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From: Janis Papanagnou <janis_papanagnou+ng@hotmail.com>
Newsgroups: comp.lang.c
Subject: Re: Buffer contents well-defined after fgets() reaches EOF ?
Date: Mon, 10 Feb 2025 02:40:28 +0100
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On 10.02.2025 02:32, Ben Bacarisse wrote:
> Janis Papanagnou <janis_papanagnou+ng@hotmail.com> writes:
> 
>> To get the last line of a text file I'm using
>>
>>     char buf[BUFSIZ];
>>     while (fgets (buf, BUFSIZ, fd) != NULL)
>>         ;   // read to last line
>>
>> If the end of the file is reached my test shows that the previous
>> contents of 'buf' are still existing (not cleared or overwritten).
> 
> Something that has not yet come up (as far as I can see) is that you
> might need to handle an empty file.  In such a case, nothing gets
> written and fgets returns NULL right away.  Processing buf in this
> situation is then undefined.

I haven't considered that at all because my context is very specific;
it's just three text lines (a comment line, an empty separator line,
and a line with the payload data that I'm interested in). If there's
a file it will have exactly these three lines (and all correctly and
consistently terminated).

> 
> One way to handle this is to put into buf something that can't get read
> by fgets.  Two newlines is a good candidate:
> 
>   char buf[BUFSIZE] = "\n\n";
> 
> You can then test for that if need be, though of course it all depends
> on what your application is doing.

Thanks for pointing it out and for the suggestion.

Janis