Better comments, enhance connect-only.c.

select/connect-only.c

@@ -1,10 +1,14 @@
 /*
- * Connect only. To show what happens if the remote side starts writing but we
- * do not read. Use with select/write-select.c.
+ * Connect only.  To show what happens if the remote side starts writing but we
+ * do not read.  Use with select/write-select.c.
  *
- * Instead of this program you can easily use netcat(1). For example:
+ * Instead of this program you can easily use BSD (also macOS) netcat(1).
+ * GNU/Linux netcat does not help us here as it uses -i for writing only.
  *
- *	nc -i 9999 localhost 2222
+ *	nc -i 2 localhost 2222
+ *
+ * If the 3rd parameter is used, the program will actually read some data each
+ * interval.  Use that if you only have a GNU netcat.
  *
  * Note that this program works with IPv4 only which is bad.
  *
@@ -25,13 +29,12 @@
 int
 main(int argc, char **argv)
 {
-	int fd;
-	struct sockaddr_in in;
+	int fd, interval = -1;
+	struct sockaddr_in in = { 0 };
 
-	if (argc != 3)
-		errx(1, "usage: %s <ip-address> <port>", argv[0]);
+	if (argc != 3 && argc != 4)
+		errx(1, "usage: %s <ip-address> <port> [interval]", argv[0]);
 
-	bzero(&in, sizeof (in));
 	in.sin_family = AF_INET;
 	in.sin_port = htons(atoi(argv[2]));
 
@@ -42,17 +45,33 @@ main(int argc, char **argv)
 	if (inet_pton(AF_INET, argv[1], &(in.sin_addr)) != 1)
 		errx(1, "inet_pton failed for '%s'", argv[1]);
 
+	if (argc == 4)
+		interval = atoi(argv[3]);
+
 	if ((fd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
 		err(1, "socket");
 
 	if (connect(fd, (struct sockaddr *) &in, sizeof (in)) == -1)
 		err(1, "connect");
 
-	/* Wait for ever. */
-	while (1)
-		pause();
+	if (interval == -1) {
+		/* Wait for ever. */
+		while (1)
+			pause();
+	}
+
+	while (1) {
+		int n;
+		char buf[4096];
 
-	/* Not reached. */
+		sleep(interval);
+		n = read(fd, buf, sizeof (buf));
+		if (n == -1)
+			err(1, "read");
+		if (n == 0)
+			break;
+		(void) write(1, buf, n);
+	}
 	close(fd);
 	return (0);
 }

select/write-select.c

@@ -1,40 +1,43 @@
 /*
  * Example on writing data with select().  Use select/connect-only.c which
- * connects but does not read anything. Alternatively, you can use netcat with
- * -i option:
+ * connects but does not read anything unless a 3rd parameter is provided.
+ * Alternatively, you can use BSD (ie. macOS as well) netcat with -i option
+ * (GNU/Linux netcat uses -i for writing only, not for reading, and is not
+ * useful in this case):
  *
- *	nc -i 9999 localhost 2222
+ *	nc -i 5 localhost 2222
  *
- * The program should stop in select() after some data has been written to the
- * kernel buffer. That's because only some data has been sent over the TCP
- * connection - as the other side does not read, we get TCP win 0 after a short
- * while and the kernel buffer is limited in size.
+ * The process should stop in select() after some data has been sent over to the
+ * remote side and filled up its kernel buffer for the connection there.  As the
+ * client side does not read (or reads slowly), the server side eventually gets
+ * a TCP window 0 (which you could see with tcpdump) after a short while as the
+ * kernel buffer holding the data to be read by the client is limited in size.
  *
  * You might need to run the client from a different machine; it might not
  * behave as expected on a localhost.  System can treat that as a special case.
  *
- * To see more, use "-i 1", and trace system calls (eg. use strace on Linux) by
- * the server from another terminal. You will see that from time to time, the
- * select() returns and the kernel buffer is filled up while the netcat slowly
- * reads the data.
+ * To see more, use "-i 1" with netcat above, and from another terminal, trace
+ * system calls (eg. use "strace -p <pid>" on Linux) performed by the server.
+ * You will see that from time to time, the select() returns and the kernel
+ * buffer is filled up while the netcat slowly reads the data.
  *
  * (c) jp@devnull.cz
  */
 
 #define	_XOPEN_SOURCE	700
 
-#include <sys/types.h>
-#include <sys/socket.h>
-#include <sys/select.h>
-#include <netinet/in.h>
-#include <netdb.h>
+#include <err.h>
 #include <fcntl.h>
+#include <libgen.h>
+#include <netdb.h>
+#include <netinet/in.h>
 #include <stdio.h>
-#include <unistd.h>
 #include <stdlib.h>
 #include <string.h>
-#include <err.h>
-#include <errno.h>
+#include <sys/select.h>
+#include <sys/socket.h>
+#include <sys/types.h>
+#include <unistd.h>
 
 #define	BUF_LEN	1024
 
@@ -47,7 +50,7 @@ main(int argc, char **argv)
 	int s, newsock, n, total = 0, optval = 1;
 
 	if (argc != 2)
-		errx(1, "usage: %s <port>", argv[0]);
+		errx(1, "usage: %s <port>", basename(argv[0]));
 
 	if ((s = socket(AF_INET, SOCK_STREAM, 0)) == -1)
 		err(1, "socket");
@@ -63,7 +66,7 @@ main(int argc, char **argv)
 		err(1, "setsockopt");
 	}
 
-	if (bind(s, (struct sockaddr *) &sa, sizeof (sa)) == -1)
+	if (bind(s, (struct sockaddr *)&sa, sizeof (sa)) == -1)
 		err(1, "bind");
 
 	if (listen(s, SOMAXCONN) == -1)
@@ -73,7 +76,7 @@ main(int argc, char **argv)
 		err(1, "accept");
 
 	/*
-	 * Just write data to the remote side. If the remote side does not read
+	 * Just write data to the remote side.  If the remote side does not read
 	 * the data we should be finally put to sleep in select().
 	 */
 	for (;;) {
@@ -85,7 +88,7 @@ main(int argc, char **argv)
 
 		if (FD_ISSET(newsock, &wrfds)) {
 			if ((n = write(newsock, buf, sizeof (buf))) == -1)
-				err(1, "write (%d)", errno);
+				err(1, "write");
 			total = total + n;
 			fprintf(stderr, "[ %d bytes written (total %d) ]\n",
 			    n, total);