fread: how to read n bytes from bin files

Hi! I have a binary file. It consists of records: 8 byte long, 4 byte float, 4 byte float, 4 byte float. I want to read the file into array Nx4. N - records number. How can I do it? I tried fread but without result.

[Alexander Voznesensky]

Is it correct?

fid = fopen('acc-angle.bin', 'rb');  % открытие файла на чтение

if fid == -1                         % проверка корректности открытия
    error('File is not opened');
end

time=[];
ax=[];
ay=[];
az=[];
records_num=0;

while ~feof(fid)

      time = [time; fread(fid,1,'int64')];        
      ax= [ax; fread(fid,1,'float32')];        
      ay= [ay; fread(fid,1,'float32')];        
      az= [az; fread(fid,1,'float32')];    
      records_num=records_num+1;

end

fclose(fid);

Answer 1

[Guillaume]

Is it correct? The easiest way for you to find out is to try.

 fid = fopen('acc-angle.bin', 'rb')

There's no 'b' option for fopen permissions. 'r' is all that is needed for binary.

Otherwise, your code looks ok to me as long as your file has little endian encoding. A faster option would be to read all records at once using the skip parameter of fread, something like:

fid = fopen('acc-angle.bin', 'r');
time = fread(fid, Inf, 'int64', 12);
fseek(fid, 'bof', 8);
ax = fread(fid, Inf, 'single', 16);
fseek(fid, 'bof', 12);
ay = fread(fid, Inf, 'single', 16);
fseek(fid, 'bof', 16);
az = fread(fid, Inf, 'single', 16);
fclose(fid);

[Walter Roberson]

After each of those fread() you should fseek() to beginning of file offset by the sum of the sizes of all of the previous fields. That is, after each fread() you are left at end of file and need to reposition.

[Guillaume]

Oops! Yes, of course. Fixed.

If the file is large, I've found that multiple calls to fread is (order of magnitude) slowers than reading the whole file all in one go, even when these calls are just reading one value after another without any seeking.

[Alexander Voznesensky]

Thanks a lot! But one mistake: In the last three lines 16 instead of 12. Am I right?

time = fread(fid, Inf, 'int64', 12);
fseek(fid, 8, 'bof');
ax = fread(fid, Inf, 'single', 16);
fseek(fid, 12, 'bof');
ay = fread(fid, Inf, 'single', 16);
fseek(fid, 16, 'bof');
az = fread(fid, Inf, 'single', 16);

[Guillaume]

Yes! Fixed as well.

[Alexander Voznesensky]

It revealed, that I have to invert byte order for each field before reading. How can I do this?

[Guillaume]

If it's the same for the whole file, in the call to fopen:

fid = fopen('acc-angle.bin', 'r', 'b');  %b for big endian

If it varies per field, in the call to fread:

time = fread(fid, Inf, 'int64', 12, 'b');

[Walter Roberson]

At the time you do the fopen() you can specify a byte order that is to apply by default to future fread().

Or, for each fread() you can specify the byte order.

Or, you can swapbytes() after doing the reading.

Answer 2

[Walter Roberson]

memmapfile allows you to declare patterns of fields and so is suitable for reading in repeated structures.