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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Fgets question
Date: Mon, 06 Jul 2015 16:29:13 -0700
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Paul N <gw7rib@aol.com> writes:
> I just had a moment of doubt about fgets, and rather to my surprise,
> neither MSDN or the C FAQ seemed to answer my question.
>
> If I read a file using fgets, which doesn't end with a newline, am I
> right in thinking that on attempting to read the last line, the
> function will read in the characters that are there, append a nul but
> no newline, and return a non-NULL value? And that next time I try
> fgets, it will return NULL?

Yes, generally.  I believe the description of the fgets() function in
the standard makes this reasonably clear (N1570 7.21.7.2p2):

    The fgets function reads at most one less than the number of
    characters specified by n from the stream pointed to by stream
    into the array pointed to by s. No additional characters are
    read after a new-line character (which is retained) or after
    end-of-file. A null character is written immediately after the
    last character read into the array.

p3:

    The fgets function returns s if successful. If end-of-file is
    encountered and no characters have been read into the array,
    the contents of the array remain unchanged and a null pointer
    is returned. If a read error occurs during the operation, the
    array contents are indeterminate and a null pointer is returned.

> So in particular, if fgets(buff, len, fp) returns a non-NULL value,
> then buff[0] is not nul?

Yes, I believe that's correct.

*However* the description of text streams in 7.21.2 says that a trailing
new-line may be required.  (By implication, if there is no new-line at
the very end of the file, the behavior is undefined.)

    Data read in from a text stream will necessarily compare equal
    to the data that were earlier written out to that stream only
    if: the data consist only of printing characters and the control
    characters horizontal tab and new-line; no new-line character is
    immediately preceded by space characters; and the last character
    is a new-line character.

On Unix-like systems, and *I think* on Windows systems, omitting the
newline at the end of the last line is acceptable.

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Working, but not speaking, for JetHead Development, Inc.
"We must do something.  This is something.  Therefore, we must do this."
    -- Antony Jay and Jonathan Lynn, "Yes Minister"