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          select, pselect - synchronous I/O multiplexing

          See select(2)

          The select() and pselect() system calls are used to effi-
          ciently 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;

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          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

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          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 wait-
          ing for data from/to a file descriptor, while another file
          descriptor is unused though ready for I/O.  select() effi-
          ciently 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 effi-
              ciency 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

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              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 time-
              out 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.

          See select(2).

          Generally speaking, all operating systems that support sock-
          ets 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 avail-
          able, 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.

          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 <stdlib.h>
          #include <stdio.h>
          #include <unistd.h>

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          #include <sys/select.h>
          #include <string.h>
          #include <signal.h>
          #include <sys/socket.h>
          #include <netinet/in.h>
          #include <arpa/inet.h>
          #include <errno.h>

          static int forward_port;

          #undef max
          #define max(x,y) ((x) > (y) ? (x) : (y))

          static int
          listen_socket(int listen_port)
              struct sockaddr_in addr;
              int lfd;
              int yes;

              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)
              struct sockaddr_in addr;

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              int cfd;

              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 fd1 = -1, fd2 = -1;
              char buf1[BUF_SIZE], buf2[BUF_SIZE];
              int buf1_avail = 0, buf1_written = 0;

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              int buf2_avail = 0, buf2_written = 0;

              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 (;;) {
                  int ready, nfds = 0;
                  ssize_t nbytes;
                  fd_set readfds, writefds, 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)

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                  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)

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                          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)

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          The above program properly forwards most kinds of TCP con-
          nections 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

          The program does not handle more than one simultaneous con-
          nection at a time, although it could easily be extended to
          do this with a linked list of buffers-one for each connec-
          tion.  At the moment, new connections cause the current con-
          nection to be dropped.

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

          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at

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