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

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

       #include <sys/select.h>

       int select(int nfds, fd_set *restrict readfds,
                  fd_set *restrict writefds, fd_set *restrict exceptfds,
                  struct timeval *restrict timeout);

       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);

       int pselect(int nfds, fd_set *restrict readfds,
                  fd_set *restrict writefds, fd_set *restrict exceptfds,
                  const struct timespec *restrict timeout,
                  const sigset_t *restrict sigmask);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           _POSIX_C_SOURCE >= 200112L

       WARNING: select() can monitor only file descriptors numbers that are less
       than FD_SETSIZE (1024)—an unreasonably low limit for many modern
       applications—and this limitation will not change.  All modern
       applications should instead use poll(2) or epoll(7), which do not suffer
       this limitation.

       select() allows a program to monitor multiple file descriptors, waiting
       until one or more of the file descriptors become "ready" for some class
       of I/O operation (e.g., input possible).  A file descriptor is considered
       ready if it is possible to perform a corresponding I/O operation (e.g.,
       read(2), or a sufficiently small write(2)) without blocking.

   File descriptor sets
       The principal arguments of select() are three "sets" of file descriptors
       (declared with the type fd_set), which allow the caller to wait for three
       classes of events on the specified set of file descriptors.  Each of the
       fd_set arguments may be specified as NULL if no file descriptors are to
       be watched for the corresponding class of events.

       Note well: Upon return, each of the file descriptor sets is modified in
       place to indicate which file descriptors are currently "ready".  Thus, if
       using select() within a loop, the sets must be reinitialized before each

       The contents of a file descriptor set can be manipulated using the
       following macros:

              This macro clears (removes all file descriptors from) set.  It
              should be employed as the first step in initializing a file
              descriptor set.

              This macro adds the file descriptor fd to set.  Adding a file
              descriptor that is already present in the set is a no-op, and does
              not produce an error.

              This macro removes the file descriptor fd from set.  Removing a
              file descriptor that is not present in the set is a no-op, and
              does not produce an error.

              select() modifies the contents of the sets according to the rules
              described below.  After calling select(), the FD_ISSET() macro can
              be used to test if a file descriptor is still present in a set.
              FD_ISSET() returns nonzero if the file descriptor fd is present in
              set, and zero if it is not.

       The arguments of select() are as follows:

              The file descriptors in this set are watched to see if they are
              ready for reading.  A file descriptor is ready for reading if a
              read operation will not block; in particular, a file descriptor is
              also ready on end-of-file.

              After select() has returned, readfds will be cleared of all file
              descriptors except for those that are ready for reading.

              The file descriptors in this set are watched to see if they are
              ready for writing.  A file descriptor is ready for writing if a
              write operation will not block.  However, even if a file
              descriptor indicates as writable, a large write may still block.

              After select() has returned, writefds will be cleared of all file
              descriptors except for those that are ready for writing.

              The file descriptors in this set are watched for "exceptional
              conditions".  For examples of some exceptional conditions, see the
              discussion of POLLPRI in poll(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 argument should be set to the highest-numbered file
              descriptor in any of the three sets, plus 1.  The indicated file
              descriptors in each set are checked, up to this limit (but see

              The timeout argument is a timeval structure (shown below) that
              specifies the interval that select() should block waiting for a
              file descriptor to become ready.  The call will block until

              • a file descriptor becomes ready;

              • the call is interrupted by a signal handler; or

              • the timeout expires.

              Note that the timeout interval will be rounded up to the system
              clock granularity, and kernel scheduling delays mean that the
              blocking interval may overrun by a small amount.

              If both fields of the timeval structure are zero, then select()
              returns immediately.  (This is useful for polling.)

              If timeout is specified as NULL, select() blocks indefinitely
              waiting for a file descriptor to become ready.

       The pselect() system call allows an application to safely wait until
       either a file descriptor becomes ready or until a signal is caught.

       The operation of select() and pselect() is identical, other than these
       three differences:

       • select() uses a timeout that is a struct timeval (with seconds and
         microseconds), while pselect() uses a struct timespec (with seconds and

       • select() may update the timeout argument to indicate how much time was
         left.  pselect() does not change this argument.

       • select() has no sigmask argument, and behaves as pselect() called with
         NULL sigmask.

       sigmask is a pointer to a signal mask (see sigprocmask(2)); if it is not
       NULL, then pselect() first replaces the current signal mask by the one
       pointed to by sigmask, then does the "select" function, and then restores
       the original signal mask.  (If sigmask is NULL, the signal mask is not
       modified during the pselect() call.)

       Other than the difference in the precision of the timeout argument, the
       following pselect() call:

           ready = pselect(nfds, &readfds, &writefds, &exceptfds,
                           timeout, &sigmask);

       is equivalent to atomically executing the following calls:

           sigset_t origmask;

           pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
           ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
           pthread_sigmask(SIG_SETMASK, &origmask, NULL);

       The reason that pselect() is needed is that if one wants to wait for
       either a signal or for a file descriptor to become ready, then an atomic
       test is needed to prevent race conditions.  (Suppose the signal handler
       sets a global flag and returns.  Then a test of this global flag followed
       by a call of select() could hang indefinitely if the signal arrived just
       after the test but just before the call.  By contrast, pselect() allows
       one to first block signals, handle the signals that have come in, then
       call pselect() with the desired sigmask, avoiding the race.)

   The timeout
       The timeout argument for select() is a structure of the following type:

           struct timeval {
               time_t      tv_sec;         /* seconds */
               suseconds_t tv_usec;        /* microseconds */

       The corresponding argument for pselect() has the following type:

           struct timespec {
               time_t      tv_sec;         /* seconds */
               long        tv_nsec;        /* nanoseconds */

       On Linux, select() modifies timeout to reflect the amount of time not
       slept; most other implementations do not do this.  (POSIX.1 permits
       either behavior.)  This causes problems both when Linux code which reads
       timeout is ported to other operating systems, and when code is ported to
       Linux that reuses a struct timeval for multiple select()s in a loop
       without reinitializing it.  Consider timeout to be undefined after
       select() returns.

       On success, select() and pselect() return the number of file descriptors
       contained in the three returned descriptor sets (that is, the total
       number of bits that are set in readfds, writefds, exceptfds).  The return
       value may be zero if the timeout expired before any file descriptors
       became ready.

       On error, -1 is returned, and errno is set to indicate the error; the
       file descriptor sets are unmodified, and timeout becomes undefined.

       EBADF  An invalid file descriptor was given in one of the sets.  (Perhaps
              a file descriptor that was already closed, or one on which an
              error has occurred.)  However, see BUGS.

       EINTR  A signal was caught; see signal(7).

       EINVAL nfds is negative or exceeds the RLIMIT_NOFILE resource limit (see

       EINVAL The value contained within timeout is invalid.

       ENOMEM Unable to allocate memory for internal tables.

       pselect() was added to Linux in kernel 2.6.16.  Prior to this, pselect()
       was emulated in glibc (but see BUGS).

       select() conforms to POSIX.1-2001, POSIX.1-2008, and 4.4BSD (select()
       first appeared in 4.2BSD).  Generally portable to/from non-BSD systems
       supporting clones of the BSD socket layer (including System V variants).
       However, note that the System V variant typically sets the timeout
       variable before returning, but the BSD variant does not.

       pselect() is defined in POSIX.1g, and in POSIX.1-2001 and POSIX.1-2008.

       An fd_set is a fixed size buffer.  Executing FD_CLR() or FD_SET() with a
       value of fd that is negative or is equal to or larger than FD_SETSIZE
       will result in undefined behavior.  Moreover, POSIX requires fd to be a
       valid file descriptor.

       The operation of select() and pselect() is not affected by the O_NONBLOCK

       On some other UNIX systems, select() can fail with the error EAGAIN if
       the system fails to allocate kernel-internal resources, rather than
       ENOMEM as Linux does.  POSIX specifies this error for poll(2), but not
       for select().  Portable programs may wish to check for EAGAIN and loop,
       just as with EINTR.

   The self-pipe trick
       On systems that lack pselect(), reliable (and more portable) signal
       trapping can be achieved using the self-pipe trick.  In this technique, a
       signal handler writes a byte to a pipe whose other end is monitored by
       select() in the main program.  (To avoid possibly blocking when writing
       to a pipe that may be full or reading from a pipe that may be empty,
       nonblocking I/O is used when reading from and writing to the pipe.)

   Emulating usleep(3)
       Before the advent of usleep(3), some code employed a call to select()
       with all three sets empty, nfds zero, and a non-NULL timeout as a fairly
       portable way to sleep with subsecond precision.

   Correspondence between select() and poll() notifications
       Within the Linux kernel source, we find the following definitions which
       show the correspondence between the readable, writable, and exceptional
       condition notifications of select() and the event notifications provided
       by poll(2) and epoll(7):

                                EPOLLHUP | EPOLLERR)
                              /* Ready for reading */
                              /* Ready for writing */
           #define POLLEX_SET  (EPOLLPRI)
                              /* Exceptional condition */

   Multithreaded applications
       If a file descriptor being monitored by select() is closed in another
       thread, the result is unspecified.  On some UNIX systems, select()
       unblocks and returns, with an indication that the file descriptor is
       ready (a subsequent I/O operation will likely fail with an error, unless
       another process reopens file descriptor between the time select()
       returned and the I/O operation is performed).  On Linux (and some other
       systems), closing the file descriptor in another thread has no effect on
       select().  In summary, any application that relies on a particular
       behavior in this scenario must be considered buggy.

   C library/kernel differences
       The Linux kernel allows file descriptor sets of arbitrary size,
       determining the length of the sets to be checked from the value of nfds.
       However, in the glibc implementation, the fd_set type is fixed in size.
       See also BUGS.

       The pselect() interface described in this page is implemented by glibc.
       The underlying Linux system call is named pselect6().  This system call
       has somewhat different behavior from the glibc wrapper function.

       The Linux pselect6() system call modifies its timeout argument.  However,
       the glibc wrapper function hides this behavior by using a local variable
       for the timeout argument that is passed to the system call.  Thus, the
       glibc pselect() function does not modify its timeout argument; this is
       the behavior required by POSIX.1-2001.

       The final argument of the pselect6() system call is not a sigset_t *
       pointer, but is instead a structure of the form:

           struct {
               const kernel_sigset_t *ss;   /* Pointer to signal set */
               size_t ss_len;               /* Size (in bytes) of object
                                               pointed to by 'ss' */

       This allows the system call to obtain both a pointer to the signal set
       and its size, while allowing for the fact that most architectures support
       a maximum of 6 arguments to a system call.  See sigprocmask(2) for a
       discussion of the difference between the kernel and libc notion of the
       signal set.

   Historical glibc details
       Glibc 2.0 provided an incorrect version of pselect() that did not take a
       sigmask argument.

       In glibc versions 2.1 to 2.2.1, one must define _GNU_SOURCE in order to
       obtain the declaration of pselect() from <sys/select.h>.

       POSIX allows an implementation to define an upper limit, advertised via
       the constant FD_SETSIZE, on the range of file descriptors that can be
       specified in a file descriptor set.  The Linux kernel imposes no fixed
       limit, but the glibc implementation makes fd_set a fixed-size type, with
       FD_SETSIZE defined as 1024, and the FD_*() macros operating according to
       that limit.  To monitor file descriptors greater than 1023, use poll(2)
       or epoll(7) instead.

       The implementation of the fd_set arguments as value-result arguments is a
       design error that is avoided in poll(2) and epoll(7).

       According to POSIX, select() should check all specified file descriptors
       in the three file descriptor sets, up to the limit nfds-1.  However, the
       current implementation ignores any file descriptor in these sets that is
       greater than the maximum file descriptor number that the process
       currently has open.  According to POSIX, any such file descriptor that is
       specified in one of the sets should result in the error EBADF.

       Starting with version 2.1, glibc provided an emulation of pselect() that
       was implemented using sigprocmask(2) and select().  This implementation
       remained vulnerable to the very race condition that pselect() was
       designed to prevent.  Modern versions of glibc use the (race-free)
       pselect() system call on kernels where it is provided.

       On Linux, select() may report a socket file descriptor as "ready for
       reading", while nevertheless a subsequent read blocks.  This could for
       example happen when data has arrived but upon examination has the wrong
       checksum and is discarded.  There may be other circumstances in which a
       file descriptor is spuriously reported as ready.  Thus it may be safer to
       use O_NONBLOCK on sockets that should not block.

       On Linux, select() also modifies timeout if the call is interrupted by a
       signal handler (i.e., the EINTR error return).  This is not permitted by
       POSIX.1.  The Linux pselect() system call has the same behavior, but the
       glibc wrapper hides this behavior by internally copying the timeout to a
       local variable and passing that variable to the system call.

       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/select.h>

           fd_set rfds;
           struct timeval tv;
           int retval;

           /* Watch stdin (fd 0) to see when it has input. */

           FD_SET(0, &rfds);

           /* Wait up to five seconds. */

           tv.tv_sec = 5;
           tv.tv_usec = 0;

           retval = select(1, &rfds, NULL, NULL, &tv);
           /* Don't rely on the value of tv now! */

           if (retval == -1)
           else if (retval)
               printf("Data is available now.\n");
               /* FD_ISSET(0, &rfds) will be true. */
               printf("No data within five seconds.\n");


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

       For a tutorial with discussion and examples, see select_tut(2).

       This page is part of release 5.13 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                              2021-03-22                          SELECT(2)