add threaddaemonize

diff --git a/src/libthread/daemonize.c b/src/libthread/daemonize.c
new file mode 100644
index 0000000..65af29a
--- /dev/null
+++ b/src/libthread/daemonize.c

@@ -0,0 +1,98 @@
+#include <u.h>
+#include <sys/signal.h>
+#include <sys/wait.h>
+#include <libc.h>
+#include <thread.h>
+#include "threadimpl.h"
+
+#undef pipe
+#undef wait
+
+static int sigpid;
+static int threadpassfd;
+
+static void
+child(void)
+{
+	int status;
+	if(wait(&status) == sigpid && WIFEXITED(status))
+		 _exit(WEXITSTATUS(status));
+}
+
+static void
+sigpass(int sig)
+{
+	if(sig == SIGCHLD)
+		child();
+	else
+		kill(sigpid, sig);
+}
+
+void
+_threadsetupdaemonize(void)
+{
+	int i, n, pid;
+	int p[2];
+	char buf[20];
+
+	sigpid = 1;
+
+	threadlinklibrary();
+
+	if(pipe(p) < 0)
+		sysfatal("passer pipe: %r");
+
+	/* hide these somewhere they won't cause harm */
+	if(dup(p[0], 98) < 0 || dup(p[1], 99) < 0)
+		sysfatal("passer pipe dup: %r");
+	close(p[0]);
+	close(p[1]);
+	p[0] = 98;
+	p[1] = 99;
+
+	/* close on exec */
+	if(fcntl(p[0], F_SETFD, 1) < 0 || fcntl(p[1], F_SETFD, 1) < 0)
+		sysfatal("passer pipe pipe fcntl: %r");
+
+	switch(pid = fork()){
+	case -1:
+		sysfatal("passer fork: %r");
+	default:
+		close(p[1]);
+		break;
+	case 0:
+		rfork(RFNOTEG);
+		close(p[0]);
+		threadpassfd = p[1];
+		return;
+	}
+
+	sigpid = pid;
+	for(i=0; i<NSIG; i++){
+		struct sigaction sa;
+
+		memset(&sa, 0, sizeof sa);
+		sa.sa_handler = sigpass;
+		sa.sa_flags |= SA_RESTART;
+		sigaction(i, &sa, nil);
+	}
+
+	for(;;){
+		n = read(p[0], buf, sizeof buf-1);
+		if(n == 0)	/* program exited */
+			child();
+		if(n > 0)
+			break;
+		sysfatal("passer pipe read: %r");
+	}
+	buf[n] = 0;
+	_exit(atoi(buf));
+}
+
+void
+threaddaemonize(void)
+{
+	write(threadpassfd, "0", 1);
+	close(threadpassfd);
+	threadpassfd = -1;
+}