Programming

FILE* lock_fname;
lock_fname = fopen ( "file.lock", "r");
/*check if file exsists*/
if (lock_fname)
{
  fclose (lock_fname);
  lock_fname = fopen ( "file.lock", "w");
  /* file is now locked */
}
else
{
  /* looks like file is locked */
}

#include <fcntl.h> // for open()
#include <cerrno> // for errno 
#include <cstdio> // for perror()
int fd;
fd=open("password.lck", O_WRONLY | O_CREAT | O_EXCL)

#include <stdio.h>
#include <sys/stat.h>
#include <fcntl.h>

int main(void)
{
        int fd;

 /*           O_EXCL:

              If O_CREAT and O_EXCL are set, open() shall fail  if  the  file
              exists.  The  check  for the existence of the file and the cre-
              ation of the file if it does not exist  shall  be  atomic  with
              respect to other threads executing open() naming the same file-
              name in the same directory with  O_EXCL  and  O_CREAT  set.  If
              O_EXCL  and  O_CREAT  are  set, and path names a symbolic link,
              open() shall fail and set errno to [EEXIST], regardless of  the
              contents  of the symbolic link. If O_EXCL is set and O_CREAT is
              not set, the result is undefined.
*/


        if ( (fd = open("./test.txt", O_RDWR|O_CREAT|O_EXCL)) < 0 )
        {
                perror("open");
        }
        else
        {
                /*here you can use fcntl in order to lock the file*/
        }

        return 0;
}