Index: usr.bin/truss/main.c
===================================================================
--- usr.bin/truss/main.c
+++ usr.bin/truss/main.c
@@ -59,8 +59,8 @@
 usage(void)
 {
 	fprintf(stderr, "%s\n%s\n",
-	    "usage: truss [-cfaedDHS] [-o file] [-s strsize] -p pid",
-	    "       truss [-cfaedDHS] [-o file] [-s strsize] command [args]");
+	    "usage: truss [-cfaedDHS] [-o file] [-s strsize] [-t timeout] -p pid",
+	    "       truss [-cfaedDHS] [-o file] [-s strsize] [-t timeout] command [args]");
 	exit(1);
 }
 
@@ -84,10 +84,11 @@
 	pid = 0;
 	trussinfo->outfile = stderr;
 	trussinfo->strsize = 32;
+	trussinfo->timeout = 0;
 	trussinfo->curthread = NULL;
 	LIST_INIT(&trussinfo->proclist);
 	init_syscalls();
-	while ((c = getopt(ac, av, "p:o:facedDs:SH")) != -1) {
+	while ((c = getopt(ac, av, "p:o:facedDs:t:SH")) != -1) {
 		switch (c) {
 		case 'p':	/* specified pid */
 			pid = atoi(optarg);
@@ -120,6 +121,9 @@
 		case 's':	/* Specified string size */
 			trussinfo->strsize = atoi(optarg);
 			break;
+		case 't':	/* Pending syscall timeout */
+			trussinfo->timeout = atoi(optarg);
+			break;
 		case 'S':	/* Don't trace signals */
 			trussinfo->flags |= NOSIGS;
 			break;
Index: usr.bin/truss/setup.c
===================================================================
--- usr.bin/truss/setup.c
+++ usr.bin/truss/setup.c
@@ -62,7 +62,7 @@
 
 SET_DECLARE(procabi, struct procabi);
 
-static sig_atomic_t detaching;
+static sig_atomic_t detaching, alarm_tripped;
 
 static void	enter_syscall(struct trussinfo *, struct threadinfo *,
 		    struct ptrace_lwpinfo *);
@@ -146,6 +146,59 @@
 }
 
 /*
+ * A signal handler so we can break out of wait after a timeout.
+ */
+static void
+trap_alarm(int signo __unused)
+{
+	alarm_tripped = 1;
+}
+
+static bool
+set_alarm(struct trussinfo *info)
+{
+	struct procinfo *p;
+	struct threadinfo *t;
+
+	LIST_FOREACH(p, &info->proclist, entries) {
+		LIST_FOREACH(t, &p->threadlist, entries) {
+			if (t->in_syscall) {
+				alarm(info->timeout);
+				alarm_tripped = 0;
+				return (true);
+			}
+		}
+	}
+	return (false);
+}
+
+
+/*
+ * Prints names and arguments of any threads currently in a syscall.
+ */
+static void
+print_sleeping_threads(struct trussinfo *info)
+{
+	struct procinfo *p;
+	struct threadinfo *t;
+
+	LIST_FOREACH(p, &info->proclist, entries) {
+		LIST_FOREACH(t, &p->threadlist, entries) {
+			if (t->in_syscall) {
+				info->curthread = t;
+				if (info->flags & (RELATIVETIMESTAMPS |
+				    ABSOLUTETIMESTAMPS))
+					clock_gettime(CLOCK_REALTIME,
+					    &t->after);
+				print_syscall(info);
+				fprintf(info->outfile, " <in progress>\n");
+			}
+		}
+	}
+}
+
+
+/*
  * Determine the ABI.  This is called after every exec, and when
  * a process is first monitored.
  */
@@ -484,6 +537,7 @@
 		}
 	}
 
+	t->in_syscall = 0;
 	print_syscall_ret(info, errorp, retval);
 	free_syscall(t);
 
@@ -669,6 +723,10 @@
 	struct ptrace_lwpinfo pl;
 	siginfo_t si;
 	int pending_signal;
+	bool alarm_set = false;
+
+	signal(SIGALRM, trap_alarm);
+	siginterrupt(SIGALRM, 1);
 
 	while (!LIST_EMPTY(&info->proclist)) {
 		if (detaching) {
@@ -676,11 +734,34 @@
 			return;
 		}
 
-		if (waitid(P_ALL, 0, &si, WTRAPPED | WEXITED) == -1) {
-			if (errno == EINTR)
+		if (info->timeout)
+			alarm_set = set_alarm(info);
+
+		while (waitid(P_ALL, 0, &si, WTRAPPED | WEXITED) == -1) {
+			if (errno == EINTR) {
+				if (detaching)
+					break;
+				if (info->timeout && alarm_tripped) {
+					/*
+					 * The timer interrupted us, so we
+					 * must be waiting on slow syscalls.
+					 * Print their names and continue
+					 * waiting, this time forever.
+					 */
+					print_sleeping_threads(info);
+					fflush(info->outfile);
+					alarm(0);
+					alarm_set = false;
+				}
 				continue;
+			}
 			err(1, "Unexpected error from waitid");
 		}
+		if (detaching)
+			continue;
+
+		if (alarm_set)
+			alarm(0);
 
 		assert(si.si_signo == SIGCHLD);
 
Index: usr.bin/truss/syscalls.c
===================================================================
--- usr.bin/truss/syscalls.c
+++ usr.bin/truss/syscalls.c
@@ -2618,8 +2618,8 @@
 		if (s_args[i] != NULL)
 			len += fprintf(trussinfo->outfile, "%s", s_args[i]);
 		else
-			len += fprintf(trussinfo->outfile,
-			    "<missing argument>");
+			len += fprintf(trussinfo->outfile, t->in_syscall ?
+			    "<unavailable>" : "<missing argument>");
 		len += fprintf(trussinfo->outfile, "%s", i < (nargs - 1) ?
 		    "," : "");
 	}
Index: usr.bin/truss/truss.h
===================================================================
--- usr.bin/truss/truss.h
+++ usr.bin/truss/truss.h
@@ -111,6 +111,7 @@
 {
 	int flags;
 	int strsize;
+	int timeout;
 	FILE *outfile;
 
 	struct timespec start_time;
Index: usr.bin/truss/truss.1
===================================================================
--- usr.bin/truss/truss.1
+++ usr.bin/truss/truss.1
@@ -11,11 +11,13 @@
 .Op Fl facedDHS
 .Op Fl o Ar file
 .Op Fl s Ar strsize
+.Op Fl t Ar timeout
 .Fl p Ar pid
 .Nm
 .Op Fl facedDHS
 .Op Fl o Ar file
 .Op Fl s Ar strsize
+.Op Fl t Ar timeout
 .Ar command Op Ar args
 .Sh DESCRIPTION
 The
@@ -77,6 +79,12 @@
 The default
 .Ar strsize
 is 32.
+.It Fl t Ar timeout
+After
+.Ar timeout
+seconds with no syscall activity, print the syscall names and arguments
+for any threads currently waiting in the kernel.
+A value of 0 (the default) will disable the timeout.
 .It Fl p Ar pid
 Follow the process specified by
 .Ar pid