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NAME
select, pselect - synchronous I/O multiplexing
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
See select(2)
DESCRIPTION
The select() and pselect() system calls are used to efficiently monitor multiple file
descriptors, to see if any of them is, or becomes, "ready"; that is, to see whether I/O
becomes possible, or an "exceptional condition" has occurred on any of the file
descriptors.
This page provides background and tutorial information on the use of these system calls.
For details of the arguments and semantics of select() and pselect(), see select(2).
Combining signal and data events
pselect() is useful if you are waiting for a signal as well as for file descriptor(s) to
become ready for I/O. Programs that receive signals normally use the signal handler only
to raise a global flag. The global flag will indicate that the event must be processed in
the main loop of the program. A signal will cause the select() (or pselect()) call to
return with errno set to EINTR. This behavior is essential so that signals can be
processed in the main loop of the program, otherwise select() would block indefinitely.
Now, somewhere in the main loop will be a conditional to check the global flag. So we
must ask: what if a signal arrives after the conditional, but before the select() call?
The answer is that select() would block indefinitely, even though an event is actually
pending. This race condition is solved by the pselect() call. This call can be used to
set the signal mask to a set of signals that are to be received only within the pselect()
call. For instance, let us say that the event in question was the exit of a child
process. Before the start of the main loop, we would block SIGCHLD using sigprocmask(2).
Our pselect() call would enable SIGCHLD by using an empty signal mask. Our program would
look like:
static volatile sig_atomic_t got_SIGCHLD = 0;
static void
child_sig_handler(int sig)
{
got_SIGCHLD = 1;
}
int
main(int argc, char *argv[])
{
sigset_t sigmask, empty_mask;
struct sigaction sa;
fd_set readfds, writefds, exceptfds;
int r;
sigemptyset(&sigmask);
sigaddset(&sigmask, SIGCHLD);
if (sigprocmask(SIG_BLOCK, &sigmask, NULL) == -1) {
perror("sigprocmask");
exit(EXIT_FAILURE);
}
sa.sa_flags = 0;
sa.sa_handler = child_sig_handler;
sigemptyset(&sa.sa_mask);
if (sigaction(SIGCHLD, &sa, NULL) == -1) {
perror("sigaction");
exit(EXIT_FAILURE);
}
sigemptyset(&empty_mask);
for (;;) { /* main loop */
/* Initialize readfds, writefds, and exceptfds
before the pselect() call. (Code omitted.) */
r = pselect(nfds, &readfds, &writefds, &exceptfds,
NULL, &empty_mask);
if (r == -1 && errno != EINTR) {
/* Handle error */
}
if (got_SIGCHLD) {
got_SIGCHLD = 0;
/* Handle signalled event here; e.g., wait() for all
terminated children. (Code omitted.) */
}
/* main body of program */
}
}
Practical
So what is the point of select()? Can't I just read and write to my file descriptors
whenever I want? The point of select() is that it watches multiple descriptors at the
same time and properly puts the process to sleep if there is no activity. UNIX
programmers often find themselves in a position where they have to handle I/O from more
than one file descriptor where the data flow may be intermittent. If you were to merely
create a sequence of read(2) and write(2) calls, you would find that one of your calls may
block waiting for data from/to a file descriptor, while another file descriptor is unused
though ready for I/O. select() efficiently copes with this situation.
Select law
Many people who try to use select() come across behavior that is difficult to understand
and produces nonportable or borderline results. For instance, the above program is
carefully written not to block at any point, even though it does not set its file
descriptors to nonblocking mode. It is easy to introduce subtle errors that will remove
the advantage of using select(), so here is a list of essentials to watch for when using
select().
1. You should always try to use select() without a timeout. Your program should have
nothing to do if there is no data available. Code that depends on timeouts is not
usually portable and is difficult to debug.
2. The value nfds must be properly calculated for efficiency as explained above.
3. No file descriptor must be added to any set if you do not intend to check its result
after the select() call, and respond appropriately. See next rule.
4. After select() returns, all file descriptors in all sets should be checked to see if
they are ready.
5. The functions read(2), recv(2), write(2), and send(2) do not necessarily read/write
the full amount of data that you have requested. If they do read/write the full
amount, it's because you have a low traffic load and a fast stream. This is not
always going to be the case. You should cope with the case of your functions managing
to send or receive only a single byte.
6. Never read/write only in single bytes at a time unless you are really sure that you
have a small amount of data to process. It is extremely inefficient not to read/write
as much data as you can buffer each time. The buffers in the example below are 1024
bytes although they could easily be made larger.
7. Calls to read(2), recv(2), write(2), send(2), and select() can fail with the error
EINTR, and calls to read(2), recv(2), write(2), and send(2) can fail with errno set to
EAGAIN (EWOULDBLOCK). These results must be properly managed (not done properly
above). If your program is not going to receive any signals, then it is unlikely you
will get EINTR. If your program does not set nonblocking I/O, you will not get
EAGAIN.
8. Never call read(2), recv(2), write(2), or send(2) with a buffer length of zero.
9. If the functions read(2), recv(2), write(2), and send(2) fail with errors other than
those listed in 7., or one of the input functions returns 0, indicating end of file,
then you should not pass that file descriptor to select() again. In the example
below, I close the file descriptor immediately, and then set it to -1 to prevent it
being included in a set.
10. The timeout value must be initialized with each new call to select(), since some
operating systems modify the structure. pselect() however does not modify its timeout
structure.
11. Since select() modifies its file descriptor sets, if the call is being used in a loop,
then the sets must be reinitialized before each call.
RETURN VALUE
See select(2).
NOTES
Generally speaking, all operating systems that support sockets also support select().
select() can be used to solve many problems in a portable and efficient way that naive
programmers try to solve in a more complicated manner using threads, forking, IPCs,
signals, memory sharing, and so on.
The poll(2) system call has the same functionality as select(), and is somewhat more
efficient when monitoring sparse file descriptor sets. It is nowadays widely available,
but historically was less portable than select().
The Linux-specific epoll(7) API provides an interface that is more efficient than
select(2) and poll(2) when monitoring large numbers of file descriptors.
EXAMPLES
Here is an example that better demonstrates the true utility of select(). The listing
below is a TCP forwarding program that forwards from one TCP port to another.
#include <arpa/inet.h>
#include <errno.h>
#include <netinet/in.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
static int forward_port;
#undef max
#define max(x, y) ((x) > (y) ? (x) : (y))
static int
listen_socket(int listen_port)
{
int lfd;
int yes;
struct sockaddr_in addr;
lfd = socket(AF_INET, SOCK_STREAM, 0);
if (lfd == -1) {
perror("socket");
return -1;
}
yes = 1;
if (setsockopt(lfd, SOL_SOCKET, SO_REUSEADDR,
&yes, sizeof(yes)) == -1)
{
perror("setsockopt");
close(lfd);
return -1;
}
memset(&addr, 0, sizeof(addr));
addr.sin_port = htons(listen_port);
addr.sin_family = AF_INET;
if (bind(lfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
perror("bind");
close(lfd);
return -1;
}
printf("accepting connections on port %d\n", listen_port);
listen(lfd, 10);
return lfd;
}
static int
connect_socket(int connect_port, char *address)
{
int cfd;
struct sockaddr_in addr;
cfd = socket(AF_INET, SOCK_STREAM, 0);
if (cfd == -1) {
perror("socket");
return -1;
}
memset(&addr, 0, sizeof(addr));
addr.sin_port = htons(connect_port);
addr.sin_family = AF_INET;
if (!inet_aton(address, (struct in_addr *) &addr.sin_addr.s_addr)) {
fprintf(stderr, "inet_aton(): bad IP address format\n");
close(cfd);
return -1;
}
if (connect(cfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
perror("connect()");
shutdown(cfd, SHUT_RDWR);
close(cfd);
return -1;
}
return cfd;
}
#define SHUT_FD1 do { \
if (fd1 >= 0) { \
shutdown(fd1, SHUT_RDWR); \
close(fd1); \
fd1 = -1; \
} \
} while (0)
#define SHUT_FD2 do { \
if (fd2 >= 0) { \
shutdown(fd2, SHUT_RDWR); \
close(fd2); \
fd2 = -1; \
} \
} while (0)
#define BUF_SIZE 1024
int
main(int argc, char *argv[])
{
int h;
int ready, nfds;
int fd1 = -1, fd2 = -1;
int buf1_avail = 0, buf1_written = 0;
int buf2_avail = 0, buf2_written = 0;
char buf1[BUF_SIZE], buf2[BUF_SIZE];
fd_set readfds, writefds, exceptfds;
ssize_t nbytes;
if (argc != 4) {
fprintf(stderr, "Usage\n\tfwd <listen-port> "
"<forward-to-port> <forward-to-ip-address>\n");
exit(EXIT_FAILURE);
}
signal(SIGPIPE, SIG_IGN);
forward_port = atoi(argv[2]);
h = listen_socket(atoi(argv[1]));
if (h == -1)
exit(EXIT_FAILURE);
for (;;) {
nfds = 0;
FD_ZERO(&readfds);
FD_ZERO(&writefds);
FD_ZERO(&exceptfds);
FD_SET(h, &readfds);
nfds = max(nfds, h);
if (fd1 > 0 && buf1_avail < BUF_SIZE)
FD_SET(fd1, &readfds);
/* Note: nfds is updated below, when fd1 is added to
exceptfds. */
if (fd2 > 0 && buf2_avail < BUF_SIZE)
FD_SET(fd2, &readfds);
if (fd1 > 0 && buf2_avail - buf2_written > 0)
FD_SET(fd1, &writefds);
if (fd2 > 0 && buf1_avail - buf1_written > 0)
FD_SET(fd2, &writefds);
if (fd1 > 0) {
FD_SET(fd1, &exceptfds);
nfds = max(nfds, fd1);
}
if (fd2 > 0) {
FD_SET(fd2, &exceptfds);
nfds = max(nfds, fd2);
}
ready = select(nfds + 1, &readfds, &writefds, &exceptfds, NULL);
if (ready == -1 && errno == EINTR)
continue;
if (ready == -1) {
perror("select()");
exit(EXIT_FAILURE);
}
if (FD_ISSET(h, &readfds)) {
socklen_t addrlen;
struct sockaddr_in client_addr;
int fd;
addrlen = sizeof(client_addr);
memset(&client_addr, 0, addrlen);
fd = accept(h, (struct sockaddr *) &client_addr, &addrlen);
if (fd == -1) {
perror("accept()");
} else {
SHUT_FD1;
SHUT_FD2;
buf1_avail = buf1_written = 0;
buf2_avail = buf2_written = 0;
fd1 = fd;
fd2 = connect_socket(forward_port, argv[3]);
if (fd2 == -1)
SHUT_FD1;
else
printf("connect from %s\n",
inet_ntoa(client_addr.sin_addr));
/* Skip any events on the old, closed file
descriptors. */
continue;
}
}
/* NB: read OOB data before normal reads. */
if (fd1 > 0 && FD_ISSET(fd1, &exceptfds)) {
char c;
nbytes = recv(fd1, &c, 1, MSG_OOB);
if (nbytes < 1)
SHUT_FD1;
else
send(fd2, &c, 1, MSG_OOB);
}
if (fd2 > 0 && FD_ISSET(fd2, &exceptfds)) {
char c;
nbytes = recv(fd2, &c, 1, MSG_OOB);
if (nbytes < 1)
SHUT_FD2;
else
send(fd1, &c, 1, MSG_OOB);
}
if (fd1 > 0 && FD_ISSET(fd1, &readfds)) {
nbytes = read(fd1, buf1 + buf1_avail,
BUF_SIZE - buf1_avail);
if (nbytes < 1)
SHUT_FD1;
else
buf1_avail += nbytes;
}
if (fd2 > 0 && FD_ISSET(fd2, &readfds)) {
nbytes = read(fd2, buf2 + buf2_avail,
BUF_SIZE - buf2_avail);
if (nbytes < 1)
SHUT_FD2;
else
buf2_avail += nbytes;
}
if (fd1 > 0 && FD_ISSET(fd1, &writefds) && buf2_avail > 0) {
nbytes = write(fd1, buf2 + buf2_written,
buf2_avail - buf2_written);
if (nbytes < 1)
SHUT_FD1;
else
buf2_written += nbytes;
}
if (fd2 > 0 && FD_ISSET(fd2, &writefds) && buf1_avail > 0) {
nbytes = write(fd2, buf1 + buf1_written,
buf1_avail - buf1_written);
if (nbytes < 1)
SHUT_FD2;
else
buf1_written += nbytes;
}
/* Check if write data has caught read data. */
if (buf1_written == buf1_avail)
buf1_written = buf1_avail = 0;
if (buf2_written == buf2_avail)
buf2_written = buf2_avail = 0;
/* One side has closed the connection, keep
writing to the other side until empty. */
if (fd1 < 0 && buf1_avail - buf1_written == 0)
SHUT_FD2;
if (fd2 < 0 && buf2_avail - buf2_written == 0)
SHUT_FD1;
}
exit(EXIT_SUCCESS);
}
The above program properly forwards most kinds of TCP connections including OOB signal
data transmitted by telnet servers. It handles the tricky problem of having data flow in
both directions simultaneously. You might think it more efficient to use a fork(2) call
and devote a thread to each stream. This becomes more tricky than you might suspect.
Another idea is to set nonblocking I/O using fcntl(2). This also has its problems because
you end up using inefficient timeouts.
The program does not handle more than one simultaneous connection at a time, although it
could easily be extended to do this with a linked list of buffers—one for each connection.
At the moment, new connections cause the current connection to be dropped.
SEE ALSO
accept(2), connect(2), poll(2), read(2), recv(2), select(2), send(2), sigprocmask(2),
write(2), epoll(7)