File Locks not Preventing File Overwrite - c++

Question

I have the following c++ code which writes "Line from #" to a file while managing a file lock. I am running this code on two different computers, which share at least some of their memory. That is I can access my files by logging onto either of these computers.
On the first computer I run the program as ./test 1 (e.g. so it will print Line from 1 20,000 times) and on the second computer I run the program as ./test 17. I am starting these programs close enough in time so that the writes to file.txt should be interleaved and controlled by the file locks.
The problem is that I am losing output as the file has 22,770 newlines, but it should have exactly 40,000 newlines.
wc file.txt
22770 68310 276008 file.txt
Also,
cat -n file.txt | grep 18667
18667 ne from 17
My question is why are my file locks not preventing file overwriting, and how can I fix my code so that multiple processes can write to the same file without file loss.
#include <unistd.h>
#include <fcntl.h>
#include <cstdio>
#include <cstdlib>
#include <fstream>
#include <sstream>
#include <iostream>
using namespace std;
void inline Set_Lck(struct flock &flck, const int fd)
{
flck.l_type = F_WRLCK;
if (fcntl(fd, F_SETLKW, &flck) == -1) {
perror("fcntl");
exit(1);
}
}
void inline Release_Lck(struct flock &flck, const int fd)
{
flck.l_type = F_UNLCK;
if (fcntl(fd,F_SETLK,&flck) == -1) {
perror("fcntl");
exit(1);
}
}
void Print_Spec(fstream &fout, ostringstream &oss,struct flock &flck, const int fd)
{
Set_Lck(flck,fd);
fout.seekp(0,ios_base::end);
fout << oss.str() << endl;
flush(fout);
Release_Lck(flck,fd);
}
int main(int argc, char **argv)
{
int fd_cd;
struct flock flock_cd;
ostringstream oss;
fstream comp_data;
const string s_cd_lck = "file_lock.txt";
const string s_cd = "file.txt";
int my_id;
if (argc == 1) {
my_id = 0;
} else if (argc == 2) {
my_id = atoi(argv[1]);
} else {
fprintf(stderr,"error -- usage ./test [my_id]\n");
exit(1);
}
/* Open file computed_data.txt for writing; create it if non-existent.*/
comp_data.open(s_cd.c_str(),ios::app|ios::out);
if (comp_data.fail()) {
perror("comp_data.open");
exit(1);
}
/* Open file that we will be locking. */
fd_cd = open(s_cd_lck.c_str(),O_CREAT|O_WRONLY,0777);
if (fd_cd == -1) {
perror("fd_cd = open");
exit(1);
}
/* Set up the lock. */
flock_cd.l_type = F_WRLCK;
flock_cd.l_whence = SEEK_SET;
flock_cd.l_start = 0;
flock_cd.l_len = 0;
flock_cd.l_pid = getpid();
for (int i = 0; i < 20000; ++i) {
oss.str(""); /* Yes, this can be moved outside the loop. */
oss << "Line from " << my_id << endl;
Print_Spec(comp_data,oss,flock_cd,fd_cd);
}
return 0;
}
I am using c++ and this program is running on Red Hat Enterprise Linux Server release 7.2 (Maipo).
My Research
I am not sure if part of the answer comes from the following Stackoverflow post (https://stackoverflow.com/a/2059059/6417898) where they state that "locks are bound to processes."
At this website (http://perl.plover.com/yak/flock/samples/slide005.html), the author dissuades against using LOCK_UN with flock and suggests closing the file each time and reopening it as needed, so as to flush the file buffer. I don't know if this carries over with fcntl or if this is even necessary if flush the file buffer manually.