select_tut

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
       "exceptional 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
       select() 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,
       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 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;
       }

       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
       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 managing to send or receive only 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.  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
           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
           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
           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.

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
       efficient 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
       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/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
       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.

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)