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:
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:
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.