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

@@ -165,8 +165,8 @@ static int WflagChars;
 static char *zflag = NULL;		/* compress each savefile using a specified command (like gzip or bzip2) */
 static int immediate_mode;
 
-static int infodelay;
 static int infoprint;
+static int gotalarm;
 
 char *program_name;
 
@@ -1962,6 +1962,8 @@ main(int argc, char **argv)
 #elif defined(HAVE_ALARM)
 		(void)setsignal(SIGALRM, verbose_stats_dump);
 		alarm(1);
+		/* signal alarm to get the first iteration ASAP */
+		gotalarm = 1;
 #endif
 	}
 
@@ -2274,8 +2276,6 @@ 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;
 
 	/*
@@ -2464,41 +2464,52 @@ dump_packet_and_trunc(u_char *user, const struct pcap_pkthdr *h, const u_char *s
 		pcap_dump_flush(dump_info->p);
 #endif
 
-	--infodelay;
-	if (infoprint)
+	if (infoprint) {
 		info(0);
+		infoprint = 0;
+	}
+
+	if (gotalarm) {
+		fprintf(stderr, "Got %u\r", packets_captured);
+		gotalarm = 0;
+		alarm(1);
+	}
 }
 
 static void
 dump_packet(u_char *user, const struct pcap_pkthdr *h, const u_char *sp)
 {
 	++packets_captured;
 
-	++infodelay;
-
 	pcap_dump(user, h, sp);
 #ifdef HAVE_PCAP_DUMP_FLUSH
 	if (Uflag)
 		pcap_dump_flush((pcap_dumper_t *)user);
 #endif
 
-	--infodelay;
-	if (infoprint)
+	if (infoprint) {
 		info(0);
+		infoprint = 0;
+	}
+
+	if (gotalarm) {
+		fprintf(stderr, "Got %u\r", packets_captured);
+		gotalarm = 0;
+		alarm(1);
+	}
 }
 
 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;
+	}
 }
 
 #ifdef _WIN32
@@ -2531,10 +2542,7 @@ print_packet(u_char *user, const struct pcap_pkthdr *h, const u_char *sp)
 #ifdef SIGNAL_REQ_INFO
 RETSIGTYPE requestinfo(int signo _U_)
 {
-	if (infodelay)
-		++infoprint;
-	else
-		info(0);
+	infoprint = 1;
 }
 #endif
 
@@ -2545,15 +2553,12 @@ RETSIGTYPE requestinfo(int signo _U_)
 void CALLBACK verbose_stats_dump (UINT timer_id _U_, UINT msg _U_, DWORD_PTR arg _U_,
 				  DWORD_PTR dw1 _U_, DWORD_PTR dw2 _U_)
 {
-	if (infodelay == 0)
-		fprintf(stderr, "Got %u\r", packets_captured);
+	gotalarm = 1;
 }
 #elif defined(HAVE_ALARM)
 static void verbose_stats_dump(int sig _U_)
 {
-	if (infodelay == 0)
-		fprintf(stderr, "Got %u\r", packets_captured);
-	alarm(1);
+	gotalarm = 1;
 }
 #endif