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 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 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

       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 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 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 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 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)