select_tut(2) — Linux manual page


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

NAME         top

       select, pselect - synchronous I/O multiplexing

SYNOPSIS         top

       See select(2)

DESCRIPTION         top

       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;

       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 situa‐

   Select law
       Many people who try to use select() come across behavior that is dif‐
       ficult 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 non‐
       blocking mode.  It is easy to introduce subtle errors that will re‐
       move 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 dif‐
           ficult 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 appro‐
           priately.  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 nec‐
           essarily read/write the full amount of data that you have re‐
           quested.  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 (EWOULD‐
           BLOCK).  These results must be properly managed (not done prop‐
           erly above).  If your program is not going to receive any sig‐
           nals, 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 be‐
           low, 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         top

       See select(2).

NOTES         top

       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         top

       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) {
               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 simultane‐
       ously.  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 inef‐
       ficient 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         top

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

COLOPHON         top

       This page is part of release 5.09 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

Linux                            2020-04-11                    SELECT_TUT(2)

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