reading fortran binary file in c++

Question

The problem of reading (with c++ program) binary file generated by fortran code has been asked many times and the satisfactory description of conventions in fortran records has been given (e.g. http://local.wasp.uwa.edu.au/~pbourke/dataformats/fortran/ )

However when I try to implement c++ program, keeping in mind fortran conventions it still does not work. Here I assume we that the binary file "test.bin" contains 1 integer and is written in binary format by fortran routines. Here is how I try to read it in c++:

#include <iostream>
#include <fstream>
using namespace std;

int main () {
  ifstream file;
  file.open("test.bin", ios::in|ios::binary);
  if (file.is_open())
  {
    int ival;
    file >> ival >> ival;  cout<< ival<<endl;
   }
   return 0;
}

Here the double >>ival construction first reads the header of the fortran record (which contains the size of the record in bytes) and the second >>ival supposed to extract the value. The integer written in file is 8, but the program outputs 0, so it does not read the data properly.

Here is a content of the binary file: ^D^@^@^@^@^@^@^@^H^@^@^@^D^@^@^@^@^@^@^@

So my question - what am I doing wrong?

Here is what hex editor shows:

0000000: 0400 0000 0000 0000 0800 0000 0400 0000  ................
0000010: 0000 0000 0a                             .....

Any idea what that means?

figure out if it is big endian or little endian. Intel is little endian. ^@ = 0, ^D = 4, ^H = 8. Looks to me like little endian, record size is 4, value is 8. I do not know what the last 8 bytes are. — JimR
The >> operator reads textual data (i.e., it tries to parse an decimal representation of an integer). Use the read member function to read binary data. — Philipp
Well, the second integer, assuming 32-bit little-endian, is actually 0, next 8 and then 4. If the first integer written in the file is actually 8, it may be that the size header is a 64-bit integer - quite reasonable with large disk files. — Martin James

Robᵩ Robᵩ · Accepted Answer · 2012-04-17T19:55:53

operator>> is the formatted input operator. It is used to read text files, converting the textual representation to binary.

You should be reading using unformatted input operations. Try this:

file.read(reinterpret_cast<char*>(&ival), sizeof ival);

Of course, after you read it, you may need to byte swap for correct endian representation.

reading fortran binary file in c++

2 Answers