Question

I have a binary file (test.bin) which has 2 unsigned int values respectively 1000 and 4000 in it.
With the below code, I want to change the first number to 5000 with the first write function and then I want to read second number and rewrite it with 4000-2000 = 2000.
However, the program changes 1000 to 5000 but it does not change 4000 to 2000. Even though I use file.flush() or file.sync(), it does not have any effect.
Interestingly, when I put file.tellg() or file.tellp(), it works as I want. (I found it out by coincidence)
This happens both on Linux and Windows. On Linux I try to compile it with g++.
sizeof(unsigned int)=4 and I’m sure program can open test.bin.

#include <fstream>
#include <iostream>

using namespace std;

int main(){
    fstream file;
    unsigned int data, buffer;
    data=5000;
    file.open("test.bin", ios::binary | ios::in | ios::out);

    file.write((char*)&data,4); // will change first number to 5000

    // file.flush();  // Nothing changes if I delete comment signs.

    // file.tellp();  // Program works correctly if I uncomment this. 
    // file.tellg();  // Program works correctly if I uncomment this.

    file.read((char*)&buffer, 4);  // position pointer should be at the beginning of the 2nd number

    file.seekp(-4, ios::cur); // Since internal pointer is at the end of the file after the read(), I manually put it back to the beginning of the 2nd number.

    buffer-=2000;
    file.write((char*)&buffer,4); // Now, it should rewrite 2nd number with 2000. 

    file.close();
    return 0;
}

Answer 1

Bingo! You have stumbled upon a very obscure, almost unwritten and at best ambiguous requirement of the C++ language. I tried to argued for removing it from G++, and wrote a patch to do so, but met a lukewarm response from the maintainers which was accepted after I showed it could be done without a heavy redesign.

If memory serves, the C++ specification mentions that file streams work with the same semantics as C <stdio.h> streams, and the C spec says that you must seek between reading and writing to the same file.

Here is the bug: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45708

Discussion #1: http://gcc.gnu.org/ml/libstdc++/2010-09/msg00079.html

Discussion #2: http://gcc.gnu.org/ml/libstdc++/2010-09/msg00104.html

EDIT: I need a better memory! Actually that was committed on September 22, 2010, into GCC 4.6.0. The remaining edits which weren’t committed related to changing the locale codecvt facet during I/O.

The bug tracking the accepted changes: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45628