#include <stdio.h>
#include <sys/file.h>

/* file locking! */

int main()
{
	int fd;
	
	if ((fd = open("text.txt", O_RDWR)) < 0 ) { /* get file descriptor */
		perror("\nOpening error: ");
		exit(2);
	}
	
	if (flock(fd, LOCK_EX) < 0) { /* lock file */
		perror("\nLocking error: ");
		exit(1);
	}
	
	char tmp; /* wait for keypress */
	scanf("\n", &tmp);
		
	flock(fd, LOCK_UN); /* unlock file */
	
	return 0;
}

Quote from "man fcntl" 

   Mandatory locking
       (Non-POSIX.)   The above record locks may be either advisory or mandatory, and are advi-
       sory by default.  To make use of mandatory locks,  mandatory  locking  must  be  enabled
       (using  the  "-o mand" option to mount(8)) for the file system containing the file to be
       locked and enabled on the file itself (by disabling group execute permission on the file
       and enabling the set-GID permission bit).

       Advisory  locks  are  not  enforced  and  are useful only between cooperating processes.
       Mandatory locks are enforced for all processes.

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

int lock(int fd)
{
    struct flock lock;

    lock.l_type = F_WRLCK;
    lock.l_whence = SEEK_SET;
    lock.l_start = 0;
    lock.l_len = 0;
    lock.l_pid = -1;
    if (fcntl(fd, F_SETLK, &lock) < 0) {
        perror("Locking file");
        return 0;
    }
    return 1;
}