multithreaded file copying - c++

Question

I wrote a c++ code to multithreaded copying a file to another directory in linux. but doesn't work(just it made an empty file in directory).
I don't know what's the problem? I think my tread has no right access to write in the shared file. but don't know what should I do.
It should work when typed in terminal :
$./a.out <file name> <dir> <thread number (default 4)>
This is my code:
/*Multithreads file copier*/
#include <fcntl.h>
#include <pthread.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <sys/sendfile.h>
#include <unistd.h>
#include <cstdio>
char* file;
char* fileout_path;
int fin, fout;
// part of each thread
struct PART{
off_t* offset =0;
size_t size;
};
//multithreading
void *Copy (void * data)
{
struct PART *mypart;
mypart = (struct PART *) data;
//open file to read and write
fin = open(file, O_RDONLY,0);
fout = open(fileout_path, O_WRONLY|O_CREAT, 0644);
unsigned long a = static_cast<unsigned long>(mypart->size);
lseek(fout, a, SEEK_SET); //set offset by size of the part
//use sendfile instead read and write to easier code
sendfile(fin, fout, mypart->offset, mypart->size);
printf("threading....\n");//to know the thread ran
pthread_exit(0);
}
int main(int argc, char *argv[])
{
int threads_number;
if (argv[3]!= NULL)
{
threads_number = atoi(argv[3]);
}
else
{
threads_number = 4;//default thread number
}
//multithreading datatypes
pthread_t tid[threads_number];
pthread_attr_t attr;
pthread_attr_init(&attr);
struct stat f_stat;
struct PART part[threads_number];
//allocation size of each part
unsigned long part_size = f_stat.st_size / threads_number;
for(int i =0; i <number_threads; i++)
{
if ( i == threads_number -1)
{
part[threads_number].size = f_stat.st_size - (part_size * (threads_number -1));
}
else
{
part[i].size = part_size;
}
}
file = argv[1];
stat(file, &f_stat);
fileout_path = argv[2];
int fin1 = open(file, O_RDONLY,0);
int fout1 = open(fileout_path, O_WRONLY|O_CREAT, 0644);
for (int j = 0; j < threads_number; j++)
{
pthread_create(&tid[j], NULL, Copy, (void *)&part[j]);
pthread_join(tid[j],NULL);
}
printf("thread is done.\n");
close(fout);
close(fin);
return 0;
}