[PATCH] select_tut.2: fix issues in example

[Peter Wu <peter@xxxxxxxxxxxxx>]

Avoid closing cfd if it is -1. Initialize buf1_avail, etc. to avoid
uninitialized memory access in the event that accept() fails. Remove
redundant setting of nfds. Fix tabs with spaces.

Do not try to read/write from/to file descriptors once an existing
connection is overwritten, the select() states are stale now.
Avoid writing zero bytes from the buffer and then closing the fd.

Signed-off-by: Peter Wu <peter@xxxxxxxxxxxxx>
---
 man2/select_tut.2 | 38 ++++++++++++++++++--------------------
 1 file changed, 18 insertions(+), 20 deletions(-)

diff --git a/man2/select_tut.2 b/man2/select_tut.2
index 25dd3c6..7e4f41d 100644
--- a/man2/select_tut.2
+++ b/man2/select_tut.2
@@ -594,7 +594,6 @@ connect_socket(int connect_port, char *address)
     cfd = socket(AF_INET, SOCK_STREAM, 0);
     if (cfd == \-1) {
         perror("socket");
-        close(cfd);
         return \-1;
     }
 
@@ -641,8 +640,8 @@ main(int argc, char *argv[])
     int h;
     int fd1 = \-1, fd2 = \-1;
     char buf1[BUF_SIZE], buf2[BUF_SIZE];
-    int buf1_avail, buf1_written;
-    int buf2_avail, buf2_written;
+    int buf1_avail = 0, buf1_written = 0;
+    int buf2_avail = 0, buf2_written = 0;
 
     if (argc != 4) {
         fprintf(stderr, "Usage\\n\\tfwd <listen\-port> "
@@ -660,7 +659,7 @@ main(int argc, char *argv[])
 
     for (;;) {
         int ready, nfds = 0;
-	ssize_t nbytes;
+        ssize_t nbytes;
         fd_set readfds, writefds, exceptfds;
 
         FD_ZERO(&readfds);
@@ -668,22 +667,18 @@ main(int argc, char *argv[])
         FD_ZERO(&exceptfds);
         FD_SET(h, &readfds);
         nfds = max(nfds, h);
-        if (fd1 > 0 && buf1_avail < BUF_SIZE) {
+
+        if (fd1 > 0 && buf1_avail < BUF_SIZE)
             FD_SET(fd1, &readfds);
-            nfds = max(nfds, fd1);
-        }
-        if (fd2 > 0 && buf2_avail < BUF_SIZE) {
+            /* Note: nfds is updated below, when fd1 is added to exceptfds. */
+        if (fd2 > 0 && buf2_avail < BUF_SIZE)
             FD_SET(fd2, &readfds);
-            nfds = max(nfds, fd2);
-        }
-        if (fd1 > 0 && buf2_avail \- buf2_written > 0) {
+
+        if (fd1 > 0 && buf2_avail \- buf2_written > 0)
             FD_SET(fd1, &writefds);
-            nfds = max(nfds, fd1);
-        }
-        if (fd2 > 0 && buf1_avail \- buf1_written > 0) {
+        if (fd2 > 0 && buf1_avail \- buf1_written > 0)
             FD_SET(fd2, &writefds);
-            nfds = max(nfds, fd2);
-        }
+
         if (fd1 > 0) {
             FD_SET(fd1, &exceptfds);
             nfds = max(nfds, fd1);
@@ -706,9 +701,9 @@ main(int argc, char *argv[])
         if (FD_ISSET(h, &readfds)) {
             socklen_t addrlen;
             struct sockaddr_in client_addr;
-	    int fd;
+            int fd;
 
-	    addrlen = sizeof(client_addr);
+            addrlen = sizeof(client_addr);
             memset(&client_addr, 0, addrlen);
             fd = accept(h, (struct sockaddr *) &client_addr, &addrlen);
             if (fd == \-1) {
@@ -725,6 +720,9 @@ main(int argc, char *argv[])
                 else
                     printf("connect from %s\\n",
                             inet_ntoa(client_addr.sin_addr));
+
+                /* Skip any events on the old, closed file descriptors. */
+                continue;
             }
         }
 
@@ -764,7 +762,7 @@ main(int argc, char *argv[])
             else
                 buf2_avail += nbytes;
         }
-        if (fd1 > 0 && FD_ISSET(fd1, &writefds)) {
+        if (fd1 > 0 && FD_ISSET(fd1, &writefds) && buf2_avail > 0) {
             nbytes = write(fd1, buf2 + buf2_written,
                        buf2_avail \- buf2_written);
             if (nbytes < 1)
@@ -772,7 +770,7 @@ main(int argc, char *argv[])
             else
                 buf2_written += nbytes;
         }
-        if (fd2 > 0 && FD_ISSET(fd2, &writefds)) {
+        if (fd2 > 0 && FD_ISSET(fd2, &writefds) && buf1_avail > 0) {
             nbytes = write(fd2, buf1 + buf1_written,
                        buf1_avail \- buf1_written);
             if (nbytes < 1)
-- 
2.9.2

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