Expected file position after FSCANF conversion failure

Suppose the file data.txt contains the lines

--VarX
0.0 0.0 1.5 10.3

Is there an expected output of the following script

fid    = fopen('data.txt');
[v, n] = fscanf(fid, '%f');
ftell(fid)
fclose(fid);

? Obviously, the FSCANF call will fail to convert any floating point numbers upon encountering the '--' string whence isempty(v) and n==0. However, I'm currently most concerned about the FTELL call. In MATLABs 7.7 (R2008b) and 7.9 (R2009b), FTELL returns 1 while MATLABs 7.10 (R2010a) and 7.13 (R2011b) both return 0.

In other words, R2009b (and earlier editions) advances the file pointer one byte, presumably because '-' is a valid leading character sequence of a (negative) floating point number. Later M-editions do apparently not advance the file pointer however, presumably because '--' is not a valid leading character sequence of a floating point number.

So, to repeat my above question: is there any defined behaviour with respect to FTELL in case of FSCANF conversion failure and, if so, did this behaviour change between R2009b and later editions of MATLAB? I cannot find anything in doc fscanf, doc ftell or the release notes.

As far as I can tell, MathWorks Technical Solution ID 1-426ARH is related to this question, but does not appear to be exactly the same thing. Am I just misreading the information therein?

PS Before you ask: For various reasons (unrelated to this particular issue) I cannot simply use

textscan(fid, '%f', 'CommentStyle', '--')

in the general case I'm trying to solve.

Answer 1

[Bård Skaflestad]

In case anyone else encounters the issue, I asked this question to the MathWorks support team. MATLAB R2010a corrected a bug whereby fscanf would consume characters from the input stream even in the case of a conversion failure.

Officially, ftell is supposed to return 0 in the example I provided originally. Do be aware, however, that releases prior to R2010a may return 1. As such, I think it is better to not write code that depends on a particular return value from ftell in case of an fscanf input failure.