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SELECT_TUT(2)              Linux Programmer's Manual             SELECT_TUT(2)

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

       /* According to POSIX.1-2001 */
       #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 manip-
       ulated  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  con-
       tents  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 non-zero 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

              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

              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  pseudo-terminals  in  packet
              mode;  see  tty_ioctl(4).)   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  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 descrip-
       tor(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 con-
       dition  is  solved by the pselect() call.  This call can be used to set the siognal
       mask to a set of signals that are only to be received within  the  pselect()  call.
       For  instance,  let  us say that the event in question was the exit of a child pro-
       cess.  Before the start of the main loop, we would  block  SIGCHLD  using  sigproc-
       mask(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  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  non-portable  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 non-blocking 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 only managing to send or receive 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 exam-
           ple 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  non-blocking  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.  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 re-initialized 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 only guaranteed to work 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 con-
       tents are undefined and should not  be  used.   pselect()  however  never  modifies

       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

       #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 a;
           int s;
           int yes;

           if ((s = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
               return -1;
           yes = 1;
           if (setsockopt(s, SOL_SOCKET, SO_REUSEADDR,
                   (char *) &yes, sizeof(yes)) == -1) {
               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;

           if ((s = socket(AF_INET, SOCK_STREAM, 0)) == -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;                   \
                            }                               \
                        } 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, 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) {
                   FD_SET(fd2, &er);
                   nfds = max(nfds, fd2);

               r = select(nfds + 1, &rd, &wr, &er, NULL);

               if (r == -1 && errno == EINTR)

               if (r == -1) {

               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)
                           buf1_avail += r;
               if (fd2 > 0)
                   if (FD_ISSET(fd2, &rd)) {
                       r = read(fd2, buf2 + buf2_avail,
                                 BUF_SIZE - buf2_avail);
                       if (r < 1)
                           buf2_avail += r;
               if (fd1 > 0)
                   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 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 non-blocking 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),   sig-
       fillset(3), sigismember(3), epoll(7)

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-

Linux                             2009-01-26                     SELECT_TUT(2)

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