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

Question

James Riehl 2011-7-26

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/12477-fwrite-sometimes-uses-5-bytes-to-write-a-4-byte-single

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?

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Answer 1

Walter Roberson 2011-7-27

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https://ww2.mathworks.cn/matlabcentral/answers/12477-fwrite-sometimes-uses-5-bytes-to-write-a-4-byte-single#answer_17050

在 MATLAB Online 中打开

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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James Riehl 2011-7-27

Thanks! I figured it was something simple, but I didn't realize 'wt' was specific for text.

Jan 2011-7-27

@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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