2-28lock_set.c

/*lock_set.c*/
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>

int lock_set(int fd, int type){
	struct flock old_lock, lock;
	lock.l_whence = SEEK_SET;
	lock.l_start = 0;
	lock.l_len = 0;
	lock.l_type = type;
	lock.l_pid = -1;

	fcntl(fd, F_GETLK, &lock);

	if(lock.l_type != F_UNLCK){
		if(lock.l_type == F_RDLCK){
			printf("Read lock already set by %d\n", lock.l_pid);
		}
		else if(lock.l_type == F_WRLCK){
			printf("Write lock already set by %d\n", lock.l_pid);
		}
	}

	lock.l_type = type;

	if((fcntl(fd, F_SETLKW, &lock)) < 0){
		printf("Lock failed:type = %d\n", lock.l_type);
		return -1;
	}

	switch(lock.l_type){
		case F_RDLCK:
			printf("Read lock set by %d\n", getpid());
			break;
		case F_WRLCK:
			printf("Write lock set by %d\n", getpid());
			break;
		case F_UNLCK:
			printf("Release lock by %d\n", getpid());
			return 1;
			break;
	}
	return 0;
}