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select_tut.2: fix issues in example 

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@lekensteyn.nl>
This commit is contained in:
Peter Wu 2016-08-10 17:23:16 +02:00 committed by Michael Kerrisk
parent 519303a6a4
commit 3c3d1f83f9
1 changed files with 18 additions and 20 deletions
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38
man2/select_tut.2
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@ -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)