CODEWITHSUNDEEP
c++ifstream
mans
mans

Reputation: 18168

Can not read enough data from a file when the file has enough data c++

I have this code in c++ ( it is after I did some tests to see why I can not read enough data from file, so it is not final code and I am looking to find why I am getting this result)

size_t readSize=629312;
_rawImageFile.seekg(0,ifstream::end);
size_t s=_rawImageFile.tellg();
char *buffer=(char*) malloc(readSize);
_rawImageFile.seekg(0);
int p=_rawImageFile.tellg();
_rawImageFile.read(buffer,readSize);
size_t extracted = _rawImageFile.gcount();
cout << "s="<< s <<endl;
cout << "p="<< p <<endl;
cout << "readsize="<< readSize<<endl;
cout << "extracted="<< extracted <<endl;
cout << "eof ="<< _rawImageFile.eofbit<<endl;
cout << "fail="<< _rawImageFile.failbit <<endl;

The output is as follow:

s=3493940224
p=0
readsize=629312
extracted=2085
eof =1
fail=2

As you can see the file size is 3493940224 and I am at the start of file (p=0) and I am trying to read 629312 bytes, but I can only read 2085?

What is the problem with this code? I did open this file in other methods and read some data out of it, but am using seekg to move pointer to the beginning of file.

The file was opened as binary.

edit 1

To find a solution, I put all code inside a function and here is it:

    _config=config;
    ifstream t_rawImageFile;
    t_rawImageFile.open(rawImageFileName,std::ifstream::in || std::ios::binary );
    t_rawImageFile.seekg (0);
    size_t readSize=629312;
    t_rawImageFile.seekg(0,ifstream::end);
    size_t s=t_rawImageFile.tellg();
    char *buffer=(char*) malloc(readSize);
    t_rawImageFile.seekg(0);
    size_t p=t_rawImageFile.tellg();
    t_rawImageFile.read(buffer,readSize);
    size_t x=t_rawImageFile.tellg();
    size_t extracted = t_rawImageFile.gcount();
    cout << "s="<< s <<endl;
    cout << "p="<< p <<endl;
    cout << "x="<< x <<endl;
    cout << "readsize="<< readSize<<endl;
    cout << "extracted="<< extracted <<endl;
    cout << "eof ="<< t_rawImageFile.eof()<<endl;
cout << "fail="<< t_rawImageFile.fail() <<endl;

and the result is: 

s=3493940224
p=0
x=4294967295
readsize=629312
extracted=2085
eof =1
fail=1

Interestingly, after read the file pointer moves to a very big value. is it possible that since the file size is very big, the application fails?

edit 2

Tested the same code with another file. the result is as follow:

s=2993007872
p=0
x=4294967295
readsize=629312
extracted=1859
eof =1
fail=1

What I can read from this test is that: after read the file pointer moves to a big number which is always the same. The amount that it reads depend on file (!).

edit 3

After changing the size_t to fstream::pos_type the result is as follow:

s=2993007872
p=0
x=-1
readsize=629312
extracted=1859
eof =1
fail=1

Why file position goes to -1 after a read?

Upvotes: 0

Views: 947

Answers (3)

Joachim Isaksson
Joachim Isaksson

Reputation: 180917

t_rawImageFile.open(rawImageFileName, std::ifstream::in || std::ios::binary );

...does not open the file in binary mode. Since || is the lazy or operator and std::ifstream::in is non zero, the whole expression has the value 1.

t_rawImageFile.open(rawImageFileName, std::ifstream::in | std::ios::binary );

...will surely work better.

Upvotes: 2

Wacek
Wacek

Reputation: 4346

Change this line:

t_rawImageFile.open(rawImageFileName,std::ifstream::in || std::ios::binary );

into this:

t_rawImageFile.open(rawImageFileName,std::ifstream::in | std::ios::binary );

Upvotes: 1

Mats Petersson
Mats Petersson

Reputation: 129374

You don't show the part where your file is being opened, but I'm pretty sure it is missing ios::binary to make sure the C runtime code doesn't interpret CTRL-Z (or CTRL-D) as end of file.

Upvotes: 1

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