diff --git a/radvd.c b/radvd.c

index 82efe0b..508abd1 100644

--- a/radvd.c

+++ b/radvd.c

@@ -17,6 +17,10 @@

 #include "includes.h"

 #include "radvd.h"

 #include "pathnames.h"

+#include <libgen.h>

+#include <sys/file.h>

+#include <stdio.h>

+

 #ifdef HAVE_NETLINK

 #include "netlink.h"

@@ -99,6 +103,9 @@ int readin_config(char *);

 int check_conffile_perm(const char *, const char *);

 const char *get_pidfile(void);

 void main_loop(void);

+static int write_pid_file(char const *daemon_pid_file_ident, pid_t pid);

+static int open_and_lock_pid_file(char const *daemon_pid_file_ident);

+static void check_pid_file(char const *daemon_pid_file_ident);

 int

 main(int argc, char *argv[])

@@ -110,6 +117,7 @@ main(int argc, char *argv[])

 	char *chrootdir = NULL;

 	int configtest = 0;

 	int daemonize = 1;

+       int pidfd = 0;

 #ifdef HAVE_GETOPT_LONG

 	int opt_idx;

 #endif

@@ -332,6 +340,14 @@ main(int argc, char *argv[])

 			exit(1);

 		}

 		daemon_retval_send(0);

+	} else {

+		pidfd = open_and_lock_pid_file(pidfile);

+		if (0 != write_pid_file(pidfile, getpid())) {

+			flog(LOG_ERR, "failure writing pid file detected");

+			exit(-1);

+		}

+		check_pid_file(pidfile);

+

 	}

 	/*

@@ -347,11 +363,11 @@ main(int argc, char *argv[])

 	main_loop();

 	flog(LOG_INFO, "sending stop adverts", pidfile);

 	stop_adverts();

-	if (daemonize) {

-		flog(LOG_INFO, "removing %s", pidfile);

-		unlink(pidfile);

+	if (!daemonize){

+		close(pidfd);

 	}

-

+	flog(LOG_INFO, "removing %s", pidfile);

+	unlink(pidfile);

 	return 0;

 }

@@ -841,3 +857,105 @@ usage(void)

 	exit(1);

 }

+static int open_pid_file(char const *daemon_pid_file_ident)

+{

+	int pidfd = open(daemon_pid_file_ident, O_SYNC | O_CREAT | O_RDWR, 0644);

+	if (-1 == pidfd) {

+	        flog(LOG_ERR, "unable to open pid file, %s: %s", daemon_pid_file_ident, strerror(errno));

+		exit(-1);

+	} else {

+	        dlog(LOG_DEBUG, 5, "opened pid file %s", daemon_pid_file_ident);

+	}

+       return pidfd;

+}

+

+static int open_and_lock_pid_file(char const *daemon_pid_file_ident)

+{

+	dlog(LOG_DEBUG, 3, "radvd startup PID is %d", getpid());

+

+	int pidfd = open_pid_file(daemon_pid_file_ident);

+

+	int lock = flock(pidfd, LOCK_EX | LOCK_NB);

+	if (0 != lock) {

+		flog(LOG_ERR, "unable to lock pid file, %s: %s", daemon_pid_file_ident, strerror(errno));

+		exit(-1);

+	} else {

+		dlog(LOG_DEBUG, 4, "locked pid file %s", daemon_pid_file_ident);

+	}

+

+	return pidfd;

+}

+

+

+static int write_pid_file(char const *daemon_pid_file_ident, pid_t pid)

+{

+ 	int pidfd = open_pid_file(daemon_pid_file_ident);

+	char pid_str[20] = {""};

+	sprintf(pid_str, "%d", pid);

+	dlog(LOG_DEBUG, 3, "radvd PID is %s", pid_str);

+	size_t len = strlen(pid_str);

+	int rc = write(pidfd, pid_str, len);

+        if (rc != (int)len) {

+	        return -1;

+	}

+        char newline[] = {"\n"};

+	len = strlen(newline);

+	rc = write(pidfd, newline, len);

+	if (rc != (int)len) {

+               close(pidfd);

+		return -1;

+	}

+        rc = fsync(pidfd);

+	if (rc != 0) {

+		dlog(LOG_DEBUG, 4, "failed to fsync pid file: %s", daemon_pid_file_ident);

+	}

+	rc = close(pidfd);

+        if (rc != 0) {

+                dlog(LOG_DEBUG, 4, "failed to close pid file: %s", daemon_pid_file_ident);

+        }

+        char *dirstrcopy = strdup(daemon_pid_file_ident);

+        char *dirstr = dirname(dirstrcopy);

+        int dirfd = open(dirstr, O_RDONLY);

+        if (dirfd == -1){

+           dlog(LOG_DEBUG, 4, "Failed to open directory: %s", dirstr);

+        }

+        rc = fsync(dirfd);

+        if (rc != 0) {

+                dlog(LOG_DEBUG, 4, "failed to fsync pid dir: %s", dirstr);

+        }

+        rc = close(dirfd);

+        if (rc != 0) {

+                dlog(LOG_DEBUG, 4, "failed to close pid dir: %s", dirstr);

+        }

+        free(dirstrcopy);

+        dlog(LOG_DEBUG, 4, "wrote pid %d to pid file: %s", pid, daemon_pid_file_ident);

+        return rc;

+}

+

+

+static void check_pid_file(char const *daemon_pid_file_ident)

+{

+	FILE *pidfile = fopen(daemon_pid_file_ident, "r");

+

+	if (!pidfile) {

+		flog(LOG_ERR, "unable to open pid file, %s: %s", daemon_pid_file_ident, strerror(errno));

+		exit(-1);

+	}

+

+	pid_t pid = -1;

+

+	int rc = fscanf(pidfile, "%d", &pid;;

+	fclose(pidfile);

+

+	if (rc != 1) {

+		flog(LOG_ERR, "unable to read pid from pid file: %s", daemon_pid_file_ident);

+		exit(-1);

+	}

+

+	if (pid != getpid()) {

+		flog(LOG_ERR, "pid in file, %s, doesn't match getpid(): %d != %d", daemon_pid_file_ident, pid, getpid());

+		exit(-1);

+	}

+	dlog(LOG_DEBUG, 4, "validated pid file, %s: %d", daemon_pid_file_ident, pid);

+}

+