Re: Buffer contents well-defined after fgets() reaches EOF ?

On Fri, 14 Feb 2025 19:34:59 -0000 (UTC)
Kaz Kylheku <643-408-1753@kylheku.com> wrote:

On 2025-02-14, Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:

Kaz Kylheku <643-408-1753@kylheku.com> writes:
[...] 

It would be good if fgets nuked the terminating newline.
>
Many uses of fgets, after every operation, look for the newline
and nuke it, before doing anything else.
>
There is a nice idiom for that, by the way, which avoids an
temporary variable and if test:
>
   line[strcspn(line, "\n")] = 0; 

[...]
>
Then how do you detect a partial line?  That can occur either if
the last line doesn't have a terminating newline (on systems that
permit it) or a line that's too long to fit in the array. 

 
I've seen many programs like this don't care. They have some
'char buf[4096]' and that's that.
 

IMHO, even a program that is not designed to handle long lines should
give an informative error diagnostic when it encounters one.
Your trick described below is good for that, but one has to be rather
good programmer in order to invent such trick. I believe that if one
has to be good in order to use the API it's a clear indication that the
API is *not* good.

Similarly, IMHO, the programs not designed to handle presence of null
characters in the text should give an informative error diagnostic when
they are encountered. And in that fgets() is especially unhelpful.

In a program not required or designed to handle arbitrarily
long lines, you can do something very simple (prior to the
above line[strcspn(line, "\n")] = 0 expression).
>
- zero-initialize the buffer.
 
- after every call to fgets, inspect the value of the second-to-last
  array element.  If the value is neither zero, nor '\n', then somehow
  diagnose that a too-long line has been presented to the program,
  contrary to its documented limitations.
 
This will yield a false positive on an unterminated last line.  That
issue can be added as a documented limitation, or else the buffer can
be sized one greater than what the documented line length limit
requires, so that the program allows inner lines to be one character
longer than the documented limit, but is strict with regard to an
unterminated last line.