Text_IO - GNAT Reference Manual

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11.5 Text_IO

Text_IO files consist of a stream of characters containing the following special control characters:

    LF (line feed, 16#0A#) Line Mark
    FF (form feed, 16#0C#) Page Mark

A canonical Text_IO file is defined as one in which the following conditions are met:

The character LF is used only as a line mark, i.e., to mark the end of the line.
The character FF is used only as a page mark, i.e., to mark the end of a page and consequently can appear only immediately following a LF (line mark) character.
The file ends with either LF (line mark) or LF-FF (line mark, page mark). In the former case, the page mark is implicitly assumed to be present.

A file written using Text_IO will be in canonical form provided that no explicit LF or FF characters are written using Put or Put_Line. There will be no FF character at the end of the file unless an explicit New_Page operation was performed before closing the file.

A canonical Text_IO file that is a regular file (i.e., not a device or a pipe) can be read using any of the routines in Text_IO. The semantics in this case will be exactly as defined in the Ada Reference Manual, and all the routines in Text_IO are fully implemented.

A text file that does not meet the requirements for a canonical Text_IO file has one of the following:

The file contains FF characters not immediately following a LF character.
The file contains LF or FF characters written by Put or Put_Line, which are not logically considered to be line marks or page marks.
The file ends in a character other than LF or FF, i.e., there is no explicit line mark or page mark at the end of the file.

Text_IO can be used to read such non-standard text files but subprograms to do with line or page numbers do not have defined meanings. In particular, a FF character that does not follow a LF character may or may not be treated as a page mark from the point of view of page and line numbering. Every LF character is considered to end a line, and there is an implied LF character at the end of the file.