flock-ing a C++ ifstream on Linux (GCC 4.6)

Question

context

I\'m slowly writing a specialized web server application in C++ (using the C onion http server library and the JSONCPP library for JSON serialization, if t

Answer 1

My solution to this problem is derived from this answer: https://stackoverflow.com/a/19749019/5899976

I've only tested it with GCC 4.8.5.

#include   // for strerror()
#include  // for std::cerr
#include 
#include 

extern "C" {
#include 
#include   // for flock()
}

    // Atomically increments a persistent counter, stored in /tmp/counter.txt
int increment_counter()
{
    std::fstream file( "/tmp/counter.txt" );
    if (!file) file.open( "/tmp/counter.txt", std::fstream::out );

    int fd = static_cast< __gnu_cxx::stdio_filebuf< char > * const >( file.rdbuf() )->fd();
    if (flock( fd, LOCK_EX ))
    {
        std::cerr << "Failed to lock file: " << strerror( errno ) << "\n";
    }

    int value = 0;
    file >> value;
    file.clear();   // clear eof bit.
    file.seekp( 0 );
    file << ++value;

    return value;

    // When 'file' goes out of scope, it's closed.  Moreover, since flock() is
    //  tied to the file descriptor, it gets released when the file is closed.
}