select(2) — Linux manual page


select(2)                  System Calls Manual                 select(2)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/select.h>

       typedef /* ... */ fd_set;

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

   Feature Test Macro Requirements for glibc (see

           _POSIX_C_SOURCE >= 200112L

DESCRIPTION         top

       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.

       A structure type that can represent a set of file descriptors.
       According to POSIX, the maximum number of file descriptors in an
       fd_set structure is the value of the macro FD_SETSIZE.

   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 call.

       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-

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

              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

              The file descriptors in this set are watched for
              "exceptional conditions".  For examples of some
              exceptional conditions, see the discussion of POLLPRI in

              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 BUGS).

              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 either:

              •  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

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

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

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

       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 nanoseconds).

       •  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

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

       The corresponding argument for pselect() is a timespec(3)

       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.

RETURN VALUE         top

       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

ERRORS         top

       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 getrlimit(2)).

       EINVAL The value contained within timeout is invalid.

       ENOMEM Unable to allocate memory for internal tables.

VERSIONS         top

       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.

STANDARDS         top


HISTORY         top

              POSIX.1-2001, 4.4BSD (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.

              Linux 2.6.16.  POSIX.1g, POSIX.1-2001.

              Prior to this, it was emulated in glibc (but see BUGS).

       fd_set POSIX.1-2001.

NOTES         top

       The following header also provides the fd_set type: <sys/time.h>.

       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 flag.

   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

   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 the 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

       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.

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

BUGS         top

       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

       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 glibc 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

       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.

EXAMPLES         top

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

           int             retval;
           fd_set          rfds;
           struct timeval  tv;

           /* 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");


SEE ALSO         top

       accept(2), connect(2), poll(2), read(2), recv(2),
       restart_syscall(2), send(2), sigprocmask(2), write(2),
       timespec(3), epoll(7), time(7)

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

COLOPHON         top

       This page is part of the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨⟩.  If you have a bug report
       for this manual page, see
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
       fetched from
       ⟨⟩ on
       2024-06-26.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
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       part of the original manual page), send a mail to

Linux man-pages 6.9.1          2024-06-15                      select(2)

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