ifstream 的问题

Question

Look at this small code, it open a ifstream:

std::ifstream _fcs; 

bool openFile(char* path)
{
    istream::pos_type pos; 
    int tmp = 0;

    _fcs.open(path, fstream::binary | fstream::in);

    if(!_fcs.is_open())
        return false;

    tmp = 0;
    pos = 0x404;

    _fcs.seekg(0x404);
    pos = _fcs.tellg(); /// return zero

    _fcs >> tmp; /// 
    _fcs.read((char*)&tmp, 4);

    return true;
}

i have two problems.

1. after seekg, tellg return zero and when I read data it reads from beginning of file.
2. operator >> seems doesn't work. always return zero!

////------------------------------------------------
thanks for your attentions. I found a crazy solution, but I get confused, if I call seekg two times, it works: see this code:

 bool openFile(char* path)
 {
    istream::pos_type pos; 
    int tmp;
    bool fail;

    _fcs.open(path, fstream::binary | fstream::in);

    if(!_fcs.is_open())
        return false;

    _fcs.seekg(0x402);
    _fcs.seekg(0x402); /// When it comments, the tellg returns 0. am i crazy!?

    fail = _fcs.fail();
    assert(!fail);

    pos = _fcs.tellg(); /// return 0x402!!!

     /// _fcs >> tmp;
    _fcs.read((char*)&tmp, 4);

    return true;
}

really, what's happened?
////------------------------------------------------

please help me...
thanks in advanced.

Answer 1

Check the failbit using _fcs.fail() after your seekg call to make sure you haven't specified an invalid file position.

To double check the size use

 _fcs.seekg(0,ios::end);
 int length = _fcs.tellg();

You also need to use .read() to get the len value, as your file is binary

Answer 2

In binary mode, >> is not supposed to work, you have to use ostream::write.

Does your file actually exists and have a size? if note, you can't "move" to an arbitrary point in an empty file.