File locking via boost::interprocess

block1.cc

#include <boost/interprocess/sync/file_lock.hpp>
#include <boost/interprocess/sync/scoped_lock.hpp>

#include <fstream>

int main( int argc, char ** argv )
{
  std::ofstream o("monkey.txt",std::ios_base::app);

  boost::interprocess::file_lock ol("monkey.txt");
  boost::interprocess::scoped_lock<boost::interprocess::file_lock> sol(ol);
  while(1){}
}

block1C.cc

//Another implementation of block1, but using the lower-level C functions in <fcntl.h>, which work via file descriptors.
#include <fcntl.h>
#include <cstdio>
#include <cstdlib>
#include <fstream>
#include <iostream> 
int main( int argc, char ** argv )
{
  FILE * fp = fopen("monkey.txt","a");
  int fd = fileno(fp);

  struct flock lock;
  lock.l_type = F_WRLCK;/*Write lock*/
  lock.l_whence = SEEK_SET;
  lock.l_start = 0;
  lock.l_len = 0;/*Lock whole file*/


  if (fcntl(fd,F_SETLKW,&lock) == -1)
    {
      std::cerr << "ERROR: could not obtain lock on monkey.txt\n";
      std::exit(10);
    }

 
  while(1){}
}

block2.cc

#include <boost/interprocess/sync/file_lock.hpp>
#include <boost/interprocess/sync/scoped_lock.hpp>
#include <iostream>
#include <fstream>

int main( int argc, char ** argv )
{
  std::ofstream o("monkey.txt",std::ios_base::app);

  boost::interprocess::file_lock ol("monkey.txt");
  boost::interprocess::scoped_lock<boost::interprocess::file_lock> sol(ol);
  std::cerr<<"unlocked\n";
}

block2C.cc

//block2, but with C-language API to locking
#include <fcntl.h>
#include <cstdio>
#include <cstdlib>
#include <fstream>
#include <iostream> 
int main( int argc, char ** argv )
{
  FILE * fp = fopen("monkey.txt","a");
  int fd = fileno(fp);

  struct flock lock;
  lock.l_type = F_WRLCK;/*Write lock*/
  lock.l_whence = SEEK_SET;
  lock.l_start = 0;
  lock.l_len = 0;/*Lock whole file*/


  if (fcntl(fd,F_SETLKW,&lock) == -1)
    {
      std::cerr << "ERROR: could not obtain lock on monkey.txt\n";
      std::exit(10);
    }
}