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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.os.linux.misc
Subject: Re: End of file mark
Date: Wed, 19 Aug 2015 17:38:41 -0700
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Richard Kettlewell <rjk@greenend.org.uk> writes:
> John Hasler <jhasler@newsguy.com> writes:
>> T writes:
>>> Under VFS and EXT4, do text files have an End Of File
>>> marker?
>>
>> No.
>>
>>> How do programs know when they are at the end of a file?
>>
>> The filesystem knows how long the file is.  It returns EOF ("end of
>> file").
>
> The EOF return value is synthesized by stdio.  The kernel just stops
> returning bytes when the end of the file is reached.

The read() system call takes an argument specifying how many bytes it
should attempt to read, and returns the number of bytes it was able to
read.  It returns 0 to indicate end-of-file.  (Returning a positive
count smaller than the size requested does not necessarily indicate
end-of-file.)  read() returns -1 on error.

The stdio input functions (on POSIX systems) are implemented on top of
read().  Each function (getchar(), fgets(), fread(), etc.) has its own
way to indicate an end-of-file condition.  Read the man page for each
input function to see how it does this.

-- 
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"