How do I lock files using fopen()?

Question

I wonder if there is any way to lock and unlock a file in Linux when I open a file using fopen (not open)?

Based on Stack Overflow question

Answer 1

The below code is not letting me lock the file using lockf, flock works fine though

    #include 
    #include 
    #include
    #include 
    #include 
    #include 
    #include 

using namespace std;

void append()
{
    FILE *fp=fopen("a.txt","a");
    if(fp)
    {
        cout< v;
    //#pragma omp parallel for
    for(int i=0;i<1000;++i)
    {
        v.push_back(new thread(append));

        //append();
    }

    for(auto y:v)
    {
        y->join();
        delete y;
    }

    return 0;
}