file_lock.cpp

#include <unistd.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <boost/interprocess/sync/file_lock.hpp>
#include <boost/interprocess/sync/scoped_lock.hpp>

using std::cout;
using std::cerr;
using std::endl;
using std::ifstream;
using std::ofstream;
using std::flush;
using std::vector;
using boost::interprocess::file_lock;
using boost::interprocess::scoped_lock;

constexpr int CHILDREN_NUM = 2048;

void count_up() {
    file_lock flock("lock");
    scoped_lock<file_lock> slock(flock);

    int count;
    ifstream("count") >> count;
    ofstream("count") << (count + 1);
}

int main(){
    vector<pid_t> pids;
    for (int i = 0; i < CHILDREN_NUM; ++i) {
        pid_t child_pid = fork();
        if (child_pid == -1) {
            cerr << "cannot create a child process" << endl;
            return 1;
        } else if (child_pid == 0) {
            count_up();
            return 0;
        } else {
            pids.push_back(child_pid);
        }
    }

    for (pid_t pid : pids) waitpid(pid, nullptr, 0);

    int count;
    { ifstream("count") >> count; }
    cout << count << endl;

    return 0;
};