Reading binary data with fstream method

Question

I need to read binary data to buffer, but in the fstreams I have read function reading data into char buffer, so my question is:

How to transport/cast binary data into unsigned char buffer and is it best solution in this case?

Example

  char data[54];
  unsigned char uData[54];
  fstream file(someFilename,ios::in | ios::binary);
  file.read(data,54);
  // There to transport **char** data into **unsigned char** data (?)
  // How to?

score 4 · Accepted Answer · answered Nov 23 '13 at 21:47

4

Just read it into unsigned char data in the first place

unsigned char uData[54];
fstream file(someFilename,ios::in | ios::binary);
file.read((char*)uData, 54);

The cast is necessary but harmless.

answered Nov 23 '13 at 21:47

john

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score 2 · Answer 2 · edited Nov 23 '13 at 21:57

2

You don't need to declare the extra array uData. The data array can simply be cast to unsigned:

unsigned char* uData = reinterpret_cast<unsigned char*>(data);

When accessing uData you instruct the compiler to interpret the data different, for example data[3] == -1, means uData[3] == 255

edited Nov 23 '13 at 21:57

Dietmar Kühl

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answered Nov 23 '13 at 21:47

Jasper

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Although a `reinterpret_cast(data)` works and is, as far as I know, portable using `static_cast(data)` does _not_ work. ... and I think your last statement is not correct in general: for one, `char` may be `unsigned` and I don't think two's complement is strictly required. – Dietmar Kühl Nov 23 '13 at 21:51

score 1 · Answer 3 · answered Nov 23 '13 at 21:48

1

You could just use

std::copy(data, data + n, uData);

where n is the result returned from file.read(data, 54). I think, specifically for char* and unsigned char* you can also portably use

std::streamsize n = file.read(reinterpret_cast<char*>(uData));

answered Nov 23 '13 at 21:48

Dietmar Kühl

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Reading binary data with fstream method

3 Answers3