select_tut(2) — Linux manual page


SELECT_TUT(2)              System Calls Manual             SELECT_TUT(2)

NAME         top

       select, pselect - synchronous I/O multiplexing

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       See select(2)

DESCRIPTION         top

       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

       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;

       main(int argc, char *argv[])
           sigset_t sigmask, empty_mask;
           struct sigaction sa;
           fd_set readfds, writefds, exceptfds;
           int r;

           sigaddset(&sigmask, SIGCHLD);
           if (sigprocmask(SIG_BLOCK, &sigmask, NULL) == -1) {

           sa.sa_flags = 0;
           sa.sa_handler = child_sig_handler;
           if (sigaction(SIGCHLD, &sa, NULL) == -1) {


           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 */

       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         top

       See select(2).

NOTES         top

       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         top

       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 <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) {
               return -1;

           yes = 1;
           if (setsockopt(lfd, SOL_SOCKET, SO_REUSEADDR,
                          &yes, sizeof(yes)) == -1)
               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) {
               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) {
               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");
               return -1;

           if (connect(cfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
               shutdown(cfd, SHUT_RDWR);
               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

       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");

           signal(SIGPIPE, SIG_IGN);

           forward_port = atoi(argv[2]);

           h = listen_socket(atoi(argv[1]));
           if (h == -1)

           for (;;) {
               nfds = 0;

               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)

               if (ready == -1) {

               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) {
                   } else {
                       buf1_avail = buf1_written = 0;
                       buf2_avail = buf2_written = 0;
                       fd1 = fd;
                       fd2 = connect_socket(forward_port, argv[3]);
                       if (fd2 == -1)
                           printf("connect from %s\n",

                       /* Skip any events on the old, closed file
                          descriptors. */


               /* 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)
                       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)
                       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)
                       buf1_avail += nbytes;
               if (fd2 > 0 && FD_ISSET(fd2, &readfds)) {
                   nbytes = read(fd2, buf2 + buf2_avail,
                                 BUF_SIZE - buf2_avail);
                   if (nbytes < 1)
                       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)
                       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)
                       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)
               if (fd2 < 0 && buf2_avail - buf2_written == 0)

       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         top

       accept(2), connect(2), poll(2), read(2), recv(2), select(2),
       send(2), sigprocmask(2), write(2), epoll(7)

Linux man-pages (unreleased)     (date)                    SELECT_TUT(2)

Pages that refer to this page: poll(2)select(2)