When should endfile be used, before or after reading? - vhdl

Question

While looking for examples of reading text from a file (using readline and read), I constantly see code like the following, but haven't found information on what exactly endfile does.
while not endfile(in_file) loop
readline(in_file, in_line);
read(in_line, in_data);
end loop;
In all other languages I know, end of file would be checked after reading the line from the file, not before.
What exactly are the semantics of endfile that make this usage correct?

Answer 1

From the IEEE VHDL Standard:
3.4.1 File operations ... Function ENDFILE returns FALSE
if a subsequent READ operation on an open file object whose access mode is read-only can retrieve another value from the file; otherwise, it returns TRUE.
So checking endfile prior to reading from the file is correct, since endfile returns false as long as another value/line can be read.

Answer 2

There are several languages which use an eof function or statement to determine the end of a file. These languages or library have no special return value (-1, null, nil, ...) or even a exception mechanism to report a failed read(..), so in this case its necessary to test eof before a read(..) and proof that one more read(..) won't crash the program.
Edit 1
zennehoy asked for some language examples, which use this kind of EOF statement/function:
Example 1: QBasic
open "file.txt" for input as #1
do until eof(1)
input #1,name$,street$, ...
loop
close #1
Example 2: Windows Scripting Host (WSH), alias VBA, alias VB Script:
do until fileHandle.atEndOfStream
[...]
loop
Example 3: if I remember correctly this would be Pascal
Ansi-C has also a eof functionality, but in many cases its not necessary to use it, because you can determine the end of a file, if read(..) returns less bytes than the requested bytes.
But this won't function if you are reading from streams/pipes/... because read can return less bytes and after several milliseconds there are more bytes and you can read again. A test for EOF will assure you, that a writer on a stream is closed.

Answer 3

Another example language where the end of file status is tested first is Ada as in while not End_Of_File loop, where End_Of_File is a function in package Ada.Text_IO and the default value for it's single argument of type File_Mode is In_file. As of Ada95, End_Of_File only operates on files of mode In_File or Inout_File otherwise generating an exception. (While endfile will return FALSE for WRITE_MODE in VHDL).
Early implementations of VHDL included preprocessors for Ada in the days of big iron. You can see the influence throughout the LRM. For instance there is no coupling between bit positions and integer values or description of a float format. These happen to allow VHDL to operate on a computer with native decimal ALUs.