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

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

       /* According to POSIX.1-2001, POSIX.1-2008 */
       #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

       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 file 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 pseudoterminals in packet mode;
              see ioctl_tty(2).)  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

   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 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

   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 guaranteed to work only 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

       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 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
       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)

       This page is part of release 5.05 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                              2019-03-06                      SELECT_TUT(2)