fwrite sometimes uses 5 bytes to write a 4-byte single

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For me, the following code
fid = fopen('test.dat','wt');
x = single(1.28);
fwrite(fid,x,'single');
bytes = ftell(fid)
produces the result
bytes =
5
instead of the expected result
bytes =
4
which I get for most other values of x. The value of 1.28 is one example, but not an isolated case. In general, about 1 to 2 percent of values take 5 bytes instead of 4. For example,
fid = fopen('test.dat','wt');
x = single(rand(10000,1));
fwrite(fid,x,'single');
bytes = ftell(fid)
produces the result
bytes =
40127
Does anyone know what is going on here and why?

Accepted Answer

Walter Roberson
Walter Roberson on 27 Jul 2011
If you want to use fwrite() you should not be opening the file as a text file ('wt'). Opening as a text file tells the file system to convert the line terminator.
>> typecast(single(1.28),'uint8')
ans =
10 215 163 63
10 happens to correspond to newline, so that is going to be converted to the byte sequence 13 10 215 163 63 (CR LF then some bytes)
When you are working with binary, leave the 't' off of 'rt' and 'wt'.
  2 Comments
James Riehl
James Riehl on 27 Jul 2011
Thanks! I figured it was something simple, but I didn't realize 'wt' was specific for text.
Jan
Jan on 27 Jul 2011
@Walter: The next text-mode file access problem. I hope this strange feature (I do not claim that it is never useful) will disappear in the next 50 years...

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