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       select, pselect - synchronous I/O multiplexing


       See select(2)


       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

       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

       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

       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 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.


       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>
       #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;
           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;
           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)

               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.


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


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