c++ - File Stream failing on read after seeking to end of file -

i need read last 64kb large file, after seeking end of file, 64kb, reads on file fail.

here's snippet:

string filename = "test.txt"; ifstream::streampos pos; // used keep track of position in file ifstream fs;  fs.open(filename, ios::binary); fs.seekg(0, ios::end); pos = fs.tellg();  // ....  fs.seekg(-16, ios::cur);  char buff[16];  fs.read(buff, 16);  // ...  fs.close(); 

the problem on fs.read() eofbit, failbit, , badbit set , file stream becomes unusable. worth noting file i'm reading being continuously appended to. also, approach works , doesn't, makes debugging difficult.

is there reason why happening?

your problem call tellg after seeking end of file. tellg causes pointer advance invoking eof state on stream: here issue:

fs.open(filename, ios::binary); fs.seekg(0, ios::end); pos = fs.tellg();   // <== causes eof on stream 

your next call fs.seekg should clear eof state, better call fs.clear() , check return insure succeeds.