Index: usr.bin/truss/setup.c
===================================================================
--- usr.bin/truss/setup.c
+++ usr.bin/truss/setup.c
@@ -144,6 +144,39 @@
 }
 
 /*
+ * A no-op signal handler so we can break out of wait.
+ */
+static void
+trapalrm(int signo __unused)
+{
+	return;
+}
+
+/*
+ * Prints names and arguments of any threads currently in a syscall.
+ */
+static void
+print_sleeping_threads(struct trussinfo *info)
+{
+	struct procinfo *p;
+	LIST_FOREACH(p, &info->proclist, entries) {
+		struct threadinfo *t;
+		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, " ...sleeping\n");
+			}
+		}
+	}
+}
+
+
+/*
  * Determine the ABI.  This is called after every exec, and when
  * a process is first monitored.
  */
@@ -482,6 +515,7 @@
 		}
 	}
 
+	t->in_syscall = 0;
 	print_syscall_ret(info, errorp, retval);
 	free_syscall(t);
 
@@ -610,17 +644,55 @@
 	siginfo_t si;
 	int pending_signal;
 
+	signal(SIGALRM, trapalrm);
+	siginterrupt(SIGALRM, 1);
+
 	while (!LIST_EMPTY(&info->proclist)) {
+		struct procinfo *p;
+		int in_syscall = 0;
+
 		if (detaching) {
 			detach_all_procs(info);
 			return;
 		}
 
-		if (waitid(P_ALL, 0, &si, WTRAPPED | WEXITED) == -1) {
-			if (errno == EINTR)
+		/* Count up all threads waiting in a syscall */
+		LIST_FOREACH(p, &info->proclist, entries) {
+			struct threadinfo *t;
+			LIST_FOREACH(t, &p->threadlist, entries) {
+				if (t->in_syscall)
+					in_syscall++;
+			}
+		}
+
+		if (in_syscall)
+			alarm(2);
+
+		while (waitid(P_ALL, 0, &si, WTRAPPED | WEXITED) == -1) {
+			if (errno == EINTR) {
+				if (detaching)
+					break;
+				if (in_syscall) {
+					/*
+					 * The 2-second 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);
+				}
 				continue;
+			}
 			err(1, "Unexpected error from waitid");
 		}
+		if (detaching)
+			continue;
+
+		if (in_syscall)
+			alarm(0);
 
 		assert(si.si_signo == SIGCHLD);
 
Index: usr.bin/truss/syscalls.c
===================================================================
--- usr.bin/truss/syscalls.c
+++ usr.bin/truss/syscalls.c
@@ -2134,7 +2134,7 @@
 			len += fprintf(trussinfo->outfile, "%s", s_args[i]);
 		else
 			len += fprintf(trussinfo->outfile,
-			    "<missing argument>");
+			    t->in_syscall ? "" : "<missing argument>");
 		len += fprintf(trussinfo->outfile, "%s", i < (nargs - 1) ?
 		    "," : "");
 	}