#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
int main()
{
char block[1024];
int in, out;
int nread;
in = open("filecopy.c", O_RDONLY);
out = open("file.out", O_WRONLY|O_CREAT, S_IRUSR|S_IWUSR);
while((nread = read(in,block,sizeof(block))) > 0)
write(out,block,nread);
exit(0);}

-rwxr-xr-x    1 root     root         18348 Apr 17 14:14 file.o

#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/file.h>
int main (int argc, char* argv[])
{ char* file = argv[1];
 int fd;
 struct flock lock;
 printf ("opening %s\n", file);
 /* Open a file descriptor to the file. */
 fd = open (file, O_WRONLY);
// acquire shared lock
if (flock(fd, LOCK_SH) == -1) {
    printf("error");
}else
{printf("Acquiring shared lock using flock");
}
getchar();
// non-atomically upgrade to exclusive lock
// do it in non-blocking mode, i.e. fail if can't upgrade immediately
if (flock(fd, LOCK_EX | LOCK_NB) == -1) {
    printf("error");
}else
{printf("Acquiring exclusive lock using flock");}
getchar();
// release lock
// lock is also released automatically when close() is called or process exits
if (flock(fd, LOCK_UN) == -1) {
    printf("error");
}else{
printf("unlocking");
}
getchar();
close (fd);
return 0;
}

-rwxr-xr-x    1 root     root         18376 Apr 17 14:20 text.o