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


       /* 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 the pivot function of most C  programs  that
       handle more than one simultaneous file descriptor (or socket handle) in
       an efficient manner.  Its principal arguments are three arrays of  file
       descriptors:  readfds,  writefds, and exceptfds.  The way that select()
       is usually used is to block while waiting for a "change of  status"  on
       one or more of the file descriptors.  A "change of status" is when more
       characters become available from the file  descriptor,  or  when  space
       becomes  available  within the kernel’s internal buffers for more to be
       written to the file descriptor, or when a  file  descriptor  goes  into
       error  (in  the  case of a socket or pipe this is when the other end of
       the connection is closed).

       In summary, select() just watches multiple file descriptors, and is the
       standard Unix call to do so.

       The  arrays  of file descriptors are called file descriptor sets.  Each
       set is declared as type fd_set, and its contents can  be  altered  with
       the macros FD_CLR(), FD_ISSET(), FD_SET(), and FD_ZERO().  FD_ZERO() is
       usually the first  function  to  be  used  on  a  newly  declared  set.
       Thereafter,  the individual file descriptors that you are interested in
       can be added one by one with FD_SET().  select() modifies the  contents
       of  the  sets  according  to  the  rules described below; after calling
       select() you can test if your file descriptor is still present  in  the
       set  with  the  FD_ISSET()  macro.   FD_ISSET() returns non-zero if the
       descriptor is present and zero if it is not.  FD_CLR() removes  a  file
       descriptor from the 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 with a  recv(2)  (for
              sockets) or read(2) (for pipes, files, and sockets) call.

              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 with a  send(2)
              (for  sockets) or write(2) (for pipes, files, and sockets) call.

              This set is watched for exceptions or errors on any of the  file
              descriptors.   However,  that is actually just a rumor.  How you
              use exceptfds is to watch for out-of-band (OOB) data.  OOB  data
              is  data  sent  on  a  socket  using the MSG_OOB flag, and hence
              exceptfds only really  applies  to  sockets.   See  recv(2)  and
              send(2) about this.  After select() has returned, exceptfds will
              be cleared of all file descriptors except  for  those  that  are
              available for reading OOB data.  You can only ever read one byte
              of OOB data though (which is done with recv(2)), and writing OOB
              data  (done  with  send(2)) can be done at any time and will not
              block.  Hence there is no need for a fourth set to  check  if  a
              socket is available for writing OOB data.

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

              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 an event.
              utimeout can be set to zero seconds, which  causes  select()  to
              return immediately.  The structure struct timeval is defined as:

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

              This argument 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 to allow while performing a
              pselect() call (see sigaddset(3) and sigprocmask(2)).  It can be
              passed  as  NULL,  in  which  case it does not modify the set of
              allowed signals on entry and exit to the function.  It will then
              behave just like select().

   Combining Signal and Data Events
       pselect()  must be used if you are waiting for a signal as well as data
       from a file  descriptor.   Programs  that  receive  signals  as  events
       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 mask out
       signals that are not to be received except 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 the virgin signal mask.  Our program would look like:

       int child_events = 0;

       child_sig_handler(int x)
           signal(SIGCHLD, child_sig_handler);

       main(int argc, char **argv)
           sigset_t sigmask, orig_sigmask;

           sigaddset(&sigmask, SIGCHLD);
           sigprocmask(SIG_BLOCK, &sigmask, &orig_sigmask);

           signal(SIGCHLD, child_sig_handler);

           for (;;) { /* main loop */
               for (; child_events > 0; child_events--) {
                   /* do event work here */
               r = pselect(nfds, &rd, &wr, &er, 0, &orig_sigmask);

               /* 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.  It does this while enabling you to
       handle multiple simultaneous pipes and sockets.  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
       available for data.  select() efficiently copes with this situation.

       A simple example of the use of select() can be found in  the  select(2)
       manual page.

   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  at  all  (see  ioctl(2)).   It is easy to introduce
       subtle errors that will remove the advantage of using select(), hence I
       will  present a list of essentials to watch for when using the 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 example above 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.  Nonetheless you should still cope with  these  errors  for

       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 above example, 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. I  have  heard that the Windows socket layer does not cope with OOB
           data properly.  It also does not cope with select() calls  when  no
           file descriptors are set at all.  Having no file descriptors set is
           a useful way to sleep the  process  with  sub-second  precision  by
           using the timeout.  (See further on.)

   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 contents are undefined and  should  not  be
       used.  pselect() however never modifies ntimeout.


       Generally  speaking,  all  operating systems that support sockets, also
       support select().  Many types of programs become extremely  complicated
       without  the  use  of  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 a;
           int s;
           int yes;

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

           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 < 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 < 0) {
                   } else {
                       buf1_avail = buf1_written = 0;
                       buf2_avail = buf2_written = 0;
                       fd1 = r;
                       fd2 = connect_socket(forward_port, argv[3]);
                       if (fd2 < 0)
                           printf("connect from %s\n",

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

               if (fd1 > 0)
                   if (FD_ISSET(fd1, &er)) {
                       char c;
                       errno = 0;
                       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;
                       errno = 0;
                       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  an
       ioctl(2)  call.   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)


       This  page  is  part of release 3.15 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at