A.10.2 Text File Management
Static Semantics
The only allowed file
modes for text files are the modes In_File, Out_File, and Append_File.
The subprograms given in subclause
A.8.2
for the control of external files, and the function End_Of_File given
in subclause
A.8.3 for sequential input-output,
are also available for text files. There is also a version of End_Of_File
that refers to the current default input file. For text files, the procedures
have the following additional effects:
For the procedures Create and Open: After a file
with mode Out_File or Append_File is opened, the page length and line
length are unbounded (both have the conventional value zero). After a
file (of any mode) is opened, the current column, current line, and current
page numbers are set to one. If the mode is Append_File, it is implementation
defined whether a page terminator will separate preexisting text in the
file from the new text to be written.
Reason: For a file with mode Append_File,
although it may seem more sensible for Open to set the current column,
line, and page number based on the number of pages in the file, the number
of lines on the last page, and the number of columns in the last line,
we rejected this approach because of implementation costs; it would require
the implementation to scan the file before doing the append, or to do
processing that would be equivalent in effect.
For similar reasons, there is no requirement
to erase the last page terminator of the file, nor to insert an explicit
page terminator in the case when the final page terminator of a file
is represented implicitly by the implementation.
For the procedure Close: If the file has the current
mode Out_File or Append_File, has the effect of calling New_Page, unless
the current page is already terminated; then outputs a file terminator.
For the procedure Reset: If the file has the current
mode Out_File or Append_File, has the effect of calling New_Page, unless
the current page is already terminated; then outputs a file terminator.
The current column, line, and page numbers are set to one, and the line
and page lengths to Unbounded. If the new mode is Append_File, it is
implementation defined whether a page terminator will separate preexisting
text in the file from the new text to be written.
Reason: The behavior of Reset should
be similar to closing a file and reopening it with the given mode
The exception Mode_Error is propagated by the procedure
Reset upon an attempt to change the mode of a file that is the current
default input file, the current default output file, or the current default
error file.
22 An implementation
can define the Form parameter of Create and Open to control effects including
the following:
the interpretation of line and column
numbers for an interactive file, and
the interpretation of text formats
in a file created by a foreign program.
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