/* 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 || _XOPEN_SOURCE >= 600

       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 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

              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,
              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 sec-
              onds, which causes select() to return immediately, with informa-
              tion  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, other-
       wise 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

       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
       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
       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 pro-
           gram 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  appropri-
           ately.  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 neces-
           sarily  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.  The  functions  read(2),  recv(2), write(2), and send(2) as well as
           the select() call can return -1 with errno set to  EINTR,  or  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
           descriptor to select() again.  In the example below,  I  close  the
           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.  pse-
           lect() 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

       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 appropri-
       ately.  In the case of an error, the contents of the returned sets  and
       the struct timeout contents are undefined and should not be used.  pse-
       lect() 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)
               return -1;
           memset(&a, 0, sizeof(a));
           a.sin_port = htons(listen_port);
           a.sin_family = AF_INET;
           if (bind(s, (struct sockaddr *) &a, sizeof(a)) == -1) {
               return -1;
           printf("accepting connections on port %d\n", listen_port);
           listen(s, 10);
           return s;

       static int
       connect_socket(int connect_port, char *address)
           struct sockaddr_in a;
           int s;

           s = socket(AF_INET, SOCK_STREAM, 0);
           if (s == -1) {
               return -1;

           memset(&a, 0, sizeof(a));
           a.sin_port = htons(connect_port);
           a.sin_family = AF_INET;

           if (!inet_aton(address, (struct in_addr *) &a.sin_addr.s_addr)) {
               perror("bad IP address format");
               return -1;

           if (connect(s, (struct sockaddr *) &a, sizeof(a)) == -1) {
               shutdown(s, SHUT_RDWR);
               return -1;
           return s;

       #define SHUT_FD1 do {                                \
                            if (fd1 >= 0) {                 \
                                shutdown(fd1, SHUT_RDWR);   \
                                close(fd1);                 \
                                fd1 = -1;                   \

       main(int argc, char *argv[])
           int h;
           int fd1 = -1, fd2 = -1;
           char buf1[BUF_SIZE], buf2[BUF_SIZE];
           int buf1_avail, buf1_written;
           int buf2_avail, buf2_written;

           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 r, nfds = 0;
               fd_set rd, wr, er;

               FD_SET(h, &rd);
               nfds = max(nfds, h);
               if (fd1 > 0 && buf1_avail < BUF_SIZE) {
                   FD_SET(fd1, &rd);
                   nfds = max(nfds, fd1);
               if (fd2 > 0 && buf2_avail < BUF_SIZE) {
                   FD_SET(fd2, &rd);
                   nfds = max(nfds, fd2);
               if (fd1 > 0 && buf2_avail - buf2_written > 0) {
                   FD_SET(fd1, &wr);
                   nfds = max(nfds, fd1);
               if (fd2 > 0 && buf1_avail - buf1_written > 0) {
                   FD_SET(fd2, &wr);
                   nfds = max(nfds, fd2);
               if (fd1 > 0) {
                   FD_SET(fd1, &er);
                   nfds = max(nfds, fd1);
               if (fd2 > 0) {

               if (FD_ISSET(h, &rd)) {
                   unsigned int l;
                   struct sockaddr_in client_address;

                   memset(&client_address, 0, l = sizeof(client_address));
                   r = accept(h, (struct sockaddr *) &client_address, &l);
                   if (r == -1) {
                   } else {
                       buf1_avail = buf1_written = 0;
                       buf2_avail = buf2_written = 0;
                       fd1 = r;
                       fd2 = connect_socket(forward_port, argv[3]);
                       if (fd2 == -1)
                           printf("connect from %s\n",

               /* NB: read oob data before normal reads */

               if (fd1 > 0)
                   if (FD_ISSET(fd1, &er)) {
                       char c;

                       r = recv(fd1, &c, 1, MSG_OOB);
                       if (r < 1)
                           send(fd2, &c, 1, MSG_OOB);
               if (fd2 > 0)
                   if (FD_ISSET(fd2, &er)) {
                       char c;

                       r = recv(fd2, &c, 1, MSG_OOB);
                       if (r < 1)
                           send(fd1, &c, 1, MSG_OOB);
               if (fd1 > 0)
                   if (FD_ISSET(fd1, &rd)) {
                       r = read(fd1, buf1 + buf1_avail,
                                 BUF_SIZE - buf1_avail);
                       if (r < 1)

                   if (FD_ISSET(fd1, &wr)) {
                       r = write(fd1, buf2 + buf2_written,
                                  buf2_avail - buf2_written);
                       if (r < 1)
                           buf2_written += r;
               if (fd2 > 0)
                   if (FD_ISSET(fd2, &wr)) {
                       r = write(fd2, buf1 + buf1_written,
                                  buf1_avail - buf1_written);
                       if (r < 1)
                           buf1_written += r;

               /* 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 simultane-
       ously.  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 time-

       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  connec-
       tions 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),  sigemp-
       tyset(3), sigfillset(3), sigismember(3), epoll(7)
Man Pages Copyright Respective Owners. Site Copyright (C) 1994 - 2019 Hurricane Electric. All Rights Reserved.