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       select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O multiplexing


       /* According to POSIX.1-2001, POSIX.1-2008 */
       #include <sys/select.h>

       /* According to earlier standards */
       #include <sys/time.h>
       #include <sys/types.h>
       #include <unistd.h>

       int select(int nfds, fd_set *readfds, fd_set *writefds,
                  fd_set *exceptfds, struct timeval *utimeout);

       void FD_CLR(int fd, fd_set *set);
       int  FD_ISSET(int fd, fd_set *set);
       void FD_SET(int fd, fd_set *set);
       void FD_ZERO(fd_set *set);

       #include <sys/select.h>

       int pselect(int nfds, fd_set *readfds, fd_set *writefds,
                   fd_set *exceptfds, const struct timespec *ntimeout,
                   const sigset_t *sigmask);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       pselect(): _POSIX_C_SOURCE >= 200112L


       select()  (or  pselect()) is 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.

       Its  principal  arguments  are  three  "sets"  of file descriptors: readfds, writefds, and
       exceptfds.  Each set is declared as type fd_set, and its contents can be manipulated  with
       the  macros  FD_CLR(),  FD_ISSET(),  FD_SET(), and FD_ZERO().  A newly declared set should
       first be cleared using FD_ZERO().  select() modifies the contents of the sets according to
       the  rules  described  below;  after calling select() you can test if a file descriptor is
       still present in a set with  the  FD_ISSET()  macro.   FD_ISSET()  returns  nonzero  if  a
       specified  file  descriptor is present in a set and zero if it is not.  FD_CLR() removes a
       file descriptor from a set.

              This set is watched to see if data is available for reading from any  of  its  file
              descriptors.   After  select()  has  returned,  readfds will be cleared of all file
              descriptors except for those that are immediately available for reading.

              This set is watched to see if there is space to write  data  to  any  of  its  file
              descriptors.   After  select()  has  returned, writefds will be cleared of all file
              descriptors except for those that are immediately available for writing.

              This set is watched for "exceptional  conditions".   In  practice,  only  one  such
              exceptional  condition  is  common:  the availability of out-of-band (OOB) data for
              reading from a TCP socket.  See recv(2), send(2), and tcp(7) for more details about
              OOB  data.   (One  other  less common case where select(2) indicates an exceptional
              condition occurs with pseudoterminals in packet  mode;  see  ioctl_tty(2).)   After
              select() has returned, exceptfds will be cleared of all file descriptors except for
              those for which an exceptional condition has occurred.

       nfds   This is an integer one more than the maximum of any file descriptor in any  of  the
              sets.   In other words, while adding file descriptors to each of the sets, you must
              calculate the maximum integer value of all of them, then increment  this  value  by
              one, and then pass this as nfds.

              This  is  the  longest  time  select()  may  wait before returning, even if nothing
              interesting happened.  If this value  is  passed  as  NULL,  then  select()  blocks
              indefinitely waiting for a file descriptor to become ready.  utimeout can be set to
              zero seconds, which causes select() to return immediately, with  information  about
              the  readiness  of  file descriptors at the time of the call.  The structure struct
              timeval is defined as:

                  struct timeval {
                      time_t tv_sec;    /* seconds */
                      long tv_usec;     /* microseconds */

              This argument for pselect() has the same meaning as utimeout, but  struct  timespec
              has nanosecond precision as follows:

                  struct timespec {
                      long tv_sec;    /* seconds */
                      long tv_nsec;   /* nanoseconds */

              This  argument  holds a set of signals that the kernel should unblock (i.e., remove
              from the signal mask of the calling thread), while the caller is blocked inside the
              pselect()  call  (see  sigaddset(3)  and sigprocmask(2)).  It may be NULL, in which
              case the call does not modify the signal mask on entry and exit  to  the  function.
              In this case, pselect() will then behave just like select().

   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.

   Usleep emulation
       On systems that do not have a usleep(3) function, you can  call  select()  with  a  finite
       timeout and no file descriptors as follows:

           struct timeval tv;
           tv.tv_sec = 0;
           tv.tv_usec = 200000;  /* 0.2 seconds */
           select(0, NULL, NULL, NULL, &tv);

       This is guaranteed to work only on UNIX systems, however.


       On  success,  select()  returns  the total number of file descriptors still present in the
       file descriptor sets.

       If select() timed out, then the return value will  be  zero.   The  file  descriptors  set
       should be all empty (but may not be on some systems).

       A  return value of -1 indicates an error, with errno being set appropriately.  In the case
       of an error, the contents of the  returned  sets  and  the  struct  timeout  contents  are
       undefined and should not be used.  pselect() however never modifies ntimeout.


       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/time.h>
       #include <sys/types.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)) {
               perror("bad IP address format");
               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),   ioctl(2),   poll(2),   read(2),  recv(2),  select(2),  send(2),
       sigprocmask(2),  write(2),  sigaddset(3),  sigdelset(3),  sigemptyset(3),   sigfillset(3),
       sigismember(3), epoll(7)


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       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at