- At the beginning of program execution the default input and output files
are the so-called standard input file and standard output file. These files
are open, have respectively the current modes In_File and Out_File, and are
associated with two implementation-defined external files. Procedures are
provided to change the current default input file and the current default
- At the beginning of program execution a default file for
program-dependent error-related text output is the so-called standard error
file. This file is open, has the current mode Out_File, and is associated
with an implementation-defined external file. A procedure is provided to
change the current default error file.
- From a logical point of view, a text file is a sequence of pages, a page
is a sequence of lines, and a line is a sequence of characters; the end of a
line is marked by a line terminator; the end of a page is marked by the
combination of a line terminator immediately followed by a page terminator;
and the end of a file is marked by the combination of a line terminator
immediately followed by a page terminator and then a file terminator.
Terminators are generated during output; either by calls of procedures
provided expressly for that purpose; or implicitly as part of other
operations, for example, when a bounded line length, a bounded page length,
or both, have been specified for a file.
- The actual nature of terminators is not defined by the language and hence
depends on the implementation. Although terminators are recognized or
generated by certain of the procedures that follow, they are not necessarily
implemented as characters or as sequences of characters. Whether they are
characters (and if so which ones) in any particular implementation need not
concern a user who neither explicitly outputs nor explicitly inputs control
characters. The effect of input (Get) or output (Put) of control characters
(other than horizontal tabulation) is not specified by the language.
- The characters of a line are numbered, starting from one; the number of a
character is called its column number. For a line terminator, a column
number is also defined: it is one more than the number of characters in the
line. The lines of a page, and the pages of a file, are similarly numbered.
The current column number is the column number of the next character or line
terminator to be transferred. The current line number is the number of the
current line. The current page number is the number of the current page.
These numbers are values of the subtype Positive_Count of the type Count (by
convention, the value zero of the type Count is used to indicate special
type Count is range 0 .. implementation-defined;
subtype Positive_Count is Count range 1 .. Count'Last;
- For an output file or an append file, a maximum line length can be
specified and a maximum page length can be specified. If a value to be
output cannot fit on the current line, for a specified maximum line length,
then a new line is automatically started before the value is output; if,
further, this new line cannot fit on the current page, for a specified
maximum page length, then a new page is automatically started before the
value is output. Functions are provided to determine the maximum line length
and the maximum page length. When a file is opened with mode Out_File or
Append_File, both values are zero: by convention, this means that the line
lengths and page lengths are unbounded. (Consequently, output consists of a
single line if the subprograms for explicit control of line and page
structure are not used.) The constant Unbounded is provided for this
- The Package Text_IO
- Text File Management
- Default Input, Output, and Error Files
- Specification of Line and Page Lengths
- Operations on Columns, Lines, and Pages
- Get and Put Procedures
- Input-Output of Characters and Strings
- Input-Output for Integer Types
- Input-Output for Real Types
- Input-Output for Enumeration Types
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