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

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

       /* 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 oc-
       curred 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

              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  excep-
              tional 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  de-
              scriptor  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 un-
              block (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
       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 diffi-
       cult 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 ad-
       vantage 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  ex-
           plained 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 re-
           ally 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 re-
           sults 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
           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 se-
           lect(), 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
           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 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  ef-
       ficient  than  select(2)  and  poll(2) when monitoring large numbers of
       file descriptors.

       Here is an example that better demonstrates the  true  utility  of  se-
       lect().   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)) {
               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  in-
       cluding  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  sus-
       pect.   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  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)

       This  page  is  part of release 5.05 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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