Question

I’ve tried many versions of the following code in g++ ( version 4.6.3 ).

int main(int argc,char *argv[])
{
   std::string fname(argv[1]);
   std::cout<<"fname is"<<fname<<std::endl;

   //  std::fstream f(fname.c_str(),
                      std::fstream::in|std::fstream::out|std::fstream::binary);
   std::fstream f(fname.c_str(),std::ios::in|std::ios::out|std::ios::binary);    
   unsigned char b[512];
   memset(b,0xff,512);
   std::cout<<f.tellg()<<std::endl;
   f.seekg(0,std::ios::beg);
   std::cout<<f.tellg()<<std::endl;
   f.read((char *)b,512);
   for(int ii=0;ii<sizeof(sector0);ii++) 
       std::cout<<std::hex<<(int )(b[ii]) <<"  ";
   std::cout<<std::endl; 
}

In each case the result is the same I pass in the file name of the source code ( sample ascii ), tellg reports a 1 and the buffer matches the file. If I pass in the filename of the executable ( my sample binary ), tellg reports -1 and the dump is all ff’s.

Am I doing something wrong or is this a compiler bug?

Answer 1

Sorry for the short answer, but don’t have much time.

You are opening the file for writing (std::ios::out) which requires an exclusive lock on the file. If the file is already in use, acquiring the lock fails, therefor opening and reading will fail too.