SELECT_TUT(2)



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

NAME
       select, pselect - synchronous I/O multiplexing

SYNOPSIS
       See select(2)

DESCRIPTION
       The select() and pselect() system calls are 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 "excep-
       tional condition" has occurred on any of the file descriptors.

       This page provides background and tutorial information on  the  use  of
       these  system calls.  For details of the arguments and semantics of se-
       lect() and pselect(), see select(2).

   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 condi-
       tional,  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 pse-
       lect() 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;
       }

       int
       main(int argc, char *argv[])
       {
           sigset_t sigmask, empty_mask;
           struct sigaction sa;
           fd_set readfds, writefds, exceptfds;
           int r;

           sigemptyset(&sigmask);
           sigaddset(&sigmask, SIGCHLD);
           if (sigprocmask(SIG_BLOCK, &sigmask, NULL) == -1) {
               perror("sigprocmask");
               exit(EXIT_FAILURE);
           }

           sa.sa_flags = 0;
           sa.sa_handler = child_sig_handler;
           sigemptyset(&sa.sa_mask);
           if (sigaction(SIGCHLD, &sa, NULL) == -1) {
               perror("sigaction");
               exit(EXIT_FAILURE);
           }

           sigemptyset(&empty_mask);

           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 */
           }
       }

   Practical
       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.

RETURN VALUE
       See select(2).

NOTES
       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.

EXAMPLES
       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/select.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) {
               perror("socket");
               return -1;
           }

           yes = 1;
           if (setsockopt(lfd, SOL_SOCKET, SO_REUSEADDR,
                   &yes, sizeof(yes)) == -1) {
               perror("setsockopt");
               close(lfd);
               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) {
               perror("bind");
               close(lfd);
               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) {
               perror("socket");
               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");
               close(cfd);
               return -1;
           }

           if (connect(cfd, (struct sockaddr *) &addr, sizeof(addr)) == -1) {
               perror("connect()");
               shutdown(cfd, SHUT_RDWR);
               close(cfd);
               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

       int
       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");
               exit(EXIT_FAILURE);
           }

           signal(SIGPIPE, SIG_IGN);

           forward_port = atoi(argv[2]);

           h = listen_socket(atoi(argv[1]));
           if (h == -1)
               exit(EXIT_FAILURE);

           for (;;) {
               int ready, nfds = 0;
               ssize_t nbytes;
               fd_set readfds, writefds, exceptfds;

               FD_ZERO(&readfds);
               FD_ZERO(&writefds);
               FD_ZERO(&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)
                   continue;

               if (ready == -1) {
                   perror("select()");
                   exit(EXIT_FAILURE);
               }

               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) {
                       perror("accept()");
                   } else {
                       SHUT_FD1;
                       SHUT_FD2;
                       buf1_avail = buf1_written = 0;
                       buf2_avail = buf2_written = 0;
                       fd1 = fd;
                       fd2 = connect_socket(forward_port, argv[3]);
                       if (fd2 == -1)
                           SHUT_FD1;
                       else
                           printf("connect from %s\n",
                                   inet_ntoa(client_addr.sin_addr));

                       /* Skip any events on the old, closed file
                          descriptors. */

                       continue;
                   }
               }

               /* 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)
                       SHUT_FD1;
                   else
                       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)
                       SHUT_FD2;
                   else
                       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)
                       SHUT_FD1;
                   else
                       buf1_avail += nbytes;
               }
               if (fd2 > 0 && FD_ISSET(fd2, &readfds)) {
                   nbytes = read(fd2, buf2 + buf2_avail,
                             BUF_SIZE - buf2_avail);
                   if (nbytes < 1)
                       SHUT_FD2;
                   else
                       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)
                       SHUT_FD1;
                   else
                       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)
                       SHUT_FD2;
                   else
                       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)
                   SHUT_FD2;
               if (fd2 < 0 && buf2_avail - buf2_written == 0)
                   SHUT_FD1;
           }
           exit(EXIT_SUCCESS);
       }

       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.

SEE ALSO
       accept(2), connect(2), poll(2), read(2), recv(2),  select(2),  send(2),
       sigprocmask(2), write(2), epoll(7)

COLOPHON
       This  page  is  part of release 5.07 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2020-04-11                     SELECT_TUT(2)

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