MATLAB: Fwrite sometimes uses 5 bytes to write a 4-byte single

binarybytesfwriteio

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?

Best Answer

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'.
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