ifstream in text mode + seekg problem

[Vadim Volevach]

Hi,

// Under C++Builder 2007 (+ Dinkumware)
#include <fstream>
using namespace std;

int main (int argc, char* argv[])
{
// Create a DOS-Text file, that has only two lines:
// - the first line is empty ("\r\n"),
// - the second line: "ABC".
// Then:

ifstream is ("c:\\abc.txt");

while (is.peek() != 'A') // It's a dead loop here... Why?
is.seekg (1, ios::cur);

return 0;
}

Thanks,
Vadim

 

[James Kanze]

// Under C++Builder 2007 (+ Dinkumware)
#include <fstream>
using namespace std;

int main (int argc, char* argv[])
{
// Create a DOS-Text file, that has only two lines:
// - the first line is empty ("\r\n"),
// - the second line: "ABC".
// Then:

ifstream is ("c:\\abc.txt");

while (is.peek() != 'A') // It's a dead loop here... Why?
is.seekg (1, ios::cur);
return 0;
}

It's undefined behavior. The only seeking you can do in a text
file is to the start of the file, to the current position, or to
a streampos returned from a call to tell[gp]().