fprintf(3) should not be used by signal handlers

On most OS platforms fprintf() et.al. are considered to be signal unsafe. I've
noticed the signal handlers in tcpdump call fprintf(). The patch below defers
call to fprintf() triggered by signal to pcap_loop callback.

Submitted to upstream:
the-tcpdump-group#638

tcpdump.c

@@ -217,8 +217,8 @@ static char *zflag = NULL;		/* compress each savefile using a specified command
 static int immediate_mode;
 #endif
 
-static int infodelay;
 static int infoprint;
+static int gotalarm;
 
 char *program_name;
 
@@ -2387,6 +2387,9 @@ DIAG_ON_CLANG(assign-enum)
 		 */
 		struct itimerval timer;
 		(void)setsignal(SIGALRM, verbose_stats_dump);
+		alarm(1);
+		/* signal alarm to get the first iteration ASAP */
+		gotalarm = 1;
 		timer.it_interval.tv_sec = 1;
 		timer.it_interval.tv_usec = 0;
 		timer.it_value.tv_sec = 1;
@@ -2738,8 +2741,6 @@ dump_packet_and_trunc(u_char *user, const struct pcap_pkthdr *h, const u_char *s
 
 	++packets_captured;
 
-	++infodelay;
-
 	dump_info = (struct dump_info *)user;
 
 	/*
@@ -2941,9 +2942,16 @@ dump_packet_and_trunc(u_char *user, const struct pcap_pkthdr *h, const u_char *s
 	if (dump_info->ndo != NULL)
 		pretty_print_packet(dump_info->ndo, h, sp, packets_captured);
 
-	--infodelay;
-	if (infoprint)
+	if (infoprint) {
 		info(0);
+		infoprint = 0;
+	}
+
+	if (gotalarm) {
+		fprintf(stderr, "Got %u\r", packets_captured);
+		gotalarm = 0;
+		alarm(1);
+	}
 }
 
 static void
@@ -2953,8 +2961,6 @@ dump_packet(u_char *user, const struct pcap_pkthdr *h, const u_char *sp)
 
 	++packets_captured;
 
-	++infodelay;
-
 	dump_info = (struct dump_info *)user;
 
 	pcap_dump((u_char *)dump_info->pdd, h, sp);
@@ -2966,33 +2972,36 @@ dump_packet(u_char *user, const struct pcap_pkthdr *h, const u_char *sp)
 	if (dump_info->ndo != NULL)
 		pretty_print_packet(dump_info->ndo, h, sp, packets_captured);
 
-	--infodelay;
-	if (infoprint)
+	if (infoprint) {
 		info(0);
+		infoprint = 0;
+	}
 }
 
 static void
 print_packet(u_char *user, const struct pcap_pkthdr *h, const u_char *sp)
 {
 	++packets_captured;
 
-	++infodelay;
-
 	pretty_print_packet((netdissect_options *)user, h, sp, packets_captured);
 
-	--infodelay;
-	if (infoprint)
+	if (infoprint) {
 		info(0);
+		infoprint = 0;
+	}
+
+	if (gotalarm) {
+		fprintf(stderr, "Got %u\r", packets_captured);
+		gotalarm = 0;
+		alarm(1);
+	}
 }
 
 #ifdef SIGNAL_REQ_INFO
 static void
 requestinfo(int signo _U_)
 {
-	if (infodelay)
-		++infoprint;
-	else
-		info(0);
+	infoprint = 1;
 }
 #endif
 
@@ -3010,7 +3019,7 @@ print_packets_captured (void)
 {
 	static u_int prev_packets_captured, first = 1;
 
-	if (infodelay == 0 && (first || packets_captured != prev_packets_captured)) {
+	if (first || packets_captured != prev_packets_captured) {
 		fprintf(stderr, "Got %u\r", packets_captured);
 		first = 0;
 		prev_packets_captured = packets_captured;
@@ -3024,12 +3033,12 @@ print_packets_captured (void)
 static void CALLBACK verbose_stats_dump(PVOID param _U_,
     BOOLEAN timer_fired _U_)
 {
-	print_packets_captured();
+	gotalarm = 1;
 }
 #else /* _WIN32 */
 static void verbose_stats_dump(int sig _U_)
 {
-	print_packets_captured();
+	gotalarm = 1;
 }
 #endif /* _WIN32 */