Get file size when in buffer

This is a discussion on Get file size when in buffer within the C++ Programming forums, part of the General Programming Boards category; I gota BYTE* buffer, which contains the contents of a file. Problem is when I want to output the contents ...

  1. #1
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    Get file size when in buffer

    I gota BYTE* buffer, which contains the contents of a file.

    Problem is when I want to output the contents of the buffer, into a new file (copy).
    I need to retrieve the size of the BYTE* buffer, and casting it to char* and using strlen() doesn't work since it breaks on EOF-bytes, which there sometimes are in files.

    Any solutions?

  2. #2
    C++まいる!Cをこわせ! Elysia's Avatar
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    None. You need to know the size beforehand.
    Although if it's a text file, then strlen might work to get the length of the text inside the file.
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    Confused Magos's Avatar
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    Consider using std::vector<BYTE> instead of BYTE* as it contains more features (like buffer size and automatic memory deallocation).
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  4. #4
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    Quote Originally Posted by Magos View Post
    Consider using std::vector<BYTE> instead of BYTE* as it contains more features (like buffer size and automatic memory deallocation).
    and I'm pretty sure vector doesn't care about NULLs or EOF bytes, the size it reports is simply the size of the data you put in it.

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    Yes, I agree with that. The problem, however, with using vector<BYTE> and file-reading functions is that you either need to use another buffer to store temporary results, or read one byte at a time. Reading one byte at a time is inefficient, but would technically work. Reading multiple bytes still presents the problem of knowing how many byte was read, which is what the original post (sort of) is asking for.

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    Confused Magos's Avatar
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    Quote Originally Posted by cpjust View Post
    and I'm pretty sure vector doesn't care about NULLs or EOF bytes, the size it reports is simply the size of the data you put in it.
    If that size isn't what you need, then what size do you need?
    A vector of size 0 is equal to a NULL pointer.
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    Quote Originally Posted by matsp View Post
    Yes, I agree with that. The problem, however, with using vector<BYTE> and file-reading functions is that you either need to use another buffer to store temporary results, or read one byte at a time.
    I'm not sure the impact would be that great, since C++ iostreams are buffered. It would be easy to do some time trials to be sure. By default, I wouldn't worry about it. Maybe something like this:

    Code:
    std::ifstream input_file(...);
    input_file >> std::noskipws;
    std::vector<BYTE> file_data(std::istream_iterator<char>(input_file), std::istream_iterator<char>());

  8. #8
    CSharpener vart's Avatar
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    Quote Originally Posted by matsp View Post
    Yes, I agree with that. The problem, however, with using vector<BYTE> and file-reading functions is that you either need to use another buffer to store temporary results, or read one byte at a time. Reading one byte at a time is inefficient, but would technically work. Reading multiple bytes still presents the problem of knowing how many byte was read, which is what the original post (sort of) is asking for.

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    What is wrong with
    Code:
    std::stringstream ss;
    // this copies the entire contents of the file into the string stream
    ss << fin.rdbuf();
    // get the string out of the string stream
    std::string contents = ss.str();
    // construct the vector from the string.
    std::vector<char> buff(contents.begin(), contents.end());
    ??
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  9. #9
    Malum in se abachler's Avatar
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    What is wrogn with keepign track of how many bytes you read form th source file, you need to do this anyway, sicne a read request for X bytes is not guaranteed to return X bytes even if there are more than X bytes left in the file.
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  10. #10
    C++ Witch laserlight's Avatar
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    Would this be more efficient than vart's example?
    Code:
    std::vector<char> buff;
    std::copy(std::istreambuf_iterator<char>(fin),
              std::istreambuf_iterator<char>(),
              std::back_inserter(buff));
    rdbuf() is very efficient, but there's a temporary string involved.
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  11. #11
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    If you look back at a thead I posted in a few months ago, this method was significantly faster:
    Code:
    void Test4( ifstream&  file )
    {
    	char block[BUFSIZ + 1];
    	string fileData;
    	while ( file.read( block, BUFSIZ ) )
    	{
    		fileData += block;
    	}
    	cout << "Test4 Bytes Read: " << fileData.size() << endl;
    }

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