Why does FEOF return false when opening an empty text file in MATLAB?

I am using the following code to read a text file line by line. I have noticed that if the file is empty, FEOF returns a 0 and I run through the loop at least one time.

fid = fopen('foo.txt');
while ~feof(fid)
     s=fgetl(fid);
     % do something with s
end
fclose(fid);

Answer 1

[MathWorks Support Team]

This behavior comes from the ANSI spec for C/C++'s "feof". FEOF does not return true until you actively read past the end of the file. This means that even after reading the last byte of a file, FEOF would return false. It would only return true if you then tried to read another byte.

As a result a better way to read the file is:

fid = fopen('foo.txt');
s=fgetl(fid)
while ~feof(fid)
     % do something with s
     s=fgetl(fid)
end
fclose(fid);

[Walter Roberson]

For C++ there are arguments to be made that it should be referred to as the ISO spec rather than the ANSI spec. ANSI X3J16 committee formally adopted ISO/IEC 14882:1998 which was a joint effort between ANSI X3J16 and ISO WG21.

My guess is that formally you are relying on POSIX's definition of feof() via IEEE 1003.1, which in turn defers to ISO C rather than to ANSI C.

Not that it really matters except to some of us old-timers who lived and breathed K&R ;-)