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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 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,
              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  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 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  only  to  be  received  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

       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 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 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 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.
           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 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 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().   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)) == -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  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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       description of the project, and information about reporting  bugs,  can
       be found at