C++ Why doesn't my code print updates made to the give file

Question

I was trying to write a code in C++ the does something like tail -f in linux. I found this question : How to read a growing text file in C++? and implemented the same. I created a temp.txt and started doing echo "temp" >> temp.txt. But my program is not printing the updates made to the file .What am I doing wrong? This is the code I'm using

#include <iostream>
#include <string>
#include <fstream>
#include <unistd.h>

int main()
{
    std::ifstream ifs("temp.txt");

    if (ifs.is_open())
    {
        std::string line;
        while (true)
        {
            while (std::getline(ifs, line)) std::cout << line << "\n";
            if (!ifs.eof()) break; // Ensure end of read was EOF.
            ifs.clear();
            sleep(3);
        }
    }

    return 0;
}

UPDATE

I've tried the same code on a linux machine and it was working fine, but it is not working on Mac. I was using gcc to compile the code.

gcc -v gives

Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.1.0 (clang-602.0.49) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.3.0
Thread model: posix

UPDATE 2
I've investigated further and realised that I was not using gcc after all. I've installed gcc separately and it is working fine now. Is this a bug in clang?

Answer 1

It is quite possible that cout buffer did not flush in your tests because buffer size did not reach the overflow limit. You could try flushing the buffer by either doing std::cout << line << std::endl; instead of std::cout << line << "\n"; or calling std::cout.flush()l before sleep(1);. Both ways should work reliably with clang and gcc.

Answers to these questions explain buffering really well:

C++ cout and cin buffers, and buffers in general

Strange behaviour of std::cout in Linux