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Date: Wed, 27 Aug 2014 12:52:45 -0400
From: Rich Felker <>
Subject: Re: fgets behaviour after eof

On Wed, Aug 27, 2014 at 01:16:20PM +0200, Szabolcs Nagy wrote:
> the C standard requires that
> "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."
> but musl's fgets always terminates the buffer with \0 even after EOF,
> this is easy to fix:
> -       *p = 0;
> +       if (s) *p = 0;

This is wrong and should be fixed, yes.

> However the behaviour of fgets(s, 1, f) is unclear if feof(f) is true,
> in this case nothing is read so fgets cannot "encounter" end-of-file,
> so it may set s[0]=0 and return s or it could check feof and return 0.
> (glibc does not check feof)

This is a known WONTFIX bug in glibc from the Drepper era. Remember,
all stdio read operations are defined in terms of fgetc, for which it
is specified:

"Upon successful completion, fgetc() shall return the next byte from
the input stream pointed to by stream. If the end-of-file indicator
for the stream is set, or if the stream is at end-of-file, the
end-of-file indicator for the stream shall be set and fgetc() shall
return EOF."

Thus, fgets encounters EOF from fgetc.


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